MOBILE PHONE PULSE OXIMETER

A Thesis Presented

By

Paul Aaron Bohn

to

The Department of Electrical and Computer Engineering

in partial fulfillment of the requirements for the degree of

Master of Science

in the field of

Electrical Engineering

Northeastern University Boston, Massachusetts

May 2015

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DEDICATION

To my wife Erin and to my family for their life long support of all my engineering endeavors.

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ACKNOWLEDGMENTS I would like to thank the following individuals for their support: Prof. Mark Niedre my thesis advisor for his guidance and for allowing me work independently towards mutual goals. Prof. Gunar Schirner for helping with brainstorming and providing resources to the project. J.P. Laine, my group leader at the Charles Stark Draper Laboratory, Inc., for his encouragement to continue my pursuit of education and for his constant career mentoring. Walter Foley, a colleague, for his friendship and for sharing some of the knowledge that he obtained over his forty-plus year career in engineering. Mary Beth Weissman, Elaine Arnold, and Steve Panagakos at Precision Graphics Inc. for assembling the prototype printed circuit boards. Janki Bhimani and Phanindhar Repala for their contributions towards the development of an Android application. The Senior Capstone Group consisting of Dan Thompson, Dan Abel, Dan Huehner, Hunt Graham, Portia Stephens, and Kerrianne O'Brien for continuing the project after my thesis work and taking it to the next level.

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TABLE OF CONTENTS

LIST OF TABLES ............................................................................................................................... vii

LIST OF FIGURES ............................................................................................................................ viii

LIST OF ACRONYMS .......................................................................................................................... x

ABSTRACT ....................................................................................................................................... xii

CHAPTER 1: INTRODUCTION ............................................................................................................ 1

1.1 Overview ................................................................................................................................ 1

1.2 Intro to Pulse Oximetry .......................................................................................................... 1

1.3 Low Cost P-Ox for the Developing World ............................................................................ 3

1.4 Proposed / Design .................................................................................................................. 4

1.5 Candidates Contribution ........................................................................................................ 5

CHAPTER 2: SYSTEM DESIGN ........................................................................................................... 6

2.1 Design Goals .......................................................................................................................... 6

2.2 System Architecture ............................................................................................................... 9

2.3 System Behavior .................................................................................................................. 13

2.3.1 MCU ............................................................................................................................. 13

2.3.2 Mobile Phone Application ............................................................................................ 14

CHAPTER 3: HARDWARE DESIGN ................................................................................................... 19

3.1 Audio Breakout Board Description ...................................................................................... 19

3.2 Testbed Board Design Description ...................................................................................... 21

3.2.1 Probes ............................................................................................................................ 22

3.2.2 Microcontroller ............................................................................................................. 22

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3.2.3 Audio Communication .................................................................................................. 23

3.2.4 Power Management ...................................................................................................... 23

CHAPTER 4: SOFTWARE DESIGN .................................................................................................... 25

4.1 Testbed Code ....................................................................................................................... 25

CHAPTER 5: TESTING AND ANALYSIS ............................................................................................. 28

5.1 Test Setup One ..................................................................................................................... 28

5.1.1 Hardware ....................................................................................................................... 28

5.1.2 Software ........................................................................................................................ 30

5.1.3 Testing and Analysis ..................................................................................................... 32

5.2 Test Setup Two .................................................................................................................... 42

5.2.1 Hardware ....................................................................................................................... 42

5.2.2 Software ........................................................................................................................ 44

5.2.3 Testing and Analysis ..................................................................................................... 45

CHAPTER 6: CONCLUSION .............................................................................................................. 51

REFERENCES ................................................................................................................................... 53

APPENDIX ....................................................................................................................................... 54

I. SCHEMATIC ................................................................................................................................. 55

II. BILL OF MATERIALS .................................................................................................................... 60

III. PCB LAYOUTS ............................................................................................................................ 65

IV. ELECTRONICS CALCULATIONS ................................................................................................... 73

V. MATLAB SCRIPTS ........................................................................................................................ 82

VI. WORK BREAKDOWN STRUCTURE ............................................................................................. 90

VII. WORK STATUS UPDATES ....................................................................................................... 101

VIII. PART NUMBERING SYSTEM .................................................................................................. 119

ABOUT THE AUTHOR.................................................................................................................... 121

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LIST OF TABLES Table 1: General Design Goals ........................................................................................................ 6

Table 2: Specific Design Goals ........................................................................................................ 8

Table 3: Pulse Oximeter Device Components ............................................................................... 10

Table 4: Mobile Phone Components .............................................................................................. 11

Table 5: Mobile Phone Pulse Oximeter Testbed Board Feature List ............................................. 24

Table 6: Variable Descriptions For pox.m ..................................................................................... 26

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LIST OF FIGURES Figure 1: Molar Extinction Coefficient for Hemoglobin in Water vs. Wavelength ........................ 2

Figure 2: System Diagram ............................................................................................................. 12

Figure 3: MCU Firmware Component Diagram ............................................................................ 13

Figure 4: Mobile Phone Functional Diagram with Android Application Components ................. 15

Figure 5: Pictorial Start-Up Instructions ........................................................................................ 16

Figure 6: Conceptual GUI (Top Left) Loading Screenshot (Top Right) Icon (Bottom Left)

Android GUI Prototype (Bottom Right) Early Interface ............................................................... 17

Figure 7: Audio Breakout Board .................................................................................................... 20

Figure 8: Audio Breakout Board Close Up .................................................................................... 21

Figure 9: Mobile Phone Pulse Oximeter Testbed .......................................................................... 22

Figure 10: Silicon Labs EFM32TG MCU Tiny Gecko Starter Kit ................................................ 23

Figure 11: Test Setup One Block Diagram .................................................................................... 29

Figure 12: Test Setup One Components ........................................................................................ 29

Figure 13: AFE4400SPO2EVM Evaluation Board with SPI Lead Wires ..................................... 30

Figure 14: Texas Instruments AFE44x0SPO2EVM GUI .............................................................. 31

Figure 15: Total Phase Data Center GUI ....................................................................................... 31

Figure 16: Percent SpO2 & HR Display ........................................................................................ 32

Figure 17: ADC Capture & Analysis ............................................................................................. 33

Figure 18: Save Data to CSV File .................................................................................................. 34

Figure 19: Save Low Level Configuration .................................................................................... 35

Figure 20: Oscilloscope Capture SPI SCLK Frequency & Period ................................................ 36

Figure 21: Register Setting Screen Capture from AFE44x0SPO2EVM GUI ............................... 37

Figure 22: Register Setting Screen Capture from Protocol Analyzer GUI Data Center ................ 37

Figure 23: Oscilloscope Read Register LED2STC Address: 0x0A Data: 0x17C0 ........................ 37

Figure 24: pox.m MATLAB Command Window Output for AFE4400SPO2EVM ..................... 38

Figure 25: RAW Data from AFE4400SPO2EVM ......................................................................... 39

Figure 26: Normalized Data from AFE4400SPO2EVM ............................................................... 39

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Figure 27: SpO2 (red); SpO2 Ambient Subtraction (blue) from AFE4400SPO2EVM ................. 40

Figure 28: High Pass Filter Response ............................................................................................ 41

Figure 29: IR Ambient Subtraction Before (red); After Filter (blue) from AFE4400SPO2EVM . 42

Figure 30: Test Setup Two Block Diagram ................................................................................... 43

Figure 31: Hardware Setup Two Pulse Oximeter Testbed & Silicon Labs MCU ......................... 43

Figure 32: Pulse Oximeter Testbed Close Up ................................................................................ 44

Figure 33: Data Flow Diagram ...................................................................................................... 45

Figure 34: pox.m MATLAB Command Window Output for Testbed .......................................... 46

Figure 35: RAW Data from Testbed .............................................................................................. 47

Figure 36: RAW Data Comparison for AFE4400SPO2EVM & Testbed ...................................... 48

Figure 37: Normalized Data from Testbed .................................................................................... 49

Figure 38: Calculated SpO2 (red) SpO2 Ambient Subtraction (blue) from Testbed ..................... 50

Figure 39: IR Ambient Subtraction Before Filter (red); After Filter (blue) from Testbed ............ 50

Figure 40: Android Application Hardware Demo by Undergraduate Capstone Team .................. 51

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LIST OF ACRONYMS ADC Analog to Digital Converter

AFE Analog Front-End

AGC Automatic Gain Control

COTS Commercial off-the-shelf

CS Chip Select

CSV Comma separated values

DAC Digital to Analog Converter

ECG Electrocardiogram

GPIO General Purpose Input Output

GUI Graphical User Interface

HR Heart Rate

Hb Deoxyhemoglobin

HbO2 Oxyhemoglobin

LED Light Emitting Diode

MCU Microcontroller Unit

MISO Master Input Slave Output

MOSI Master Output Slave Input

NIR Near Infrared

PC Personal Computer

PCB Printed Circuit Board

Pleth Photoplethysmogram

Pox Pulse Oximeter

PR Pulse Rate

SCLK Serial Clock

SpO2 Blood Oxygen Saturation

SPI Serial Peripheral Interface Bus

SNR Signal-to-noise Ratio

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TIA Transimpedance Amplifier

USB Universal Serial Bus

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ABSTRACT

There is global demand for low cost medical care diagnostics, and too often these life-saving tools are unavailable to low-income countries and remote areas of the world. One example of such a device is a pulse oximeter: a vital instrument that measures blood oxygen saturation. A modern medical grade pulse oximeter is often physically large and requires training and access to a mains power. All such points are contrary to operation in a remote low-income locale. This project covers the design, fabrication, and testing of a testbed pulse oximeter that is targeted for use in the developing world. While developing areas may be lacking infrastructure, mobile phones are readily available. Mobile phones provide a source of power, computation, and access to information that was previously unavailable. By connecting through the headset jack, the testbed design utilizes the phone’s capabilities run a pulse oximeter. Data was acquired from the testbed and then analyzed with good results. Global access to point-of-care medical devices can be significantly improved with further use of mobile computing. This will lead to reductions in cost, increased portability, improved patient compliance, and expanded distribution of medical knowledge.

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CHAPTER 1: INTRODUCTION

1.1 Overview

The objectives of the thesis are to i) create a record of a testbed pulse oximeter design

that is targeted for use in the developing world, ii) to freely distribute the design information, and

iii) to transfer the knowledge to the Northeastern University student body to continue the work.

1.2 Intro to Pulse Oximetry

A pulse oximeter is a device that measures blood oxygen saturation (SpO2) and pulse rate

(PR) which are important indicators of health. All organs need oxygen to function properly;

performing simple cognitive tasks becomes challenging within minutes after a loss of oxygen.

The heart and brain are particularly sensitive to a reduction in oxygen, and when those organs

begin to fail, risk of death rises dramatically (World Health Organization, 2011). The use of pulse

oximeters can greatly reduce the risk of death during medical procedures that require general

anaesthesia by monitoring a patient's blood oxygen level and intervening when the oxygen drops

below an acceptable range (World Health Organization, 2011).

