AN1525

AN1525
Pulse Oximeter Design Using Microchip’s Analog Devices
and dsPIC® Digital Signal Controllers (DSCs)
Author:
Zhang Feng
Microchip Technology Inc.
INTRODUCTION
Pulse oximeter is a non-invasive medical device that
monitors the oxygen saturation of a patient’s blood and
heart rate. This application note demonstrates the
implementation of a high-accuracy pulse oximeter
using Microchip’s analog devices and dsPIC® Digital
Signal Controllers (DSCs).
FIGURE 1:
FUNCTION BLOCK DIAGRAM
PWM1
PWM2
LED On/Off
LED Driver
Analog Signal Conditioning
Analog
Switch
IR
ADC0
Red
Transimpedance
Amplifier
Highpass
Filter
Gain Stage
Amplifier
Photodiode
LED Current Control
ADC1
DC Offset
DAC
I2C™
LCD
I/O
Computer,
WiFi® or
Bluetooth®
 2013-2015 Microchip Technology Inc.
Microcontroller
UART
DS00001525B-page 1
AN1525
THEORY OF OPERATION
A pulse oximeter monitors the oxygen saturation
(SpO2) of a human’s blood based on the red light (600750 nm wavelength) and infrared light (850-1000 nm
wavelength) absorption characteristics of oxygenated
hemoglobin (HbO2) and deoxygenated hemoglobin
(Hb). The pulse oximeter flashes the red and infrared
lights alternately through a finger to a photodiode.
HbO2 absorbs more infrared light and allows more red
FIGURE 2:
light to pass through. On the other hand, Hb absorbs
more red light and allows more infrared light to pass
through.
The photodiode receives the non-absorbed light from
each LED. This signal is inverted using inverting OpAmp and therefore the result, as shown in Figure 2,
represents the light that has been absorbed by the
finger.
REAL-TIME RED AND INFRARED (IR) PULSATION SIGNALS CAPTURED BY THE
OSCILLOSCOPE
Red Pulsation Signal
IR Pulsation Signal
The pulse amplitudes (Vpp) of the red and infrared
signals are measured and converted to Vrms to
produce a Ratio value as given by Equation 1. The
SpO2 can be determined using the Ratio value and a
look-up table that is made up of empirical formulas. The
pulse rate is calculated based on the Analog-to-Digital
converter (ADC) sample number and sampling rate.
The look-up table is an important part of the system.
Look-up tables are specific to a particular oximeter
design and are usually based on calibration curves
derived from many measurements of a healthy subject
at various SpO2 levels. Figure 3 shows a sample
calibration curve.
EQUATION 1:
Red_AC_Vrms / Red_DC
Ratio = --------------------------------------------------------------IR_AC_Vrms / IR_DC
DS00001525B-page 2
 2013-2015 Microchip Technology Inc.
AN1525
FIGURE 3:
SAMPLE CALIBRATION CURVE
Sample Calibration Curve
100
SpO2 (%)
80
60
40
20
0
0.4
1
2
3.5
Ratio
CIRCUIT DESCRIPTION
LED Driver circuit
The SpO2 probe used in this example is an off-the-shelf
Nellcor® compatible finger clip type of probe which
integrates one red LED and one IR LED and a photodiode. The LEDs are controlled by the LED driver circuit.
The red light and IR light passing through the finger are
detected by the signal conditioning circuit and are then
fed to a 12-bit ADC module of the microcontroller
where %SpO2 can be calculated.
A DUAL SPDT analog switch driven by two PWM
signals from the microcontrollers turns the red and
infrared LEDs on and off alternately. In order to acquire
the proper number of ADC samples and have enough
time to process the data before the next LED turns on,
the LEDs are switched on/off according to the timing
diagram in Figure 4:
FIGURE 4:
TIMING DIAGRAM
g
Read
ADC
Read
ADC
RED_off
1780uS
RED_on
220uS
320uS
Read
ADC
Processing data
IR_on
220uS
IR_off
1780uS
The LED current/intensity is controlled by a 12-bit
Digital-to-Analog Converter (DAC) which is driven by
the microcontroller.
