AN1525Pulse Oximeter Design Using Microchip’s Analog Devices
and dsPIC® Digital Signal Controllers (DSCs)
INTRODUCTIONPulse oximeter is a non-invasive medical device thatmonitors the oxygen saturation of a patient’s blood andheart rate. This application note demonstrates theimplementation of a high-accuracy pulse oximeterusing Microchip’s analog devices and dsPIC® DigitalSignal Controllers (DSCs).
FIGURE 1: FUNCTION BLOCK DIAGRAM
Author: Zhang FengMicrochip Technology Inc.
DC Offset
ADC1
Microcontroller
Photodiode
IR Red
DAC
Computer, WiFi® or
Bluetooth®
I2C™
I/O
UART
PWM1
Transimpedance Amplifier
Gain Stage Amplifier
ADC0
Analog Signal Conditioning
LED Current Control
Analog Switch
LED On/Off
LED Driver
PWM2
Highpass Filter
LCD
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THEORY OF OPERATIONA pulse oximeter monitors the oxygen saturation(SpO2) of a human’s blood based on the red light (600-750 nm wavelength) and infrared light (850-1000 nmwavelength) absorption characteristics of oxygenatedhemoglobin (HbO2) and deoxygenated hemoglobin(Hb). The pulse oximeter flashes the red and infraredlights alternately through a finger to a photodiode.HbO2 absorbs more infrared light and allows more red
light to pass through. On the other hand, Hb absorbsmore red light and allows more infrared light to passthrough.
The photodiode receives the non-absorbed light fromeach LED. This signal is inverted using inverting Op-Amp and therefore the result, as shown in Figure 2,represents the light that has been absorbed by thefinger.
FIGURE 2: REAL-TIME RED AND INFRARED (IR) PULSATION SIGNALS CAPTURED BY THE OSCILLOSCOPE
The pulse amplitudes (Vpp) of the red and infraredsignals are measured and converted to Vrms toproduce a Ratio value as given by Equation 1. TheSpO2 can be determined using the Ratio value and alook-up table that is made up of empirical formulas. Thepulse rate is calculated based on the Analog-to-Digitalconverter (ADC) sample number and sampling rate.
EQUATION 1:
The look-up table is an important part of the system.Look-up tables are specific to a particular oximeterdesign and are usually based on calibration curvesderived from many measurements of a healthy subjectat various SpO2 levels. Figure 3 shows a samplecalibration curve.
Red Pulsation Signal
IR Pulsation Signal
Ratio Red_AC_Vrms / Red_DCIR_AC_Vrms / IR_DC---------------------------------------------------------------=
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FIGURE 3: SAMPLE CALIBRATION CURVECIRCUIT DESCRIPTIONThe SpO2 probe used in this example is an off-the-shelfNellcor® compatible finger clip type of probe whichintegrates one red LED and one IR LED and a photodi-ode. The LEDs are controlled by the LED driver circuit.The red light and IR light passing through the finger aredetected by the signal conditioning circuit and are thenfed to a 12-bit ADC module of the microcontrollerwhere %SpO2 can be calculated.
LED Driver circuitA DUAL SPDT analog switch driven by two PWMsignals from the microcontrollers turns the red andinfrared LEDs on and off alternately. In order to acquirethe proper number of ADC samples and have enoughtime to process the data before the next LED turns on,the LEDs are switched on/off according to the timingdiagram in Figure 4:
FIGURE 4: TIMING DIAGRAM
The LED current/intensity is controlled by a 12-bitDigital-to-Analog Converter (DAC) which is driven bythe microcontroller.
0
20
40
60
80
100
0.4 1 2 3.5
SpO
2 (%
)
Ratio
Sample Calibration Curve
g
IR_off 1780uS
RED_on 220uS
RED_off 1780uS
320uS
Read ADC
Read ADC
Read ADC
Processing data
IR_on 220uS
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Analog Signal Conditioning CircuitThere are two stages in the signal conditioning circuit.The first stage is the transimpedance amplifier and thesecond stage is the gain amplifier. A Highpass filter isplaced between the two stages.TRANSIMPEDANCE AMPLIFIERThe transimpedance amplifier converts a few microamps of current generated by the photodiode to a fewmillivolts.
HIGHPASS FILTERThe signal received from the first stage amplifierpasses through a Highpass filter which is designed toreduce the background light interference.
GAIN AMPLIFIERThe output of the Highpass filter is sent to a secondstage amplifier with a gain of 22 and a DC offset of220 mV. The values for the amplifier’s gain and DCoffset are set to properly place the output signal level ofthe gain amplifier into the microcontroller’s ADC range.
DIGITAL FILTER DESIGNThe output of the analog signal conditioning circuit isconnected to the ADC module of the dsPIC DSCs. OneADC sample is taken during each LED’s on-timeperiod, and one ADC sample is taken during bothLED’s off-time period.