A pulse oximeter indirectly measures blood oxygen saturation by gauging the color

intensity of blood (Texas Instruments, 2013). Color is an indicator of the amount of oxygen that is

in blood. Red blood cells contain a protein molecule called hemoglobin that transports oxygen

(World Health Organization, 2011). There are two types of hemoglobin: oxyhemoglobin is

hemoglobin combined with oxygen, and deoxyhemoglobin is hemoglobin that lacks oxygen.

Blood transports oxygen from the lungs and heart to the extremities where it is released into

tissues. In a cyclical fashion, oxygen lacking blood returns to the heart and lungs to be

oxygenated. Blood rich with oxyhemoglobin appears red because it filters most light from the

visual spectrum except for red light. The dashed line in figure 1 represents the light absorption of

oxyhemoglobin. The line sharply dips near the red (~660nm) portion of the visual spectrum; thus,

allowing red light to transmit and scatter through the blood. The solid line in figure 1 represents

the light absorption of deoxyhemoglobin (Hb). At the red wavelength of 660nm the

deoxyhemoglobin (Hb) absorption is much greater than oxyhemoglobin (HbO2). The additional

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attenuation of light from the lower concentration of oxygen causes blood to become dark blue in

color. This explains why the skin of somebody who stops breathing starts to appear blue.

Pulse oximeters typically measure blood color intensity at two different wavelengths of

light- red and near-infrared (Texas Instruments, 2013). These wavelengths are chosen for their

light transmission characteristics through biological tissue. The common finger-clip pulse

oximeters use two LEDs one for each wavelength and a photodetector in a transmissive

arrangement. The LEDs are on located on one side of the finger while the photodetector is located

on the other side such that light is transmitted through the tissue during operation.

Figure 1: Molar Extinction Coefficient for Hemoglobin in Water vs. Wavelength

The concentration of oxy- and deoxyhemoglobin can be estimated with knowledge of the

absorption coefficients at two wavelengths (Wang & Wu, 2007). “The absorption coefficient is

defined as the probability of photon absorption in a medium per unit path length” (Wang & Wu,

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2007). Figure 1 has two vertical lines that intersect with the molar extinction coefficient curves.

One vertical line is at a wavelength of 660nm (red) and another at 895nm (NIR). An absorption

coefficient equation can be written for each of the wavelengths (Wang & Wu, 2007). The

equations are in terms of wavelength molar extinction coefficients and molar concentration of

oxy- and deoxyhemoglobin (Wang & Wu, 2007). The two equations make up a system with two

unknowns variables. Once oxy- and deoxyhemoglobin is determined. Blood oxygen saturation

can be calculated by taking the ratio oxyhemoglobin to the total hemoglobin in the blood (Wang

& Wu, 2007).

Pulse oximeters often display blood volumetric pulse waveform over time, and report

pulse rate in beats per minute. This is called a pleth waveform, which is shorthand for

photoplethysmogram. The changing blood volume is determined by shining light through tissue

while sensing the returning light with a photodetector. Some of the light will be absorbed in a

periodic fashion as blood pulsates through the body causing a fluctuation in light intensity. The

pulsation of light is sensed by the photodetector then recorded and displayed by the device. The

device then calculates pulse rate by monitoring the number of pulses that occurred in a given

time.

Most pulse oximeters determine SpO2 from an empirical formula (Texas Instruments,

2013). First, the red and IR signals are normalized to remove the DC component caused by

biological tissue and bone. Second, a ratio of the red-IR normalized signals is taken. Third, a

calibration offset and scale factor is applied to the red-IR ratio to generate the resultant SpO2.

The scale factor and offset is empirically determined through calibration and is often stored in a

lookup table.

1.3 Low Cost P-Ox for the Developing World

In the developing world, there is a great need for low-cost point-of-care medical devices

such as pulse oximeters. The lack of infrastructure, reliable source of power, transportation, and

well trained medical workers makes it difficult, if not impossible, to deploy medical equipment

designed to operate in stable hospital conditions. Limited access to medical devices \in low-

income countries leads to lesser quality of life and shortened life spans.

[For example], children with very severe or severe pneumonia should usually be

treated in hospital, but many low-income and middle-income countries do not have a

sufficient number of hospital beds for this strategy. . . .

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If pneumonia is combined with hypoxaemia, as happens in 13% of cases, children

are five-times more likely to die than are those with only pneumonia. Oxygen

concentrations should therefore be monitored and oxygen therapy should be made

available, but this approach is not always possible. Low-income and middle-income

countries need an estimated 1,000,000 pulse oximeters. (Howitt, et al., 2012)

Affordability of medical devices is of concern. Medical grade pulse oximeters can cost

hundreds or even thousands of US dollars. Low grade devices can be purchased for around 20 US

dollars, but may not meet medical standards or may not come with needed features for use in a

low income situation. Market analysis is required to obtain a true target cost that is viable for low-

income countries. One could imagine a scenario where some of the cost could be offset through

philanthropic means.

1.4 Proposed / Design

The testbed pulse oximeter design outlined in this thesis showcases an example of a

mobile phone turning it into a medical device. Mobile phones have many desirable characteristics

that make them well suited for the task. For example, they are readily available throughout the

world. As of May 2014, there were nearly seven billion mobile subscriptions worldwide

(MOBITHINKING, 2014). 5.4 billion of those subscribers are in the developing countries with a

90.2% market penetration (MOBITHINKING, 2014). Phones provide a reliable source of power.

This is directly achievable by harvesting energy from the phone. Indirectly, the phone’s charger

or a spare battery pack can serve as a power source. Mobile phones have ample computational

power and storage for many medical diagnostic applications. Mobile phones can provide an

avenue for telemedicine by storing data and sending diagnostic results to trained clinicians for

analysis and monitoring. Finally, phones can run applications to guide untrained users. These

applications can provide real-time feedback, medical advice, and operator instructions.

The audio headset jack offers a path to interface medical devices with a mobile phone

(Kuo, Verma, Schmid, & Dutta, 2010). University of Michigan’s Hijack project demonstrated

that data and power can be transferred over the common headset jack opening a method to

connect sensors to existing mobile phones. Drawing power from the headset jack eliminates the

need for batteries or alternate power sources thus keeping cost, size, and weight down. It reduces

the logistical need for batteries to keep the devices operating. Sending diagnostic results through

the audio headset jack forgoes the need for a proprietary data communication port. Almost all

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mobile phones have a headset jack so it can serve a near universal solution for transmitting low

bandwidth data to a mobile phone. In addition, a modular platform could be developed around the

headset jack interface to add other sensor modalities such as EKG, temperature, blood pressure,

etc. (Kuo, Verma, Schmid, & Dutta, 2010). The remainder of this document addresses design

approaches for the development of a testbed pulse oximeter that is powered by and sends data

through a mobile phone audio headset jack.

1.5 Candidates Contribution

The effort to develop a pulse oximeter testbed was led by the thesis candidate and this

involved the management of fellow students. A summary of work for this project included: (1)

literature research and a design trade study, (2) electronics design and layout of two custom

circuit boards: a mobile phone audio breakout board test fixture and a testbed for pulse oximetry,

(3) the assembly of three breakout boards, (4) managed vendors and manufacturers for test

equipment, parts procurement, PCB manufacturing, and assembly work, (5) wrote MCU test

software in C language and analysis software in MATLAB, (6) wrote thorough technical and

programmatic documentation, (7) tested the hardware, (8) turned over a working example to a

Northeastern University undergraduate capstone group for future improvements.

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CHAPTER 2: SYSTEM DESIGN

The pulse oximeter system design takes into account the unique environment, use cases,

and constraints presented with a design for the developing world. This chapter covers: (1) general

and specific design goals that were derived from an initial needs assessment, (2) several design

approaches that were considered, (3) a system architecture and behavioral model.

2.1 Design Goals

The needs assessment and design goals that are listed in tables 1 and 2 are forward

looking with the intention that the testbed described in this thesis will make an incremental step

towards meeting the goals listed in this chapter.

General Design Goals

Description

Public Acceptance ● The device must be accepted by users and medical providers of the developing world.

Safety ● The device must pose a very small safety risk to the users. This includes the use of non-hazardous materials/coatings, no choking hazards, no cutting hazards, no electrical hazards, etc.

Reliable ● The device must be very reliable during product lifetime and during operation.

Ease of Operation ● The device must be easily setup. This includes the uses of hardware and software

● The device will display user instructions in real-time.

Performance ● The device must operate in accordance with industry standards for pulse oximetry.

● The device may not perform at the same standards as a high-end medical device in favor for power savings, cost minimization, public acceptance, and ease of operation.

Table 1: General Design Goals

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General Design Goals

Description

Minimum Cost ● The final price of the device must be affordable to medical providers in low income countries.

Minimum Cost ● The final price of the device must be affordable to medical providers in low income countries.

Use of Standard Parts ● The device will incorporate commercial-off-the-shelf (COTS) electronics and mechanical components to minimize cost.

Ease of Construction ● The device will have minimal physical parts to simplify construction.

● The device will be designed to demonstrate a path for high volume manufacturing.

● The device will be disassemblable.

Minimum Maintenance and Ease of Maintenance

● The device will require light cleaning after each use. ● Design cost minimization will be weighted heavier than ease

of maintenance. ● The device will run self-diagnostics to determine if it is

operating within specifications. If it detects that it is not operating within specifications, it will deactivate and provide user feedback.

Reconfigurable ● The device will work with a broad range of mobile devices. ● The device will work with adults, children, and infants. ● The device itself will not be reconfigurable in favor of cost

minimization, ease of use, ease of construction, ease of maintenance.

Durability ● The device will be ruggedized for field use. ○ The device will handle repeated drops from 2 meters

off the ground onto solid pavement. ○ The device will be liquid resistant.

● The device will operate in environmental conditions unique to its operating locale.

Environmental Protection

● The device should be fully recyclable. ● Should minimize the use of materials and energy to construct

the device. ○ Low SWaP (size weight and power)

● The device should have minimal toxic materials.

Table 1: General Design Goals Continued

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Specific Design Goals

Description

Physical ● Finger size form factor ● Incorporate finger sensor (photodetector and LEDs) into device ● Robust cabling

Functional ● Must operate for the duration of a surgical procedure ● Audible feedback ● Alarm modes

○ High PR ○ Low PR ○ No PR ○ Desaturation

● Will operate in low power mode ○ Detect inactivity

● Will harvest energy from phone ● Compatible with as many mobile phones as possible ● Telemedicine ● Provide real-time help ● Bidirectional data transmission over mobile phone headset jack

Environmental ● Regional operating requirements

Economic ● Quantities in the thousands to hundred thousands

Legal ● Must meet most medical regulatory requirements as long as they do not interfere with high-level goals

● Open intellectual property ● Open source hardware and software

Human Factors & Ergonomics

● Compatible with adults and children

Table 2: Specific Design Goals

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2.2 System Architecture

At the top level the system is comprised of a mobile phone and a pulse oximeter device.

The next layer down includes the components that make up the phone and the pulse oximeter

device. The final layer is made up of the software that runs on both devices.

Several design approaches and architectures were considered for the pulse oximeter

device. The design goal to reduce the overall system cost led to some ideas to use the mobile

phone’s audio circuitry to directly drive the pulse oximeter functionality with minimal

components. This approach would reduce the universality of the solution by limiting the different

types of phones that would work with the device. The pleth measurement performance would also

vary with the phone type.

