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AN1556Blood Pressure Meter Design Using Microchip’s Analog
Devices and PIC24F Microcontrollers
INTRODUCTIONA Blood Pressure Meter (BPM) is a non-invasivedevice used to measure blood pressure. Thisapplication note demonstrates the implementation of adigital blood pressure meter using Microchip’sMCP6N16 instrumentation amplifier and aPIC24FJ128GC010 microcontroller.
FIGURE 1: BLOCK DIAGRAM
Author: Zhang Feng,Microchip Technology Inc.
Battery AAA 1.5V x 4
Power Supply
USB 5V 500mA
Voltage Regulator MCP1802
5/6V
3.3V VDD
Air Pump Control Pressure Sensor
Solenoid Valve Control
Instrumentation AmplifierMCP6N16
OPA1 Internal Op-Amp High-pass Filter
MicrocontrollerPIC24FJ128GC010
ADC1
DAC
CTMU
Touch Pads
Buzzer
Computer
Wi-Fi® Module RN171
Bluetooth® Module RN42
PICkit™ Serial
I/O
PWM 1
LCD LCD Driver
UART
SPI/UART
USB
PWM2
CTMU I/Os
Analog Signal Conditioning
ADC2
Measurement
User Interface
Debugging
Communication
Data Logging
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ANALOG FRONT-END SIGNAL CONDITIONING CIRCUIT DESCRIPTIONAs shown in Figure 1, the MCP6N16-100 singleInstrumentation Amplifier (INA) is used to condition theanalog signal acquired from the Wheatstone bridgetype of air pressure sensor. The overall INA gain is setat 101 V/V. The output signal of the MCP6N16 is splitinto two paths. One path, representing the cuffpressure, is connected to the ADC1 channel of themicrocontroller. Another path is passed through a 2-pole active high-pass filter with cutoff frequencies of0.48Hz and 4.8Hz, and a gain of 92. The high-passfilter is formed by one of the microcontroller’s internaloperational amplifiers. The output of the high-passfilter, representing the oscillation signal, is sent to theADC2 channel of the microcontroller.
PIC24FJ128GC010 MICROCONTROLLERThe microcontroller provides a 12-bit high-speedpipeline Analog-to-Digital Converter (ADC). The cuffpressure signal and the oscillation signal are bothsampled at 250 Hz by the 12-bit ADC.
As described in the previous section, the demo systemuses one of the microcontroller’s internal operationalamplifiers to build the analog high-pass filter. It alsoutilizes one of the internal 10-bit Digital-to-AnalogConverters (DAC) with buffered output voltage toprovide an adjustable DC offset level as the bias for theoperational amplifier.
Microchip’s mTouch® sensing solution is implementedin the firmware for the capacitive touch pads utilizingthe microcontroller’s Charge Time Measurement Unit(CTMU). Meanwhile, the air pressure sensor requires astable supply current of 100 uA DC, which is alsoprovided by the CTMU.
The demo system utilizes the microcontroller’s Pulse-Width Modulation (PWM) module to drive the airpump’s motor in fast or slow speed mode. The noisegenerated by a running motor can interfere with thepressure measurement while inflating. The circuitdesign, firmware and Printed Circuit Board (PCB)layout are intended to reduce the motor noise.
The results for blood pressure and pulse rate can besent to a Liquid Crystal Display (LCD) panel, USBinterface and wireless interface. The microcontrollerhas an integrated LCD controller that generates thedata and timing control required to directly drive a staticor multiplexed LCD panel. The demo system utilizesthe microcontroller’s USB On-The-Go interface for datacommunication as well as the 5V main power source.For wireless communication, the microcontroller’s SPIor UART port connects to a Microchip RN42 Bluetooth®
module or an RN171 Wi-Fi® module. A real-time clock
is also displayed on the LCD panel by using themicrocontroller’s Real-Time Clock and Calendarmodule that can operate in Deep Sleep mode.
POWER SUPPLYThe main input power is supplied by either four 1.5VAAA alkaline batteries or the USB’s VBus line. TheMCP1802 LDO converts the 5V or 6V main inputvoltage to 3.3V VDD.
SPECIAL FEATURESA 2-pass moving average filter with a cutoff frequencyof 3.5 Hz is implemented in the firmware. It filters theraw ADC samples acquired from each ADC channel.
This demo system uses a custom algorithm for bloodpressure calculation that was developed based uponpublished academic papers available in the publicdomain (see Appendix D: “References”).