Taking advantage of the powerful Digital SignalProcessing (DSP) engine integrated in dsPIC DSCs, adigital FIR Bandpass Filter is implemented to filter theADC data. The filtered data is used to calculate thepulse amplitude. Digital filter code is generated usingMicrochip’s Digital Filter Design Tool.
CONNECTIVITYThe SpO2 and pulse rate data can be sent to acomputer through a UART port with the PICkit™ SerialAnalyzer. The serial port setting is 115200-8-N-1-N.The pulse signal can be plotted out using an applicationsuch as Microchip’s Generic Serial Data Display GUIas shown in Figure 5.
The data can also be sent to a Wi-Fi® or Bluetooth®
module via UART port.
FIGURE 5: THE WAVEFORM DISPLAYING THE PULSE SIGNAL
FIR Bandpass Filter SpecificationsSampling Frequency (Hz): 500Passband Frequency (Hz): 1 and 5Stopband Frequency (Hz): 0.05 and 25FIR Window: KaiserPassband Ripple (-dB): 0.1Stopband Ripple (-dB): 50Filter Length: 513
500
550
600
650
700
750
800
850
900
950
1000
1 43 85 127
169
211
253
295
337
379
421
463
505
547
589
631
673
715
757
799
841
883
925
967
1009
1051
1093
1135
1177
1219
1261
1303
1345
1387
1429
1471
1513
1555
1597
1639
1681
1723
1765
1807
1849
1891
1933
1975
2017
2059
2101
2143
2185
2227
2269
2311
2353
2395
2437
IR
RED
DS00001525B-page 4 2013-2015 Microchip Technology Inc.
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FIGURE 6: PROGRAM FLOWCHARTInitialization
Turn On/Off RED IR LEDs
Alternately
Is the signal received from the probe valid?
Go to Sleep No
Are Red IR ADC Data Ready?
Adjust DAC to Calibrate IR LED
Yes
No
Yes
Adjust DAC to Calibrate Red LED
FIR Bandpass Digital Filtering
Find MaxMin of IR RED Filtered AC Signals
Calculate SPO2 Pulse Rate
Main Loop
Display Result
Start
Timer 3 Interrupt Occurred Read RED DC AC Signal
Timer 2 Interrupt Occurred Read IR DC AC Signal
Read DC Baseline Signal after Timer3 Interrupt
before Timer2 Interrupt
From Interrupts
Is Red ADC Data Ready?
Yes
Is IR ADC Data Ready?
Yes
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NOTES:DS00001525B-page 6 2013-2015 Microchip Technology Inc.
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APPENDIX A: SCHEMATICS This appendix shows the Microchip Pulse Oximeterschematics.
SHEET 1: MICROCHIP PULSE OXIMETER DEMO BOARD SCHEMATIC 1
GND 2
VFB
4
VIN
6VO
UT
5
EN3
SW1
U2
GND
C1
C2
R1
R2
GND
GND
GND
GND
Boos
t Reg
ulator
Microco
ntroller - dsP
IC33
FJ12
8GP8
02
VCC 3.3V
AN0
AN1
PGC2
PGD2
RB0
/DB0
OC2/RED
RB1
/DB1
RB2
/DB2
RB3
/DB3
RB4
/DB4
RB5
/DB5
RB6
/DB6
RB7
/DB7
SCL1
SDA1
OC1/IR
RA2
/RW
RA3
/RS
RA4
/E
R6
R7
C5
C6
C7
C8
VCC 3.3V
GND
DAC
_B/RED
DAC
_A/IR
DAC
_C/DC O
FFSE
T
DAC
_D
SCL1SDA1
RB0
/DB0
RB1
/DB1
RB2
/DB2
RB3
/DB3
RB4
/DB4
RB5
/DB5
RB6
/DB6
RB7
/DB7
RA4
/ERA2
/RW
RA3
/RS
DAC
VCC 3.3V
GND
MCLR
R4
VCC 3.3V
GND
MCLR
R3
VCC 3.3V
C4
C9
GND
VCC 3.3V
MCLR
ICSP
VCC 3.3V
GND
VCC 3.3V
GND
C10
C11
S2
GNDC
12
PGD2
U1T
X
PGC2
TP1
U1T
X
GND
BT1
1 2 3 4 5 6
P1
1 2 3 4 5 6
P2
L1
VDD
1
SCL
2
SDA
3
nLDAC
4
RDY/nB
SY5
VOUTA
6
VOUTB
7
VOUTC
8
VOUTD
9
VSS
10
U3
123
S1
TP2
MCLR
1AN
0/VR
EF+/CN2/RA0
2
AN1/VR
EF-/C
N3/RA1
3
PGD1/EM
UD1/AN
2/C2IN-/R
P0/CN4/RB0