Another approach was to design the pulse oximeter device circuitry using discrete

electronic components. This is certainly a valid approach that could be cost-effective depending

on parts selection. The disadvantages of this choice include an increase in the number of parts and

a modest increase in physical volume and complexity.

The approach that was selected for this thesis work included the use of a purpose-built

integrated circuit for pulse oximetry. This approach offers several advantages: increased pleth

measurement performance, a reduction in parts count, increased manufacturability, and the cost

drops significantly with volume. The disadvantages might include: a reliance on a single vendor

for the integrated circuit, and the unit cost for small production quantities. As of April 2015, the

current price for 2500 pieces of Texas Instruments part number AFE4490RHAR is 8.94 USD.

While the price may seem a little high, and it may be considered to be a disadvantage it is worth

mentioning that AFE4490 is a medical grade part that can be found in high end instruments. Also,

high production volume will offer a reduction in price and it is likely a business arrangement

could be made with Texas Instruments that would further the cause.

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Tables 3 and 4 list the pulse oximeter device and mobile phone components for the

selected design approach.

Pulse Oximeter Component

Subcomponents Description

Printed Circuit Board

Photodetector, IR LED, & red LED Sensor probe

Analog front-end for pulse oximetry Detector, LED illumination, signal

digitization, and timing electronics

Microcontroller Provides SpO2 calculation, analog

front-end control, and serial data

communication

Power management electronics Power harvesting, regulation, &

storage

Enclosure Probe/Sensor Fixture Mechanical enclosure features that

holds LEDs and photodiodes

Finger Clip Hinge mechanism and enclosure

body

Data & Power Interface Cable Assembly

Shielded four conductor cable Cable assembly connects the mobile

phone to the pulse oximeter 3.5mm audio connector

Table 3: Pulse Oximeter Device Components

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Mobile Phone Component

Description

Processor Provides computation power to process and display data.

Memory Provides data storage.

Display Provides graphical user interface.

Speakers Provides audible feedback to the user.

Data & Power Interface Audio jack provides data and power interface.

Radios Provides access to the internet and remote monitoring station.

Battery / Power Pack Provides power through an internal mobile phone battery or an external power pack.

Charger Mobile phone charger that can be solar, thermal or a standard wall adapter.

Table 4: Mobile Phone Components

Figure 2 System Diagram shows interconnect between electronic components. This

includes the communications between the analog-front end, MCU and phone. The diagram shows

a phone’s right audio channel supplying the power management block with a power signal. The

power management block supplies power to the MCU and phone while the MCU has some

control authority over the power management block.

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Figure 2: System Diagram

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2.3 System Behavior

Much of the system behavior is defined by the device code. The code will include an

Android mobile phone application for user interface and microcontroller code for hardware

management.

2.3.1 MCU

The microcontroller firmware will have many components; most of which are shown in

Figure 3 MCU Firmware Component Diagram. There will be an initialization routine that occurs

during power up plus components for power management, diagnostics, and debugging. There will

also be two control loops: the first is for ambient light cancellation, the second is for automatic

gain control and LED output. There will be routines for calculating SpO2 and pulse rate (PR).

The SpO2 calibration data will be stored in MCU memory. The MCU will handle

communications between the AFE4490 analog front-end for pulse oximetry and the mobile

phone.

Figure 3: MCU Firmware Component Diagram

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On power up, the MCU will initialize chip settings, GPIO pin settings, USART,

interrupts, etc. It will enable the primary LED voltage regulator, and then it will send the default

settings to the analog front-end. The MCU will handle the power management, sequencing, and

monitoring. It will report diagnostic values from the analog front-end and error states to the

mobile phone. Debugging code will be embedded for testing communications and test modes of

operation.

A minimum of two control loops are needed for the device to operate in a dynamic

environment. One control loop will be used for ambient light cancellation. The other control loop

will be used to automatically adjust LED current levels and amplifier gain settings. The ambient

light cancellation loop will input ambient data from the analog front-end then estimate the

ambient value to be cancelled. Finally it will adjust the cancellation current by digitally

controlling the ambient-cancellation DAC (Texas Instruments, 2014). The loop adjusts the dc-

biased signal coming from the photodiode I-V amplifier to the midpoint value of the second gain

stage amplifier (Texas Instruments, 2014). The automatic gain control (AGC) loop adjusts the

amplifier gain settings and the LED current levels to maximize the signal-to-noise ratio (SNR)

while limiting power consumption to the available energy capacity. An additional power

optimization control loop could be implemented that would change the measurement duty cycle

and current levels to meet power consumption needs.

Data communication to and from the analog front-end will be accomplished through

serial peripheral interface (SPI) bus. The microcontroller comes with an SPI hardware peripheral

that is controlled by setting MCU registers. An SPI library with functions for initialization,

reading, and writing registers will be used in the main program. The mobile phone data

communications will be sent over the phone’s headset jack by AC-coupling audio signals to

microcontroller GPIO pins. The headset jack audio signals will contain Manchester coded binary

data. The microphone channel will handle the transmission of data to the phone, and the left

stereo channel will be used to receive data from the phone. A library of interrupt driven functions

will be used for the phone audio communications.

2.3.2 Mobile Phone Application

The mobile phone Android application will contain the following components: a

graphical user interface (GUI), a real-time user help module, an alarm handler, an audible pulsed

tone generator, an audio data communication interface, a sinewave generator for device power, a

data logger, a cloud server internet client, and device communication debugging interface. Figure

4 Mobile Phone Functional Diagram details some of these components.

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Figure 4: Mobile Phone Functional Diagram with Android Application Components

The GUI will provide easy-to-understand instructions to an unfamiliar user. The device

interface will be similar to professional medical grade instruments in the operating room, so that

trained medical staff will be comfortable utilizing the device. After launching the Android

application the user will be presented with graphical instructions that explain the setup of the

device. Figure 5 Pictorial Start-Up Instructions is an example series of screens that could be

displayed to a user.

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Figure 5: Pictorial Start-Up Instructions

First the device must be plugged in, and then the Android application will generate a sine

wave audio signal on the right audio channel to power the device. Adjusting the mobile phone

volume to maximum provides the most energy to the device. After the device initializes, the

mobile phone application will initiate communications with the device over the left audio channel

and mic channel. After communications with the device is established, the main screen will

appear. If any error states exist the user will be presented with graphical instructions on how to

correct the problem. Figure 6 shows four example Android application screenshots. The two

rightmost images are concepts created with Google Draw, and the two leftmost images are

screenshots from an early Android GUI prototype application.

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Figure 6: Conceptual GUI (Top Left) Loading Screenshot (Top Right) Icon (Bottom Left) Android GUI Prototype (Bottom Right) Early Interface

During normal operation, the device will numerically display the SpO2 level and pulse

rate. It will graphically display the SpO2 time series and alert the user during alarm states. The

device will display the appropriate WHO pulse-oximetry hypoxia decision-making criteria to

provide real-time active support to the user (Howitt, et al., 2012). An audible tone will beep with

the pulse rate rhythm. The pitch of the tone will get higher with increasing SpO2 levels and will

droop as SpO2 levels fall.

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The application will have a data logging capability. SpO2 records for a given patient and

event can be saved and later retrieved. The records include information on patient identification,

time, geolocation, device id number, phone information, software/firmware version, power usage,

SpO2 raw samples, pulse rate raw samples, alarms, errors, AFE4490 register settings, and

application settings. The record data is saved in a binary format to conserve memory space and to

reduce the wireless bandwidth needed for transmission.

The application will be able to transmit the data in real-time to a remote location over the

internet. This functionality could be used in a number of different ways. For example, it could be

sent to nursing staff to monitor a patient that is in another room, or it could be used to send data

across the world for immediate expert analysis. In addition, the data can be stored to a master

database for long term medical studies.

The application will have a simple text field interface for debug and test of the

communications with the analog front-end and MCU. There will be a text field for register

address and data, a radio button to select the AFE4490 or MCU, and the following buttons: one to

read data and one to write data. If a user wants to read the AFE4490 diagnostic register they will

execute the following four steps. First, they will select the radio button for the AFE4490. Second,

they will initiate the diagnostic sequence by writing to register CONTROL0 by typing 0x00 into

the address field and 0x0004 in the data field. Then they will press the write register button.

Third, they will enable an AFE4490 SPI register read by writing to register CONTROL0 by

typing 0x00 into the address field and 0x0001 into the data field. Then they will press the write

register button. Forth, they will read the DIAG register by typing 0x30 into the address field.

Then they will press the read button to retrieve the data. Similarly, the MCU will have control,

status, and data registers that will be accessed by selecting the MCU radio button.

19

CHAPTER 3: HARDWARE DESIGN

Two custom printed circuit boards (PCBs) were designed, manufactured, and assembled

for this project. One, an audio breakout board was created to gain access to mobile phone audio

signals for testing purposes. Two, a pulse oximeter testbed board was created to experiment and

test early concepts. An analog front-end evaluation module for pulse oximetry from Texas

Instruments was used as a learning tool and reference design to aid the developmental process.

3.1 Audio Breakout Board Description

The audio breakout board consists of a standard cell phone headset cable with a plug, a

headset jack, and four BNC connectors that are attached to a printed circuit board (PCB) with

outer dimensions of 2.5 in by 3.8 in. The PCB has two layers with a ground plane and has rubber

feet so it can rest upright on a bench top. All cables are strain relieved to prevent poor electro-

mechanical connection. The electrical signals are passed one-to-one from the headset plug to the

headset jack using a standard cell phone connector pinout. There is a BNC connector for the left

stereo channel, right stereo channel, and microphone signals. The fourth BNC connector is routed

to the microphone signal through a first order RC high pass filter circuit that will block the

microphone DC bias voltage.

The purpose of the breakout board is to gain access to the audio signals between the cell

phone and an accessory, so an engineer can evaluate their performance. The audio signals can be

monitored by hooking up standard laboratory test equipment, such as an oscilloscope, to the BNC

connectors. One possible setup is to plug the breakout board cable into a cell phone, and then

plug an accessory into the breakout board headset jack.

20

Figure 7: Audio Breakout Board

21

Figure 8: Audio Breakout Board Close Up

3.2 Testbed Board Design Description

The cell phone pulse-oximeter development board is a capable testbed for evaluating (1)

pulse oximeter sensor probes (2) the AFE4490 Texas Instruments analog front-end (3) the Silicon

Labs Gecko microcontroller family (4) cell phone audio jack digital communication, and (5)

energy harvesting and storage. Some notable features include two high efficiency adjustable

voltage regulators, a super capacitor for energy storage, and an on-board pulse-oximeter sensor

probe.

The major circuit elements include (1) pulse oximeter probe, (2) analog front-end, (3)

microcontroller, (4) audio jack communication, and (5) power management. The pulse oximeter

probe detects changing light absorption of hemoglobin in blood with a photodetector. The

photodetector converts the light into an analog electronic signal that is sent to an analog front-end

where it is conditioned and digitized. The digitized signal is transmitted via SPI serial protocol to

microcontroller that calculates blood oxygen saturation (SPO2). The SPO2 result is transmitted to

a cell phone through the audio jack. The result can then be displayed on the cell phone screen. All

circuitry is powered by the cell phone’s audio port. The cell phone sends a 20 KHz sine wave

22

from the right audio channel into a step-up transformer. The signal is rectified, filtered, regulated,

and then temporarily stored in a capacitor for later use.