 2013-2015 Microchip Technology Inc.
DS00001525B-page 3
AN1525
Analog Signal Conditioning Circuit
DIGITAL FILTER DESIGN
There are two stages in the signal conditioning circuit.
The first stage is the transimpedance amplifier and the
second stage is the gain amplifier. A Highpass filter is
placed between the two stages.
The output of the analog signal conditioning circuit is
connected to the ADC module of the dsPIC DSCs. One
ADC sample is taken during each LED’s on-time
period, and one ADC sample is taken during both
LED’s off-time period.
TRANSIMPEDANCE AMPLIFIER
The transimpedance amplifier converts a few micro
amps of current generated by the photodiode to a few
millivolts.
HIGHPASS FILTER
The signal received from the first stage amplifier
passes through a Highpass filter which is designed to
reduce the background light interference.
GAIN AMPLIFIER
The output of the Highpass filter is sent to a second
stage amplifier with a gain of 22 and a DC offset of
220 mV. The values for the amplifier’s gain and DC
offset are set to properly place the output signal level of
the gain amplifier into the microcontroller’s ADC range.
Taking advantage of the powerful Digital Signal
Processing (DSP) engine integrated in dsPIC DSCs, a
digital FIR Bandpass Filter is implemented to filter the
ADC data. The filtered data is used to calculate the
pulse amplitude. Digital filter code is generated using
Microchip’s Digital Filter Design Tool.
FIR Bandpass Filter Specifications
Sampling Frequency (Hz): 500
Passband Frequency (Hz): 1 and 5
Stopband Frequency (Hz): 0.05 and 25
FIR Window: Kaiser
Passband Ripple (-dB): 0.1
Stopband Ripple (-dB): 50
Filter Length: 513
CONNECTIVITY
The SpO2 and pulse rate data can be sent to a
computer through a UART port with the PICkit™ Serial
Analyzer. The serial port setting is 115200-8-N-1-N.
The pulse signal can be plotted out using an application
such as Microchip’s Generic Serial Data Display GUI
as 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
1000
950
900
850
800
750
IR
RED
700
650
600
550
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
500
DS00001525B-page 4
 2013-2015 Microchip Technology Inc.
AN1525
FIGURE 6:
PROGRAM FLOWCHART
Start
Initialization
Turn On/Off
RED ĂŶĚ IR LEDs
Alternately
From Interrupts
Main Loop
Is the signal received
from the probe valid?
No
Goto Sleep
Yes
Are Red ĂŶĚ IR ADC
Data Ready?
No
Yes
FIR Bandpass Digital Filtering
Find MaxMin of IR ĂŶĚ RED Filtered AC Signals
Calculate SPO2 ĂŶĚ Pulse Rate
Display Result
Timer 3 Interrupt Occurred
Read RED DC ĂŶĚ AC Signal
Timer 2 Interrupt Occurred
Read IR DC ĂŶĚ AC Signal
Read DC Baseline Signal
after Timer3 Interrupt
before Timer2 Interrupt
Adjust DAC to
Calibrate Red LED
Adjust DAC to
Calibrate IR LED
Is Red ADC
Data Ready?
Is IR ADC
Data Ready?
Yes
 2013-2015 Microchip Technology Inc.
Yes
DS00001525B-page 5
AN1525
NOTES:
DS00001525B-page 6
 2013-2015 Microchip Technology Inc.
GND
%DWWHU\9[$$$
BT1
GND
C1
X)
L1
1
6
3
VFB
VOUT
GND
SW
VIN
EN
U2 0&37,&+<
 2013-2015 Microchip Technology Inc.