4
PGC1/EM
UC1/AN
3/C2IN+/RP1
/CN5/RB1
5
AN4/C1IN-/R
P2/CN6/RB2
6
AN5/C1IN+/RP3
/CN7/RB3
7
VSS
8
OSC
1/CLK
I/CN30
/RA2
9
OSC
2/CLK
O/CN29
/PMA0
/RA3
10
SOSC
I/RP4
/CN1/PM
BE/RB4
11
SOSC
O/T1C
K/CN0/PM
A1/RA4
12VD
D13
PGD3/EM
UD3/AS
DA1
/RP5
/CN27
/PMD7/RB5
14
PGC3/EM
UC3/AS
CL1
/RP6
/CN24
/PMD6/RB6
15
INT0
/RP7
/CN23
/PMD5/RB7
16
TCK/SC
L1/RP8
/CN22
/PMD4/RB8
17
TDO/SDA1
/RP9
/CN21
/PMD3/RB9
18
VSS
19
VCAP
/VDDCORE
20
PGD2/EM
UD2/TD
I/RP1
0/CN16
/PMD2/RB1
021
PGC2/EM
UC2/TM
S/RP1
1/CN15
/PMD1/RB1
122
AN12
/DAC
1RP/RP1
2/CN14
/PMD0/RB1
223
AN11
/DAC
1RN/RP1
3/CN13
/PMRD/RB1
324
AN10
/DAC
1LP/RTC
C/RP1
4/CN12
/PMWR/RB1
425
AN9/DAC
1LN/RP1
5/CN11
/PMCS1
/RB1
526
AVSS
27
AVDD
28
U1
C3
TP9
TP10
TP11
TP8
MCLR
ICSP
RB1
4
RB1
4VS
S1
VDD
2NC
3RS
4R/W
5EN
6DB0
7DB1
8DB2
9DB3
10DB4
11DB5
12DB6
13DB7
14LE
D+
15LE
D-
16LC
D1
GND
GND
BT+
BT+
GND
R17
C18
GND
11
22
33
44
55
66
77
88
99
1010
1111
1212
1313
1414
1515
1616
J2
2013-2015 Microchip Technology Inc. DS00001525B-page 7
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SHEET 2: MICROCHIP PULSE OXIMETER DEMO BOARD SCHEMATIC 2Q1
MMBT
2222
DAC
_B/RED
Q2
MMBT
2222
10 Ohm
R10
10 Ohm
R11
5.1K
R9
5.1K
R8
GND
OC2/RED
OC1/IR
GND
VCC 3.3V
100
R12
100
R13
GND
DAC
_A/IR
VCC 3.3V
LED Driv
er DB9
-3/RED
DB9
-2/IR
AN1
220K
R15
22pF
C16
10K
R5
VCC 3.3V
100K
R14
10pF
C15
AN0
0.1uF
C14
DAC
_C/DC O
FFSE
T
DB9
-9/CAT
HODE
Analog
Signa
l Con
ditio
ning
Con
nect to
SpO
2 Se
nsor
DB9
Fem
ale Con
nector
12345 6789
1110
J1 D Co
nnector 9
GND
GND
DB9
-9/CAT
HODE
DB9
-3/RED
DB9
-2/IR
0.1uF
C13
GND
TP6
TP7
TP4
TP3
V+1
NO1
2
COM1
3
IN1
4
NC1
5
NO2
10
COM2
9
IN2
8
NC2
7
GND
6
U4
ADG8
84BR
MZ Dual SPD
T
DB9
-5/Ano
de
TP5
1uF
C17
2.7K
R16
GND
GND
GND
GND
DB9
-5/Ano
de
SPO2 SE
NSO
R
+A3
-A2
OUTA
1
+A-A
OUTA
AA
A
VSS 4VDD8
MCP6002
U5A
+B5
-B6
OUTB
7
+B-B
OUTB
BB
B
VSS 4VDD8
MCP6002
U5B
DS00001525B-page 8 2013-2015 Microchip Technology Inc.
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APPENDIX B: MEDICAL DEMO WARNINGS, RESTRICTIONS AND DISCLAIMER
This demo is intended solely for evaluation anddevelopment purposes. It is not intended for medicaldiagnostic use.
APPENDIX C: REFERENCESAN1494, “Using MCP6491 Op Amps for Photodet-ection Applications”, Microchip Technology Inc.,DS01494, 2013.
2013-2015 Microchip Technology Inc. DS00001525B-page 9
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NOTES:DS00001525B-page 10 2013-2015 Microchip Technology Inc.
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• There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
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DS00001525B-page 11
headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona; Gresham, Oregon and design centers in California and India. The Company’s quality system processes and procedures are for its PIC® MCUs and dsPIC® DSCs, KEELOQ® code hopping devices, Serial EEPROMs, microperipherals, nonvolatile memory and analog products. In addition, Microchip’s quality system for the design and manufacture of development systems is ISO 9001:2000 certified.
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