Figure 9: Mobile Phone Pulse Oximeter Testbed

3.2.1 Probes

This design can operate with its onboard pulse oximeter probe or can work with an

external probe. By default, the board is configured to work with the onboard probe. An external

probe can be used by removing R40, R41, R42, and R43.

3.2.2 Microcontroller

The microcontroller resides on a separate printed circuit board (PCB) called a Gecko

starter kit made by Silicon Labs. The starter kit was made to demonstrate the microcontroller’s

capabilities and will be used to develop firmware. Silicon Labs offers six commercially available

starter kits that are plug compatible with the cell phone pulse oximeter board. Each kit has a

different version of the same microcontroller line.

23

Figure 10: Silicon Labs EFM32TG MCU Tiny Gecko Starter Kit

3.2.3 Audio Communication

This design has two audio jack communication circuits. One circuit is for transmitting

and the other circuit is for receiving digital data. The transmitter circuit includes a voltage divider

that midpoint biases the microcontroller input pin that is AC coupled to the left stereo audio jack

channel. The receiver circuit is connected to a microcontroller output pin that is low-pass filtered

and AC coupled to the microphone audio jack channel. R21 can be populated with a 1kohm

resistor to allow an iPhone to detect that a cable is plugged into the audio jack.

3.2.4 Power Management

The input impedance of the power harvester circuit can be changed by adding parallel

capacitance using DIP switch SW1. Changing the impedance will impact the power transfer

characteristics of the harvester circuit. Two high efficiency adjustable voltage regulators can be

driven by the energy harvester or Gecko starter kit power supply by selecting jumpers JP3 and

JP3.

24

Features Description

Pulse Oximeter Analog front-end

● Texas Instruments AFE4490 Integrated Analog Front End for

Pulse Oximeters

Sensors ● Support an external NellcorTM Probe

● On board pulse oximeter probe consisting of OSI

Optoelectronics DLED-660/895-CSL-2 LED pair & PIN-8.0-

LLS photodetector

Microcontroller ● Support for Silicon Labs EFM32 Gecko 32-bit Microcontroller

Family

Communications ● Audio jack communication circuitry

Power Management ● Power harvester circuitry

● Two high efficiency adjustable voltage regulators

● PCB footprint for large capacitor on the output of LED

regulator for power storage

● Test points for measuring current and power of all supplies

● The ability to switch between Gecko MCU Starter Kit power

and audio jack power

● The ability to change input impedance of audio jack power

supply (transformer/capacitor)

● Test points to hook up an external load for power testing

Connectors ● DB9 - NellCor DS-100A ● 20-pin female header - EFM32 Gecko Starter Kits

● 10-pin male header - Total Phase Protocol Analyzer (SPI)

● 3.5mm audio jack - Mobile phone connection

Table 5: Mobile Phone Pulse Oximeter Testbed Board Feature List

25

CHAPTER 4: SOFTWARE DESIGN

The project codebase consists of microcontroller code, Android mobile phone code, and

MATLAB code. This thesis work concluded with a code package for testbed hardware and a

prototype Android GUI application. Future work will be needed to create device code for the final

pulse oximeter design.

4.1 Testbed Code

The MCU test code was developed using Silicon Labs Simplicity StudioTM. Simplicity

StudioTM was chosen because it is freely provided by the Gecko EFM32TM MCU manufacturer.

Scripting and algorithm development was accomplished with Mathworks MATLAB. MATLAB

was chosen for its ease-of-use, its capability, the author’s familiarity with the tool, and its license

accessibility. Future development should use free-license software and open source tools to keep

costs down and increase the design’s accessibility to technical individuals who live in the

developing world.

An MCU test program was written to learn how the Texas Instruments AFE4490 analog

front-end works. The program initializes the MCU, enables the voltage regulator, sets up SPI

communications, and configures the analog front-end registers. The program can query analog

front-end registers such as the diagnostic register, and it can place analog front-end in data stream

mode.

Two MATLAB scripts were written to support the development effort. The first script

dataimport.m ingests binary SPI data generated by the pulse-oximeter. The script sorts the data by

register address and populates the raw measurement data into MATLAB workspace variables.

Further, it removes erroneous data that is sometimes generated due to experimental setup. The

second script pox.m uses the raw data captured by dataimport.m to calculate SpO2 and pulse rate.

Table 6 covers the variable names and descriptions used in script pox.m.

26

Variable Name Description

ChIR is the RAW IR channel

ChIRAMBIENT is the RAW IR ambient channel

ChIRIRAMBIENT is the RAW IR ambient subtracted from IR channel

ChRED is the RAW red channel

ChREDAMBIENT is the RAW red ambient channel

ChREDREDAMBIENT is the RAW red ambient subtracted from the red channel

IR is the normalized IR channel

IRas is the normalized IR channel with ambient subtraction

IRasfilt is the normalized IR channel with ambient subtraction after going through a high-pass filter

R is the normalized red channel

Ras is the normalized red channel with ambient subtraction

RIR Ratio of red to IR (RIR = R/IR)

Table 6: Variable Descriptions For pox.m

The script pox.m plots the raw data ChIR, ChIRAMBIENT, ChIRIRAMBIENT, ChRED,

ChREDAMBIENT, and ChREDREDAMBIENT. These variables correspond to the registers 42

through 47 of the AFE4490 analog front-end. Variables ChRED, ChREDAMBIENT, ChIR, and

ChIRAMBIENT are normalized by dividing each variable by its own mean on an element-wise

basis. The resultant and corresponding variables are R, Ras, IR, and IRas and are then plotted.

The ratio of the red channel to IR channel is saved to variable RIR and then substituted into a

linear approximation function that outputs SpO2. The SpO2 function is an extremely rough

approximation, and it only defines a monotonic trend. In the future, the SpO2 function will be

replaced with a lookup table filled with empirical data to get valid SpO2 values. The script plots a

time series of the calculated SpO2 data values and outputs the mean SpO2 value to the MATLAB

command window. Pulse rate is determined by analyzing the IRas variable. First, a high pass

filter with a cutoff frequency of 0.5Hz is used to smooth out low frequency artifacts that can be

caused by adjusting acquisition parameters such as amplifier gain or LED current. The resultant

27

signal is saved in variable IRasfilt, the magnitude and phase response of the cutoff filter is

plotted, and the variables IRas, and IRasfilt are plotted versus time. Second, the number of peaks

in a given amount of time are counted and then divided by time to solve for pulse rate.

28

CHAPTER 5: TESTING AND ANALYSIS

Two test setups were used to aid testing, analysis, and software development. The first

setup was made up of commercially available parts and was built around a pulse oximeter

evaluation module. This setup was used to gain familiarity with the Texas Instruments AFE4400

Integrated Analog Front-End for Heart Rate Monitors and Low-Cost Pulse Oximeters. The

second test setup included a testbed mobile phone pulse oximeter board designed to be an

intermediate step to creating a medical device. This board was used to develop the MCU code

and to test hardware functionality.

5.1 Test Setup One

Test setup one introduced the first hands-on experience with a pulse oximeter. It

provided detailed insight on how the AFE4400 analog front-end performs and operates. It

exposed how the AFE44x0 parts handle the SPI protocol and communications. It also provided

the first datasets that were used to begin the development of the MATLAB analysis script pox.m.

5.1.1 Hardware

The first setup consisted of a NellcorTM SpO2 probe, a Texas Instruments

AEF4400SPO2EVM evaluation module, a Total Phase Beagle I2C/SPI protocol analyzer, an

Agilent Technologies MSO-X-3104A oscilloscope, and a Windows 7 computer. Figure 11 Test

Setup One Block Diagram shows how the individual components were connected.

29

Figure 11: Test Setup One Block Diagram

Figure 12: Test Setup One Components

The NellcorTM SpO2 sensor probe is a typical finger clip device that can be found in a

medical office. A photodiode is embedded on one side of the clip and an IR/RED LED pair is

embedded on the opposite side. The sensor probe plugs into a DB9 connector located on the

AFE4400SPO2EVM evaluation board. The board comes with a Windows application to operate

the AFE4400. It provides a working example for an engineer to speed up development. The

AFE4400 evaluation module, the SPI protocol analyzer, and oscilloscope communicate with the

Windows 7 computer over USB. The SPI protocol analyzer and oscilloscope were used to

monitor and capture data from the AFE4400 SPI communication bus. Lead wires were soldered

to the AFE4400 evaluation module to gain access to the SPI electrical signals. Figure 13 shows

30

the SPI protocol analyzer grabber clip split cable connected to the lead wires. Oscilloscope probes

were also connected to these wires for test setup one.

Figure 13: AFE4400SPO2EVM Evaluation Board with SPI Lead Wires

5.1.2 Software

Several software packages were used to support test setup one. The Texas Instruments

AFE44x0SPO2EVM GUI was used to control the evaluation board. It allows the user to adjust

and save register settings, capture and analyze data, and generate SpO2 and pulse rate results.

Total Phase Data Center provides real-time SPI bus monitoring and data capture. Agilent

IntuiLink Data Capture was used to capture oscilloscope waveforms and screenshots. A

MATLAB script pox.m described in chapter 6 was used to analyze the data.

31

Figure 14: Texas Instruments AFE44x0SPO2EVM GUI

Figure 15: Total Phase Data Center GUI

32

5.1.3 Testing and Analysis

Several tests were performed in succession to build up to the final test setup one

configuration. The first test brought the evaluation module online with the provided commercial

software GUI. The SpO2 probe was used to take data sets to confirm that the pulse oximeter was

working. The next tests gathered information on how the AFE4400 communicates over the SPI

data bus.

The data captures were initialized using two tabs within the AFE44x0SPO2EVM GUI.

Figure 16 Percent SpO2 & HR Display shows the resultant heart rate signal, SpO2, and HR after

a sixty second acquisition using the evaluation module and GUI.

Figure 16: Percent SpO2 & HR Display

33

Figure 17 ADC Capture & Analysis shows four time series plots containing ADC codes

for IR, IR ambient, red, and red ambient photodiode sampling periods.

Figure 17: ADC Capture & Analysis

34

The data that was captured in the previously mentioned tabs was exported as a CSV file

using the save tab shown in Figure 18.

Figure 18: Save Data to CSV File

The device register settings were saved using buttons within the low level configuration

tab shown in Figure 19. The exported csv data and register settings were used to create the first

version of the MATLAB analysis script pox.m described in chapter 4. The results from the script

provided confirmation that the SpO2 probe and evaluation module was working.

35

Figure 19: Save Low Level Configuration

The next series of tests were to learn how the AFE4400 handles SPI communications.

The oscilloscope was used to determine the SPI clock frequency (see figure 20), validate SPI

signal integrity, and decode SPI messages.

36

Figure 20: Oscilloscope Capture SPI SCLK Frequency & Period

The knowledge gained from monitoring the SPI signals with the oscilloscope was used

adjust the protocol analyzer configuration for proper operation. The low level configuration tab in

the EVM GUI was used to read and write registers while the SPI bus was instrumented. Figures

21 and 22 are partial screen captures of the EVM GUI and protocol analyzer software Data

Center. Figure 23 is a scope capture of an SPI data transaction. All of the figures show an update

made to the AFE4400 register address 0x01 and the resultant data value. The simultaneous

capturing of data was used to validate that the SPI communications were working.