MEASUREMENT TECHNOLOGYA digital blood pressure meter measures systolic anddiastolic pressures by oscillometric detection.Microchip’s digital blood pressure meter demo canmeasure blood pressure and pulse rate during inflation.The Measurement While Inflating (MWI) principlereduces overall measuring time, which in turn reducesdiscomfort caused by the pressure in the cuff.
After the motor pumps the pressure up to 30 mmHg infast mode, the motor changes to slow mode graduallyand linearly inflating the cuff. When the pressure in thecuff gets high enough to just begin constricting the flowof blood, the arterial pulse becomes detectable by thepressure sensor. At this point, the waveform capturedfrom the ADC2 channel (the output of the analog high-pass filter) starts to show the onset of the bloodpressure oscillation signal (see Figure 2). Theoscillation signal is then filtered by a 2-pass movingaverage filter. The Mean Arterial Pressure (MAP) is thecuff pressure which corresponds to the maximumoscillation signal. Based on the custom algorithm forblood pressure calculation, the Systolic Pressure(SYS) and the Diastolic Pressure (DIA) can bedetermined using the MAP value and a lookup table ofempirical formulas (see Figure 3). The inflation processis automatically terminated when the pressure reachesa specified value. The Pulse Rate (PR) can becalculated based on ADC sample rate and samplenumbers obtained in multiple consecutive pulses.
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FIGURE 2: RAW ADC DATA OF THE OSCILLATION SIGNAL ACQUIRED FROM THE ADC2 AT250 SAMPLES/SECOND
FIGURE 3: THE PLOT OF ADC DATA FOR DIGITALLY FILTERED OSCILLATION SIGNAL AND CUFF PRESSURE SIGNAL
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PROGRAM FLOWCHARTFigure 4 shows the firmware process flow.
FIGURE 4: FIRMWARE PROCESS FLOWCHART
Start
Initialization
Checking Push Buttons
Inflating Cuff
Enable ADC sample list 1 interrupt for taking ADC samples from CH1 and
CH2 channels
Processing ADC data
Release Air
Calculating blood pressure and pulse rate
Display Result
Main Loop
Start Button Pressed
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APPENDIX A: BPM DEMO BOARDThis appendix shows the BPM demo board.
FIGURE 5: BPM DEMO BOARD
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APPENDIX B: SCHEMATICSThis appendix shows the Blood Pressure Meterschematics.
SHEET 1: MICROCHIP BLOOD PRESSURE METER SCHEMATIC – ANALOG FRONT-END
10K
0.1uF
1K
0.33uF
1Meg
1K
33uF
91K
100
100100
2.2uF
10K
1uF
0.1uF
DNP
2SMPP-02
0901200127
DNP
DNP
DNP
DMG1012UW-7 DMG1012UW-7
S1AB-13-F
DNP
DNP
DNP
DFLS120LQ-747uF
MCP6N16-100E/MS
100
100K
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SHEET 2: MICROCHIP BLOOD PRESSURE METER SCHEMATIC – PIC24FMICROCONTROLLER
100
0.1uF 0.1uF 1uF 0.1uF 0.1uF 0.1uF 1uF
12pF 12pF
10uF
32.768KHz
PIC24FJ128GC010-I/PT
MCU
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SHEET 3: MICROCHIP BLOOD PRESSURE METER SCHEMATIC – INTERFACE CIRCUITS10K
1KRESET
0.1uF
10K10K
10K
GEC06SBSN-M89
1K 1K 1K
CTS100001-1
GEC06SBSN-M89
801-43-022-10-012000801-43-022-10-012000
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SHEET 4: MICROCHIP BLOOD PRESSURE METER SCHEMATIC – POWER SUPPLYShield
6
Shield
666
0.1uF4.7K 56K
100K
AAA AAA AAA AAA
1uF 1uF
DS
G
IRLML2244TRPBF
MCP1802T-3302I/OT
10uF
Red
DFLS120LQ-7
DFLS120LQ-7
UX60-MB-5ST
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APPENDIX C: WARNINGS, RESTRICTIONS AND DISCLAIMER
This demo is intended solely for evaluation anddevelopment purposes. It is not intended for medicaldiagnostic use.
APPENDIX D: REFERENCESWang Wei-Wei, Pu Bao-Ming, He Bao-Yue, Li Sheng-Jin. Design and Realization of ElectronicSphygmomanometer Based on Inflation Measurement.Journal of Computer Systems & Applications,2012,21(2):150-153,97.
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2013-2015 Microchip Technology Inc.
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