R3
X)
X)
8
19
27
13
28
20
1
AN0/VREF+/CN2/RA0
AN1/VREF-/CN3/RA1
OSC1/CLKI/CN30/RA2
OSC2/CLKO/CN29/PMA0/RA3
SOSCO/T1CK/CN0/PMA1/RA4
PGD1/EMUD1/AN2/C2IN-/RP0/CN4/RB0
PGC1/EMUC1/AN3/C2IN+/RP1/CN5/RB1
AN4/C1IN-/RP2/CN6/RB2
AN5/C1IN+/RP3/CN7/RB3
SOSCI/RP4/CN1/PMBE/RB4
PGD3/EMUD3/ASDA1/RP5/CN27/PMD7/RB5
PGC3/EMUC3/ASCL1/RP6/CN24/PMD6/RB6
INT0/RP7/CN23/PMD5/RB7
TCK/SCL1/RP8/CN22/PMD4/RB8
TDO/SDA1/RP9/CN21/PMD3/RB9
PGD2/EMUD2/TDI/RP10/CN16/PMD2/RB10
PGC2/EMUC2/TMS/RP11/CN15/PMD1/RB11
AN12/DAC1RP/RP12/CN14/PMD0/RB12
AN11/DAC1RN/RP13/CN13/PMRD/RB13
AN10/DAC1LP/RTCC/RP14/CN12/PMWR/RB14
AN9/DAC1LN/RP15/CN11/PMCS1/RB15
S2
GND
0&/5BQ
MCLR ICSP
'63,&)-*3,62
VSS
VSS
AVSS
VDD
AVDD
VCAP/VDDCORE
MCLR
U1
C12
X)
GND
R4
.
VCC 3.3V
X)
C3
4
5
6
7
11
14
15
16
17
18
21
22
23
24
25
26
2
3
9
10
12
GND
.
R2
.
R1
.
R7
LEDLED+
DB7
DB6
DB5
DB4
DB3
DB2
DB1
DB0
EN
R/W
RS
NC
VDD
VSS
LCD1
.
R6
GND
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
1+'+=)6:)%:9&
RB0/DB0
RB1/DB1
RB2/DB2
RB3/DB3
RB4/DB4
RB5/DB5
RB6/DB6
RB7/DB7
SCL1
SDA1
PGD2
PGC2
OC2/RED
OC1/IR
RB14
U1TX
TP1
TP2
VCC 3.3V
AN0
AN1
RA2/RW
RA3/RS
RA4/E
GND
X)
C2
VCC 3.3V
5
4
3
2
0&30623
RDY/nBSY
nLDAC
SDA
SCL
VDD
U3
VOUTA
VOUTB
VOUTC
VOUTD
GND
VSS
DAC
'13
R17
16
15 BT+
14
RB7/DB7
GND
13
RB6/DB6
12
RB5/DB5
11
RB4/DB4
10
RB3/DB3
9
RB2/DB2
8
RB1/DB1
7
RB0/DB0
6
RA4/E
5
RA2/RW
4
RA3/RS
3
2
VCC 3.3V
1
C18
X)
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
J2
GND
C9 X)
1
VCC 3.3V
C4 X)
10
6
7
8
9
C7
Q)
GND
U1TX
RB14
GND
VCC 3.3V
PGD2
PGC2
DAC_D
3,&NLWŒ6HULDO
1
2
3
4
5
6
P2
,&63
1
2
3
4
5
6
P1
C8
Q)
DAC_C/DC OFFSET
TP10
DAC_B/RED
TP9
DAC_A/IR
TP8
VCC 3.3V
C6
Q)
MCLR ICSP
GND
C5
Q)
TP11
GND
SCHEMATICS
MCLR
GND
C11
C10
GND
VCC 3.3V
MCLR
4
5
SHEET 1:
Microcontroller - dsPIC33FJ128GP802
S1
1
BT+
Boost Regulator
GND
2
2
APPENDIX A:
SCL1
SDA1
3
AN1525
This appendix shows the Microchip Pulse Oximeter
schematics.