37

Figure 21: Register Setting Screen Capture from AFE44x0SPO2EVM GUI

Figure 22: Register Setting Screen Capture from Protocol Analyzer GUI Data Center

Figure 23: Oscilloscope Read Register LED2STC Address: 0x0A Data: 0x17C0

Test setup one data was captured and analyzed with pox.m. Figure 24 shows the

MATLAB command window output for pox.m. The variable SPO2as_mean is the calculated

mean SpO2 value for 65.536 second acquisition period. The inputs for this SpO2 calculation

included ChIRIRAMBIENT and ChREDREDAMBIENT channels from the analog front-end. A

SpO2 value of 79.735% is low and is unhealthy. The value can be explained because the

instrument was uncalibrated. A linear approximation was used in place of an empirical lookup

table to generate the SpO2 numbers. The variable Fs represents a sample rate of 500Hz. The

variable fc is a cut-off frequency of a high pass filter. The variable pulse_rate was 81.6288 pulses

per minute.

38

Figure 24: pox.m MATLAB Command Window Output for AFE4400SPO2EVM

Figure 25 shows the raw data coming from the AFE4400SPO2EVM board. The ChIR

and ChRED channels are greater in magnitude than their ambient subtracted versions

ChIRIRAMBIENT, and ChREDREDAMBIENT. There are low frequency oscillations on both

red and IR channels. The higher frequency oscillations are due to pulsatile blood flow.

Figure 26 shows normalized data. The data was normalized by dividing each channel by

its mean on an element-by-element basis.

39

Figure 25: RAW Data from AFE4400SPO2EVM

Figure 26: Normalized Data from AFE4400SPO2EVM

40

Figure 27 shows the SpO2 calculation plot. It includes the SpO2 calculation with and

without the ambient light subtraction.

Figure 27: SpO2 (red); SpO2 Ambient Subtraction (blue) from AFE4400SPO2EVM

Figure 28 shows the magnitude and phase response of a high-pass filter that has a cut-off

frequency of 0.5Hz. This filter was used to remove the low frequency oscillations that are in the

ChIRIRAMBIENT channel.

41

Figure 28: High Pass Filter Response

Figure 29 shows IR ambient subtraction signal before and after the high-pass filter. The

filter converges after ~4 seconds which is when the ringing stops. The resultant signal is used to

calculate the pulse rate by counting peaks.

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42

Figure 29: IR Ambient Subtraction Before (red); After Filter (blue) from AFE4400SPO2EVM

5.2 Test Setup Two

The purpose of test setup two was to exercise the custom pulse oximeter testbed for the

first time. The work started with an initial board check out. Then microcontroller test software

was written. Finally, data was captured over the SPI bus using a protocol analyzer in the same

fashion as test setup one. Future setups will include a mobile phone.

5.2.1 Hardware

The second setup included the testbed mobile phone pulse oximeter board, Silicon Labs

EFM32TG MCU Tiny Gecko Starter Kit, a Total Phase Beagle I2C/SPI protocol analyzer, and a

Windows 7 computer. Figure 30 Test Setup Two Block Diagram shows how the individual

components were connected.

43

Figure 30: Test Setup Two Block Diagram

Figures 31 and 32 show the testbed hardware in a powered up state. The MCU board and

protocol analyzer is visible in Figure 31. The red/IR LED is visibly lit and is located in the left

corner of the testbed board. The onboard photodiode and LEDs were used to the collect data that

is described in the test and analysis section.

Figure 31: Hardware Setup Two Pulse Oximeter Testbed & Silicon Labs MCU

44

Figure 32: Pulse Oximeter Testbed Close Up

Initial hardware check out included the installation of pin header jumpers, testing the

power supplies, experimenting with the super capacitor, and driving the power circuitry with a

sine wave from a mobile phone. A PCB layout error was discovered during first power up and

resolved by cutting a trace away from a via. After the repair was made, the voltage regulators

came up and were adjustable to the designed ranges. A second PCB error was detected by reading

out a diagnostic register from the analog front-end. The diagnostic indicated that the photodiode

detector was installed with the wrong polarity. A small wire jumper can be seen in Figure 32

located near the top portion of the DB9 connector that is embossed with the letters AMP. The

jumper was installed and two PCB traces were cut to alleviate the photodiode polarity reversal.

5.2.2 Software

Test setup two required an MCU test program, Total Phase Data Center, and MATLAB

scripts. Figure 33 shows the data flow starting at the testbed SpO2 hardware. The MCU program

configures the analog front-end to continuously stream data. The SPI signals are digitized using

the SPI protocol analyzer. The decoded SPI messages were logged with Total Phase Data Center

and then exported as a binary file. The script dataimport.m was used to decode SPI messages and

45

store the resultant data to the MATLAB workspace. The data in the workspace was analyzed with

pox.m.

Figure 33: Data Flow Diagram

5.2.3 Testing and Analysis

The measurements were taken by placing a finger between the onboard photodetector and

LED package. A dark cloth was used to cover the testbed to prevent ambient light from

interfering with the measurement. Measurements were taken and then imported into MATLAB

for analysis. Using a trial and error approach, the analog front-end registers were adjusted to get

the signals within range. A differentiation between test setup one and two was that the analog

front-end gain settings were static. If experimental conditions changed it resulted in a bad

measurement. For example, an increase in ambient light or finger position motion would often

drive the pleth signal out of range.

Figure 34 shows a MATLAB command window output for a dataset that was taken with

testbed. The SPO2_mean variable was equal to 79.9655% which is close to the results of

79.735% acquired from test setup one. The test setup two acquisition time was 18.378 seconds

while test setup one was 65.536 seconds. The sample rate of 500 Hz and high pass filter cutoff

frequency of 0.5Hz was the same for both setups. The pulse rate for test setup two was 67.8850

pulses per minute while test setup one was 81.6288 pulses per minute.

46

Figure 34: pox.m MATLAB Command Window Output for Testbed

Figure 35 shows the raw data captured from testbed pulse oximeter board. The signal

magnitudes for CHIR, CHIRIRAMBIENT, are close to 7x10^4 ADC counts and the signal

magnitudes for ChRED, ChREDREDAMBIENT are close to 3x10^4 ADC counts. Test setup two

signals for the previously mentioned channels were close to 12x10^4 and 9x10^4 ADC counts,

which is approximately a factor of 5 difference. The SNR of these signals are impacted by the

sensor characteristics, how the measurement was taken, and the analog front-end settings.

47

Figure 35: RAW Data from Testbed

Figure 36 shows a side-by-side comparison of raw data from the testbed board and TI

AFE4400SPO2EVM evaluation board. The data collected from the AFE4400SPO2EVM board

has about 3.5 more samples than the testbed board. The IR and red channels from the

AFE4400SPO2EVM board converge onto each other while the testbed channels do not. The

converging behavior is likely due to analog front-end control register updates during the

acquisition. The testbed register settings remain static during the acquisition; therefore, the

signals magnitudes remain relatively static.

48

Figure 36: RAW Data Comparison for AFE4400SPO2EVM & Testbed

49

Figure 37 shows normalized red channel data. The red ambient subtracted data is

significantly less noisy than the red channel. The test setup one data has less noise than the test

setup two.

Figure 37: Normalized Data from Testbed

Figure 38 shows the SpO2 plot. The SpO2 value converges on ~79% after 1500 samples.

This change could be a result of a physical motion artifact. Again, this data is noisy compared to

test setup one.

50

Figure 38: Calculated SpO2 (red) SpO2 Ambient Subtraction (blue) from Testbed

Figure 39 shows IR ambient subtraction signal before and after a high-pass filter

with a cutoff frequency of 0.5Hz. In the test setup two case, the filter was not necessary because

the low frequency oscillations were not present. The magnitude of the signal appears to be less

than the test setup one case.

Figure 39: IR Ambient Subtraction Before Filter (red); After Filter (blue) from Testbed

51

CHAPTER 6: CONCLUSION

The impact of mobile computing on point-of-care medical devices has not been fully

realized. The potential cost savings, the increased mobility, improved patient compliance, and

distribution of knowledge has the potential to revolutionize medical care. While many pulse

oximeters are commercially available only a few utilize mobile phone technology.

A testbed mobile phone pulse oximeter was designed, built, and tested. MCU test

software and MATLAB analysis software was written. A working reference design was turned

over to a Northeastern University undergraduate capstone group for further development. Figure

40 shows the results of the capstone group’s work. A prototype Android application is running in

conjunction with the testbed hardware.

Figure 40: Android Application Hardware Demo by Undergraduate Capstone Team

52

The final goal is to develop a medical grade patient monitor. The monitor would have a

fully integrated suite of diagnostics including electrocardiogram (ECG), respiratory, SpO2, blood

pressure, digital stethoscope, and temperature sensing. It would be portable and provide

interactive user instructions, real-time data analysis, medical record data logging, alarms, and

remote monitoring. The patient monitor device would take the form of a small ruggedized

enclosure with connectors for sensors and phone/tablet interface. The mobile device would

provide a video display, audio feedback, computing, data storage, communications, and power.

The design will be open source licensed both hardware and software. As well as being created

with open design software tools would allow engineers in the developing world to modify the

patient monitor to address needs unique to their locale.

53

REFERENCES Howitt, P., Darzi, A., Yang, G.-Z., Ashrafian, H., Atun, R., Barlow, J., . . . Conteh, L. (2012).

Technologies for global health. The Lancet Commissions, 507-535. doi:10.1016/S0140-

6736(12)61127-1

Kuo, Y.-S., Verma, S., Schmid, T., & Dutta, P. (2010). Hijacking power and bandwidth from the

mobile phone's audio interface. Proceedings of the First ACM Symposium on Computing

for Development (pp. 1-10). London, United Kingdom: ACM.

doi:10.1145/1926180.1926210

MOBITHINKING. (2014, May 16). Global mobile statistics 2014 Part A: Mobile subscribers;

handset market share; mobile operators. Retrieved April 16, 2015, from mobiForge:

http://mobiforge.com/research-analysis/global-mobile-statistics-2014-part-a-mobile-

subscribers-handset-market-share-mobile-operators#subscribers

Texas Instruments. (2013, August 28). SpO Pulse Ox Wrist Oximeter Reference Design. Dallas,

Texas, United States of America: Texas Instruments Incorporated. Retrieved from

http://www.ti.com/tool/tida-00010

Texas Instruments. (2014, June). AFE4490 Integrated Analog Front-End for Pulse Oximeters.

Rev H. Dallas, Texas, United States of America: Texas Instruments Incorporated.

Retrieved from http://www.ti.com/product/afe4490

Wang, L. V., & Wu, H. (2007). Biomedical optics: principles and imaging. Hoboken, N.J.:

Wiley-Interscience.

World Health Organization. (2011). Pulse Oximetry Training Manual. Geneva, Switzerland:

WHO Press.

APPENDIX

54

I. SCHEMATICS

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IV. ELECTRONICS CALCULATIONS

73

Nor

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http

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82

C:\Users\pab2906\Downloads\pox.m Wednesday, April 08, 2015 2:57 PM

% Title: Pulse Oximeter Calculations

% Description: A preliminary attempt to manipulate raw data from a Texas

% Instruments Part AFE4490.

% Revision: 2014-08-24_1

% Workspace Information

% ChIR: is the RAW IR channel

% ChIRAMBIENT: is the RAW IR ambient channel

% ChIRIRAMBIENT: is the RAW IR ambient subtracted from IR channel

% ChRED: is the RAW red channel

% ChREDAMBIENT: is the RAW red ambient channel

% ChREDREDAMBIENT: is the RAW red ambient subtracted from the red channel

% IR: is the normalized IR channel

% IRas: is the normalized IR channel with ambient subtraction

% R: is the normalized red channel

% Ras: is the normalized red channel with ambient subtraction

% clc

% clear

close all;

% load('codes.mat');

% plot raw data

figure

plot(ChIR,'DisplayName','ChIR');

hold all;

plot(ChIRAMBIENT,'DisplayName','ChIRAMBIENT');

plot(ChIRIRAMBIENT,'DisplayName','ChIRIRAMBIENT');

plot(ChRED,'DisplayName','ChRED');

plot(ChREDAMBIENT,'DisplayName','ChREDAMBIENT');

plot(ChREDREDAMBIENT,'DisplayName','ChREDREDAMBIENT');

title('RAW Data')

xlabel('N Samples')

ylabel('ADC Count')

legend('ChIR','ChIRAMBIENT', 'ChIRIRAMBIENT', 'ChRED', 'ChREDAMBIENT', 'ChREDREDAMBIENT');

hold off;

% pox calculation without ambient subtraction %%%%%%%%%%%%%%%%%%%%%%%%%%%%%

R = ChRED./mean(ChRED);

IR = ChIR./mean(ChIR);

RIR = R./IR;

% equation for spo2 derived from normalized (R/IR)

% will create a spo2 empirical lookup table for real application

% for now a linear approximation will do

SPO2 = -33.33*RIR+113.3;

SPO2_mean = mean(SPO2);

% pox calculation with ambient subtraction %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Ras = ChREDREDAMBIENT/mean(ChREDREDAMBIENT);

IRas = ChIRIRAMBIENT/mean(ChIRIRAMBIENT);

RIRas = Ras./IRas;

-1-

83

C:\Users\pab2906\Downloads\pox.m Wednesday, April 08, 2015 2:57 PM

SPO2as = -33.33*RIRas+113.3;

SPO2as_mean = mean(SPO2as)

% plot normalized red/ir and red/ir ambient subraction

figure

plot(IR, 'b');

hold on;

plot(IRas, 'r');

hold on;

plot(R, 'color', [0 0.75 0.75]);

hold on;

plot(Ras, 'color', [0.75 0.75 0]);

title('Normalized Red/IR and Red/IR Ambient Subraction')

xlabel('N Samples')

ylabel('Normalized')

legend('IR','IRas','R','Ras');

hold off;

% plot spo2

n=1:length(RIR);

figure

plot(n,SPO2, 'r')

hold on;

plot(n,SPO2as, 'b')

title('SPO2')

xlabel('N Samples')

ylabel('SPO2 Percent')

legend('SPO2','SPO2as');

hold off;

% calculate blood pulse rate %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% afe4490 pulse repetition count

prp_count = 7999;

% pulse repetition frequency PRF afe4490 datasheet notation

prf = 4e6/(prp_count+1);

% sample rate use standard notation

Fs = prf

% pulse repetition period (s)

prp = 1/prf;

% sample duration (s) see afe4490 datasheet for information

sample_duration = prp/4;

% number of samples

n = length(ChIRIRAMBIENT);

% acquisition duration

acq_duration = n*prp

-2-

84

C:\Users\pab2906\Downloads\pox.m Wednesday, April 08, 2015 2:57 PM

% time vector

time = linspace(0,acq_duration,n);

% unit frequency is nyquist frequency defined as

f_nyq = Fs/2;

% cutoff frequency (Hz)

fc = 0.5

% normalized cutoff frequency Wn

Wn = (fc/f_nyq);

% filter order

order = 6;

% high pass filter

[z,p,k] = butter(order,Wn,'high');

[sos,g] = zp2sos(z,p,k); % convert to sos form

Hd = dfilt.df2tsos(sos,g); % create a dfilt object

h = fvtool(Hd); % plot magnitude response

set(h,'Analysis','freq') % display frequency response

IRasfilt = sosfilt(sos,IRas);

% plot signal before and after filter

figure

plot(time, IRas, 'r');

hold on

plot(time, IRasfilt, 'b');

title('IRas and IRasfilt')

xlabel('Time (s)')

ylabel('Normalized')

hold off

filtstart = 3000;

filtend = 8000;

% plot data after filter converges

stlvls = statelevels(IRasfilt(filtstart:filtend));

% set max beats per minute

bpm_max = 120;

bps_max = bpm_max/60;

pulse_period_max = 1/bps_max;

pulse_period_max_samp = floor(pulse_period_max / Wn);

[pks,locs] = findpeaks(IRasfilt(filtstart:filtend),'MinPeakDistance', pulse_period_max_samp);

min = (acq_duration - time(filtstart)) / 60;

pulse_rate = length(pks) / min

-3-

85

C:\Users\pab2906\Downloads\dataimport.m Wednesday, April 08, 2015 3:01 PM

% imports spi binary data file from protocol analyzer software

% populates workspace with formatted data from AFE4490

% Revision: 2014-08-24_1

clc

clear

close all

fileID = fopen('real_data13.bin');

%fileID = fopen('adc_capture_no_error_20140623_194832.bin');

%fileID = fopen('adc_capture_20140623_194832.bin');

%raw = fread(fileID, [10000, 1], 'uint8'); % read a limited number of samples

raw = fread(fileID, 'uint8'); % read all samples

fclose(fileID);

% locate ones in raw data maxtrix

oneloc = find(raw==1);

readbitloc = 0;

readbitloccnt = 0;

% search for byte sequences in raw data to locate spi read bits

for i = 1 : numel(oneloc)

% check if previous three bytes are zero

if raw(oneloc(i)-1)==0 & raw(oneloc(i)-2)==0 & raw(oneloc(i)-3)==0

% check if next four bytes rawre zero OR 255

if ( raw(oneloc(i)+1)==0 & raw(oneloc(i)+2)==0 & raw(oneloc(i)+3)==0 & raw(oneloc(i)+4

)==0) | ( raw(oneloc(i)+1)==255 & raw(oneloc(i)+2)==255 & raw(oneloc(i)+3)==255 & raw(

oneloc(i)+4)==255)

readbitloccnt = readbitloccnt+1;

readbitloc(readbitloccnt) = oneloc(i);

end

end

end

address = 0;

dataH = 0;

dataM = 0;

dataL = 0;

for i = 1:numel(readbitloc)-12

address(i,1) = raw(readbitloc(i)+5);

dataH(i) = raw(readbitloc(i)+10);

dataM(i) = raw(readbitloc(i)+11);

dataL(i) = raw(readbitloc(i)+12);

end

% contcatenate hex data and convert

hexdata = [dec2hex(dataH) dec2hex(dataM) dec2hex(dataL)];

raw_data = hex2dec(hexdata);

data = bitand(raw_data,4194303); % bitmask to clear upper two bits 0x3FFFFF

-1-

86

C:\Users\pab2906\Downloads\dataimport.m Wednesday, April 08, 2015 3:01 PM

adr_rawdata = [address, raw_data];

adr_datawbitmask =[address, data];

ZEROcnt=0;

ALED2CONVENDcnt=0;

ALED1CONVENDcnt=0;

ADCRSTENDCT0cnt=0;

ADCRSTENDCT1cnt=0;

ADCRSTENDCT2cnt=0;

ADCRSTENDCT3cnt=0;

TIAGAINcnt=0;

CONTROL1cnt=0;

SPARE2cnt=0;

SPARE4cnt=0;

RESERVED2cnt=0;

ChREDcnt=0;

ChREDAMBIENTcnt=0;

ChIRcnt=0;

ChIRAMBIENTcnt=0;

ChREDREDAMBIENTcnt=0;

ChIRIRAMBIENTcnt=0;

ERRORcnt=0;

unknaddress=0;

% sort data into variables

for i = 1:length(address)

switch address(i)

case 0

ZEROcnt=ZEROcnt+1;

case 16

ALED2CONVENDcnt=ALED2CONVENDcnt+1;

ALED2CONVEND(ALED2CONVENDcnt,1) = data(i);

case 20

ALED1CONVENDcnt=ALED1CONVENDcnt+1;

ALED1CONVEND(ALED1CONVENDcnt,1) = data(i);

case 22

ADCRSTENDCT0cnt=ADCRSTENDCT0cnt+1;

ADCRSTENDCT0(ADCRSTENDCT0cnt,1) = data(i);

case 24

ADCRSTENDCT1cnt=ADCRSTENDCT1cnt+1;

ADCRSTENDCT1(ADCRSTENDCT1cnt,1) = data(i);

case 26

ADCRSTENDCT2cnt=ADCRSTENDCT2cnt+1;

ADCRSTENDCT2(ADCRSTENDCT2cnt,1) = data(i);

case 28

ADCRSTENDCT3cnt=ADCRSTENDCT3cnt+1;

ADCRSTENDCT3(ADCRSTENDCT3cnt,1) = data(i);

case 32

TIAGAINcnt=TIAGAINcnt+1;

TIAGAIN(TIAGAINcnt,1) = data(i);

case 30

CONTROL1cnt=CONTROL1cnt+1;

CONTROL1(CONTROL1cnt,1) = data(i);

-2-

87

C:\Users\pab2906\Downloads\dataimport.m Wednesday, April 08, 2015 3:01 PM

case 36

SPARE2cnt=SPARE2cnt+1;

SPARE2(SPARE2cnt,1) = data(i);

case 38

SPARE4cnt=SPARE4cnt+1;

SPARE4(SPARE4cnt,1) = data(i);

case 40

RESERVED2cnt=RESERVED2cnt+1;

RESERVED2(RESERVED2cnt,1) = data(i);

case 42 % 0x2A LED2VAL: LED2 Digital Sample Value Register

ChREDcnt=ChREDcnt+1;

ChRED(ChREDcnt,1) = data(i);

case 43 % 0x2B ALED2VAL: Ambient LED2 Digital Sample Value Register

ChREDAMBIENTcnt=ChREDAMBIENTcnt+1;

% check for sign bit

if(length(dec2bin(data(i)))>21)

% mask sign bit 0x1FFFFF

ChREDAMBIENT(ChREDAMBIENTcnt,1) = bitand(data(i),2097151)-2^21;

else

ChREDAMBIENT(ChREDAMBIENTcnt,1) = data(i);

end

case 44 % 0x2C LED1VAL: LED1 Digital Sample Value Register

ChIRcnt=ChIRcnt+1;

ChIR(ChIRcnt,1) = data(i);

case 45 % 0x2D ALED1VAL: Ambient LED1 Digital Sample Value Register

ChIRAMBIENTcnt=ChIRAMBIENTcnt+1;

% check for sign bit

if(length(dec2bin(data(i)))>21)

% mask sign bit 0x1FFFFF

ChIRAMBIENT(ChIRAMBIENTcnt,1) = bitand(data(i),2097151)-2^21;

else

ChIRAMBIENT(ChIRAMBIENTcnt,1) = data(i);

end

case 46 % 0x2E LED2-ALED2VAL: LED2-Ambient LED2 Digital Sample Value Register

ChREDREDAMBIENTcnt=ChREDREDAMBIENTcnt+1;

ChREDREDAMBIENT(ChREDREDAMBIENTcnt,1) = data(i);

case 47 % 0x2F LED1-ALED1VAL: LED1-Ambient LED1 Digital Sample Value Register

ChIRIRAMBIENTcnt=ChIRIRAMBIENTcnt+1;

ChIRIRAMBIENT(ChIRIRAMBIENTcnt,1) = data(i);

otherwise

ERRORcnt=ERRORcnt+1;

unknaddress(ERRORcnt,1)=address(i);

end

end

% code below removes array elements if the array lengths do not match

% this can happen if there are data capture errors

size = [ChREDcnt; ChREDAMBIENTcnt; ChIRcnt; ChIRAMBIENTcnt; ChREDREDAMBIENTcnt; ChIRIRAMBIENTcnt

; ];

max = max(size);

min = min(size);

diff = max - min;

-3-

88

C:\Users\pab2906\Downloads\dataimport.m Wednesday, April 08, 2015 3:01 PM

mode = mode(size);

var = var(size);

% make lengths match

for i=(ChREDcnt+1):max

ChRED(i) = 0;

end

for i=(ChREDAMBIENTcnt+1):max

ChREDAMBIENT(i) = 0;

end

for i=(ChIRcnt+1):max

ChIR(i) = 0;

end

for i=(ChIRAMBIENTcnt+1):max

ChIRAMBIENT(i) = 0;

end

for i=(ChREDREDAMBIENTcnt+1):max

ChREDREDAMBIENT(i) = 0;

end

for i=(ChIRIRAMBIENTcnt+1):max

ChIRIRAMBIENT(i) = 0;

end

for i=max:-1:min

ChRED(i) = [];

ChREDAMBIENT(i) = [];

ChIR(i) = [];

ChIRAMBIENT(i) = [];

ChREDREDAMBIENT(i) = [];

ChIRIRAMBIENT(i) = [];

end

size = [length(ChRED); length(ChREDAMBIENT); length(ChIR); length(ChIRAMBIENT); length(

ChREDREDAMBIENT); length(ChIRIRAMBIENT); ]

% clear the workspace

clear ChIRAMBIENTcnt ChIRIRAMBIENTcnt ChIRcnt ChREDAMBIENTcnt ChREDcnt

clear ZEROcnt ADCRSTENDCT0cnt ADCRSTENDCT1cnt ADCRSTENDCT2cnt ADCRSTENDCT3cnt

clear ALED1CONVENDcnt ALED2CONVENDcnt CONTROL1cnt RESERVED2cnt SPARE2cnt

clear SPARE4cnt TIAGAINcnt adr_rawdata diff max min mode var raw_data

clear address data dataH dataL dataM fileID hexdata unknaddress size adr_datawbitmask

clear i oneloc raw readbitloc readbitloccnt ans ERRORcnt ChREDREDAMBIENTcnt

disp('done')

-4-

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100

VII. WORK STATUS UPDATES

101

Mobile Phone Pulse Oximeter Project Status Updates

Northeastern University

  09/7/2014 ­ 03/31/2015  Paul Bohn 

● Worked on thesis document  08/31/2014 ­ 09/06/2014  Paul Bohn 

● Distributed information to capstone team members ● Wrote short tutorial on how to load mcu project code ● Worked on thesis document 

 08/24/2014 ­ 08/30/2014  Paul Bohn 

● Met with Mark Niedre to discuss progress ● Collected a lot data ● Created slideshow with latest results 

○ figured out register settings for AFE4490 ● Attended telecon with capstone group 

  08/17/2014 ­ 08/23/2014  Paul Bohn 

● Identified schematic error ○ On board photodiode pinout is reversed ○ Modified custom PCB to fix photodiode pin reversal 

● Updated schematic ● MCU Code 

○ Added diagnostic code  ○ Changed power up sequence 

  08/10/2014 ­ 08/16/2014  Paul Bohn 

● Successfully powered up custom device (LED lights up and ADC samples) and 

1 of 18 10003­001 

102

Mobile Phone Pulse Oximeter Project Status Updates

Northeastern University

captured real data for the first time! ● Offered to provide a bare PCB to capstone team members for hardware development 

testing ● Provided capstone team members with a list of task options that would drive to project 

forward ● Suggested that capstone team members should call me on the phone so I can help 

them with task planning ● MCU Code 

○ Configured/wrote default register settings into AFE4490 ○ Refined AFE4490 register settings and added comments ○ Fixed gpio pin assignment errors ○ Setup pin interrupt service to capture data when ADC_RDY signal goes high 

● Shared MCU code with capstone group  08/03/2014 ­ 08/09/2014  Paul Bohn 

● Completed preliminary MCU SPI code to read and write AFE4490 registers ○ Successfully wrote registers to custom board 

● Attempted to coordinate meeting with capstone team members    07/27/2014 ­ 08/02/2014  Paul Bohn 

● Completed the development of a MATLAB script that will import a binary file containing raw AFE4400 SPI data into the workspace. The script parses the raw data into matrices that correspond to AFE4400 registers. The data was captured with a Total Phase Beagle SPI protocol analyzer. 

● Processed the imported raw SPI data with analysis script pox.m ● Removed power to ground short on second custom PCB by cutting trace away  ● Tested power supplies on second custom PCB 

○ The board comes up fine     07/20/2014 ­ 07/26/2014  Paul Bohn 

● Developing a MATLAB script that will import RAW AFE4400 data from a SPI protocol 

2 of 18 10003­001 

103

Mobile Phone Pulse Oximeter Project Status Updates

Northeastern University

analyzer   07/13/2014 ­ 07/19/2014  Paul Bohn 

● Released preliminary MATLAB script for analyzing RAW AFE4400 data ● Communicated with Dan Thompson via email. Pointed him towards information that is 

pertinent to MCU cell phone communication. ● Developing a MATLAB script that will import RAW AFE4400 data from a SPI protocol 

analyzer   07/06/2014 ­ 07/12/2014  Paul Bohn 

● Developing MATLAB script pox.m to analyze RAW AFE4400 data   06/29/2014 ­ 07/05/2014  Paul Bohn 

● Developing MATLAB script to analyze RAW AFE4400 data ○ Able to calculate rough SPO2 values 

  06/22/2014 ­ 06/28/2014  Paul Bohn 

● Completed SPI data capture slide show ● Developing MATLAB script to analyze RAW AFE4400 data 

  06/15/2014 ­ 06/21/2014  Paul Bohn 

● Provided cost estimate information ● Created SPI data capture slide show 

  

3 of 18 10003­001 

104

Mobile Phone Pulse Oximeter Project Status Updates

Northeastern University

06/08/2014 ­ 06/14/2014  Paul Bohn 

● Added SPI code to project directory ● On campus meeting 

○ Turned over hardware to capstone team ● Started work to grab SPI data from development board (requires hardware mod) 

  05/01/2014 ­ 06/07/2014  Paul Bohn 

● Installed LEDs on development boards ● Development of SPI MCU code 

  05/25/2014 ­ 05/31/2014  Paul Bohn 

● Bringing capstone team up­to­speed by distributing project information ● Development of SPI MCU code 

  05/18/2014 ­ 05/24/2014  Paul Bohn 

● Updated the 10100­001_RevA_PhonePulseOx_Development_Plan ● Met with capstone team ● Bringing capstone team up­to­speed by distributing project information ● Contacted Chris Poling from ProTEQ Solutions to get a loaner Oscilloscope 

 05/11/2014 ­ 05/17/2014  Paul Bohn 

● Updated the 10100­001_RevA_PhonePulseOx_Development_Plan ● Met with Gunar and Mark to discuss project and how to bring in capstone group ● Going through MCU training material 

 05/04/2014 ­ 05/10/2014  

4 of 18 10003­001 

105

Mobile Phone Pulse Oximeter Project Status Updates

Northeastern University

Paul Bohn ● Going through MCU training material 

  04/27/2014 ­ 05/03/2014  Paul Bohn 

● Archived Janki’s reports and software to project folder ○ https://drive.google.com/?authuser=0#folders/0Bw57FuC6l3MXU1VFWUhFMllDbDg 

● Experimented with TI AFE4400 Dev Kit ● Requested, secured, and reviewed source code for TI dev kit ● Going through MCU training material 

○ http://www.silabs.com/support/training/Pages/online­training.aspx ○ Reading data sheets etc. 

    04/20/2014 ­ 04/26/2014  Paul Bohn 

● Visited NEU picked up TI AFE4400 Dev Kit ● Going through MCU training material 

○ http://www.silabs.com/support/training/Pages/online­training.aspx

 ● Located Totalphase SPI debugger cable ● Consolidated all project parts and equipment ● Requested assistance to build a future team to complete the project 

   04/06/2014 ­ 04/12/2014  Paul Bohn 

● Attended Monday team meeting 

5 of 18 10003­001 

106

Mobile Phone Pulse Oximeter Project Status Updates

Northeastern University

) ○ uploaded full project directories ○ created directories for source code development 

■ 10305­001_Firmware ■ 10400­001_Test_App ■ 10401­001_POx_App 

● Submitted solder paste gerber file to assembly house ● Supported board assembly by answering questions ● Attended Monday team meeting ● Received boards on Friday (3/28) ● Started testing board 

○ identified and fixed minor layout error ○ voltage regulators power up and adjust ○ confirmed device can be powered by a cell phone 

 03/16/2014 ­ 03/22/2014  

6 of 18 10003­001 

107

Mobile Phone Pulse Oximeter Project Status Updates

Northeastern University

Paul Bohn ● Packaged and edited PCB design files for archival ● Put parts kit together ● Shipped parts kit to assembly house ● Researched DXF to Gerber file conversion so a so a solder paste stencil can be made ● Generated a solder paste gerber file from a dxf file using linkcad7 and gerbv ● Attended Monday team meeting ● Met with Prof. Mark Niedre to give a status update 

  03/09/2014 ­ 03/15/2014  Paul Bohn 

● Purchased Gerber Files ● PCBs arrived ● Newark parts arrived ● Digi­key parts should arrive early next week ● Attended Monday team meeting 

     03/02/2014 ­ 03/08/2014  Paul Bohn 

● Submitted Digikey order (Web ID 49783172 Access ID 76224) ● Updated 10900­001_RevA_PhonePulseOx_Part_Numbering ● Submitted Newark Electronics order ● Submitted ExpressPCB order 

  02/23/2014 ­ 03/01/2014  Paul Bohn 

● Completed PCB Layout ● Updated schematic, BOM, and Digikey order (Web ID 49783172 Access ID 76224) ● Sent design to a very experienced electrical engineer for review ● Requested quote from assembly house and worked out a deal for free assembly 

service. 

7 of 18 10003­001 

108

Mobile Phone Pulse Oximeter Project Status Updates

Northeastern University

● Attended Monday team meeting   02/16/2014 ­ 02/22/2014  Paul Bohn 

● Sent information to Janki on PCB design ● Routing PCB ● Updated schematic, BOM, and Digikey order 

  02/09/2014 ­ 02/15/2014  Paul Bohn 

● Attended Monday team meeting ● Did a factory restore on Janki’s new laptop ● Made PCB footprint ● Modified schematic ● Completed rough PCB parts placement ● Worked on Digi­key order. It is basically complete at this point. ● Requested information from OSI Optics ● Started team building and recruitment 

○ Identified four potential team members ● Started routing PCB 

 

8 of 18 10003­001 

109

Mobile Phone Pulse Oximeter Project Status Updates

Northeastern University

01/26/2014 ­ 02/01/2014  Paul Bohn 

● Worked on proposal text ● Received budgetary pricing for Fluke Spo2 device $2000.00 ● Worked on design description write up ● Met with Coilcraft sales engineer.  Can get any number of free samples.  Modified and 

custom inductor designs are possible  ● Met with Janki and discussed cell phone app.  Started to development of app behavior 

model ● Researched Android market penetration in the developing world 

  01/19/2014 ­ 01/25/2014  Paul Bohn 

● Wrote draft proposal text ● Updated work breakdown structure and budget ● Worked on design description write up ● Request quote for SPO2 tester ● Supported Tier1 seed grant proposal preparations  

  01/12/2014 ­ 01/18/2014  Paul Bohn 

● Worked on pcb footprints ● Updated BOM ● Requested information from new NEU group “ENABLE” ● NEU campus meeting ● Created a Google Drive tutorial ● Developed strawman proposal document ● Created proposal budget ● Encouraged Janki to balance spring14 schedule to make time for this project 

   01/05/2014 ­ 01/11/2014  Paul Bohn 

10 of 18 10003­001 

110

Mobile Phone Pulse Oximeter Project Status Updates

Northeastern University

● Worked on pcb footprints ● Started digikey order: Web ID 49783172 Access ID 76224 ● Updated BOM 

   12/29/2013 ­ 01/04/2014  Paul Bohn 

● Parts from TI have arrived ● Consolidated parts to one box ● Worked on pcb footprints ● Updated BOM 

  12/22/2013 ­ 12/28/2013  Paul Bohn 

● Parts came in from OSI Optoelectronics ● Parts from TI have shipped 

○ Went with AFE4490 because AFE4400 was backordered  12/15/2013 ­ 12/21/2013  Paul Bohn 

● Updated 10303­001_RevA_PhonePulseOx_Main_Board_BOM to include part package dimensions 

● Making PCB footprints for the circuit board layout ● Requested samples from Texas Instruments and OSI Optoelectronics 

     12/08/2013 ­ 12/14/2013  Paul Bohn 

● Updated 10303­001_RevA_PhonePulseOx_Main_Board_BOM to include PCB footprints 

● Making PCB footprints for the circuit board layout 

11 of 18 10003­001 

111

Mobile Phone Pulse Oximeter Project Status Updates

Northeastern University

● Researched Design Spark CAD tools for next version ○ http://www.designspark.com/ 

 Mark Nakib 

● Returned Analog Discovery to Ufuk Muncuk  12/01/2013 ­ 12/07/2013  Paul Bohn 

● Worked on schematic and submitted draft for review ● Updated 10303­001_RevA_PhonePulseOx_Main_Board_BOM ● Making PCB footprints for the circuit board layout ● Started designing an integrated PCB sensor ● Secured samples from 

○ Coilcraft ○ Linear Technology 

● Parts from OSI Optoelectronics are being shipped from overseas ● Attended meeting at NEU 

  11/24/2013 ­ 11/30/2013  Paul Bohn 

● Worked on schematic and power calculations ○ Schematic is close to completion 

● Requested samples from ○ Coilcraft ○ OSI Optoelectronics ○ Linear Technology ○ Silicon Labs 

● Contacted Fluke medical and Silicon Labs ● Updated Google Doc 10101­001_RevA_PhonePulseOx_Documentation ● Organized project directory ● Installed Google Drive for PC to streamline workflow 

   11/17/2013 ­ 11/23/2013  Paul Bohn 

12 of 18 10003­001 

112

Mobile Phone Pulse Oximeter Project Status Updates

Northeastern University

● Watched WHO pulse ox training video ● Worked on schematic and bill of materials for main board ● Created main board calculations spreadsheet ● Organized project folder and added documentation ● Applied part numbers to project files ● Design research 

○ Selected parts ○ Read datasheets ○ Reviewed reference designs ○ Reviewed reference material ○ Reviewed code 

 Janki Bhimani 

● Did reverse engineering and to the operation algorithm from code of AFE4400 ● found on which formulae it works ● traced the algorithm of operation 

  11/10/2013 ­ 11/16/2013  Paul Bohn 

● Worked on development plan slideshow ● Made some mobile phone cartoons for slideshow ● Updated part numbering for documentation ● Capturing schematic for main PCB ● Absorbing training material created by the World Health Organization 

  Mark Nakib 

● Installed analog discovery software ● Began experimenting to become more familiar with system 

   11/03/2013 ­ 11/09/2013  Paul Bohn 

● Made another pass at WBS task details ● Worked on development plan slide show ● Updated audio breakout board documentation 

13 of 18 10003­001 

113

Mobile Phone Pulse Oximeter Project Status Updates

Northeastern University

● Committed audio breakout board to SVN ● Tracked down analog discovery ADI module ● Added a mobile phone study section to PhonePulseOx_Documentation ● Performed design research 

  Mark Nakib 

● Picked up analog discovery   10/27/2013 ­ 11/02/2013  Paul Bohn 

● Started a development plan slide show ● Installed TI AFE4400 development kit GUI ● Updated phone audio breakout BOM ● Assembled and tested 3 phone audio breakout boards ● Delivered phone audio breakout boards to NEU ● Updated file directory structure readme ● Created budget tracking spreadsheet ● Created configuration management part number assignment spreadsheet 

  Mark Nakib 

● Met with Paul to pick up audio breakout boards  10/20/2013 ­ 10/26/2013  Paul Bohn 

● Researched Silicon Labs Gecko MCU families ● Worked on schematic capture for cell phone pulse oximeter ● Updated nuForge wikipages ● Updated Phone Pulse Oximeter Google Document with design requirements ● Made first cut at an action item list ● Installed TortoiseSVN ● Research version control methods ● Status update meeting with Mark Niedre ● Started development of a work breakdown structure 

 Janki Bhimani 

14 of 18 10003­001 

114

Mobile Phone Pulse Oximeter Project Status Updates

Northeastern University

● Retrieved dev kit from Mark Nidre ● Installed TI AFE4400 development kit GUI ● Experimented with TI AFE4400 development kit 

 Mark Nakib 

● Researched mobile phone sinewave generator applications ● Requested source code for mobile phone application software ● Researched mobile phone software development 

    10/13/2013 ­ 10/19/2013  Paul Bohn 

● Released first version of system block diagram ● Submitted project background reading material to team members ● Created project action item list ● Created instructions to submit ExpressPCB order ● Met with Janki and divided up tasks ● Started schematic capture for cell phone pulse oximeter ● Researched Silicon Labs Gecko MCU families ● Updated nuForge wikipages ● Updated Phone Pulse Oximeter Google Document 

  10/06/2013 ­ 10/12/2013  Paul Bohn 

● Completed cell phone audio breakout board PCB layout and documentation ● Submitted breakout board design for review ● Made PCB footprint for AFE4400 ● Reviewed development board schematic ● Started to develop system block diagram 

     9/29/2013 ­ 10/05/2013 

15 of 18 10003­001 

115

Mobile Phone Pulse Oximeter Project Status Updates

Northeastern University

 Paul Bohn 

● Completed a detailed read of AFE4400 datasheet ● Completed detailed read of AFE4400 development kit user guide ● Evaluated DesignSpark CAD tools as a possible longer term solution ● Installed and used PCB Library Expert 2013 to aid layout 

  9/22/2013 ­ 9/28/2013  Paul Bohn 

● Started to layout PCB for cell phone audio breakout board ○ using express pcb for design (http://www.expresspcb.com/) 

● Met with PI on 9/23 to discuss embedded design ● Submitted items for purchasing ● Started reading AFE4400 datasheet ● Started to record design goals in design documentation  

  9/15/2013 ­ 9/21/2013  Paul Bohn 

● Performed some design research ● Started development of a cell phone audio breakout test fixture 

○ captured schematic ● Created bill of materials for audio breakout test fixture ● First parts arrived on 9/17 from allheart second order came in on 9/20 ● Started to take a closer look at the TI AFE4400 ● Read a paper AudioDAQ: Turning the Mobile Phone's Ubiquitous Headset Port into a

116

Mobile Phone Pulse Oximeter Project Status Updates

Northeastern University

● Gathered costing information for dev kits and parts ○ https://docs.google.com/spreadsheet/ccc?key=0Ag57FuC6l3MXdHFpSEZOSG

hUZnU1M3BYSXp4M2JtbVE#gid=0 ● Decided to develop the following as intermediate steps 

○ Cell phone audio jack interface test fixture ○ Digital Stethoscope  

● Purchased parts from medical supplier allheart   

 

117

Mobile Phone Pulse Oximeter Project Status Updates

Northeastern University

    9/1/20123 ­ 9/7/2013  

● Performed some design research ● Learned about mobile phone capabilities ● Read about hijack project (paper) 

○ http://web.eecs.umich.edu/~prabal/projects/hijack/   7/27/2013 ­ 8/31/2013  Paul Bohn 

● Met with PI on 8/1 and 8/28 ● Project Research and Design Ideas ● Researched PAT instrumentation ● Read “Technologies Global Health.” and many app notes on pulse ox 

 

18 of 18 10003­001 

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ABOUT THE AUTHOR

From a very early age, Paul Aaron Bohn wanted to know how things worked. Ultimately,

this led to his desire to become an engineer. He pursued hobbies and activities that involved

hands-on experience such as amateur radio, model making, building toys, and repairing products.

He has worked for electrical contractors, general contractors, a violin maker, research

organizations, and, as a teenager, started a machining business.

Paul studied Electrical Engineering in a joint undergraduate program at University of

Missouri-St. Louis and Washington University in St. Louis. In 2005, he transferred to

Northeastern University and graduated two years later with a Bachelor of Science in Electrical

Engineering. He previously worked for the Boston University Electronics Design Facility where

he developed equipment for scientific research. Paul is currently a Member Technical Staff in the

Imaging and Sensor Systems Group at The Charles Stark Draper Laboratory.

Paul enjoys developing prototypes that demonstrate new capability, and all of the

multidisciplinary aspects of engineering that are required to bring prototypes to fruition. He has

had the opportunity to work on a broad range of projects. For instance, he has developed devices

for medical research, surface physics, high energy physics, biological research, energy sector,

imaging, precision timing, communication systems, remote sensing, particle detection, guidance,

navigation, and control.

One of Paul’s other passions includes music. He is a founding member of Juventas New

Music Ensemble where he served as the President of the Board of Directors, and Chairmen of the

governance committee. He played an integral part in the incorporation and filing for IRS

501(c)(3) non-profit status. The organization still is thriving since his departure to pursue

graduate education.

121