MICROCHIP PULSE OXIMETER DEMO BOARD SCHEMATIC 1
DS00001525B-page 7
DB9-5/Anode
3
2
GND
R8
5.1K
100
R12
TP4
C15
+A
A
-A
A
GND
1
C14
0.1uF
100K
10pF
GND
MCP6002
U5A
OUTA
A
ADG884BRMZ Dual SPDT
NC1
IN1
COM1
NO1
V+
U4
GND
NC2
IN2
COM2
NO2
GND
TP5
C17
R16
2.7K
GND
1uF
TP6
AN0
10K 6
C16
+B
B
-B
B
R15
GND
5
DAC_C/DC OFFSET
R5
TP3
R11
10 Ohm
VCC 3.3V
Q2
MMBT2222
6
7
8
9
10
Analog Signal Conditioning
Q1
MMBT2222
5
4
3
2
1
R10
10 Ohm
GND
VCC 3.3V
R14
OC2/RED
DB9-3/RED
DB9-9/CATHODE
DAC_A/IR
GND
C13
0.1uF
VCC 3.3V
8
VDD
VSS
4
220K
22pF
100
R13
MCP6002
U5B
7
OC1/IR
DB9-2/IR
OUTB
B
8
DS00001525B-page 8
VDD
VSS
R9
5.1K
TP7
AN1
DAC_B/RED
GND
11
10
5
9
4
8
3
7
2
6
1
DB9-2/IR
DB9-3/RED
DB9-9/CATHODE
DB9-5/Anode
SPO2 SENSOR
GND
DB9 Female Connector
Connect to SpO2 Sensor
D Connector 9
J1
SHEET 2:
4
LED Driver
AN1525
MICROCHIP PULSE OXIMETER DEMO BOARD SCHEMATIC 2
 2013-2015 Microchip Technology Inc.
AN1525
APPENDIX B:
MEDICAL DEMO
WARNINGS,
RESTRICTIONS AND
DISCLAIMER
APPENDIX C:
REFERENCES
AN1494, “Using MCP6491 Op Amps for Photodetection Applications”, Microchip Technology Inc.,
DS01494, 2013.
This demo is intended solely for evaluation and
development purposes. It is not intended for medical
diagnostic use.
 2013-2015 Microchip Technology Inc.
DS00001525B-page 9
AN1525
NOTES:
DS00001525B-page 10
 2013-2015 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
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.
•
Microchip is willing to work with the customer who is concerned about the integrity of their code.
•
Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding device
applications and the like is provided only for your convenience
and may be superseded by updates. It is your responsibility to
ensure that your application meets with your specifications.
MICROCHIP MAKES NO REPRESENTATIONS OR
WARRANTIES OF ANY KIND WHETHER EXPRESS OR
IMPLIED, WRITTEN OR ORAL, STATUTORY OR
OTHERWISE, RELATED TO THE INFORMATION,
INCLUDING BUT NOT LIMITED TO ITS CONDITION,
QUALITY, PERFORMANCE, MERCHANTABILITY OR
FITNESS FOR PURPOSE. Microchip disclaims all liability
arising from this information and its use. Use of Microchip
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hold harmless Microchip from any and all damages, claims,
suits, or expenses resulting from such use. No licenses are
conveyed, implicitly or otherwise, under any Microchip
intellectual property rights.
Trademarks
The Microchip name and logo, the Microchip logo, dsPIC,
FlashFlex, flexPWR, JukeBlox, KEELOQ, KEELOQ logo, Kleer,
LANCheck, MediaLB, MOST, MOST logo, MPLAB,
OptoLyzer, PIC, PICSTART, PIC32 logo, RightTouch, SpyNIC,
SST, SST Logo, SuperFlash and UNI/O are registered
trademarks of Microchip Technology Incorporated in the
U.S.A. and other countries.
The Embedded Control Solutions Company and mTouch are
registered trademarks of Microchip Technology Incorporated
in the U.S.A.
Analog-for-the-Digital Age, BodyCom, chipKIT, chipKIT logo,
CodeGuard, dsPICDEM, dsPICDEM.net, ECAN, In-Circuit
Serial Programming, ICSP, Inter-Chip Connectivity, KleerNet,
KleerNet logo, MiWi, MPASM, MPF, MPLAB Certified logo,
MPLIB, MPLINK, MultiTRAK, NetDetach, Omniscient Code
Generation, PICDEM, PICDEM.net, PICkit, PICtail,
RightTouch logo, REAL ICE, SQI, Serial Quad I/O, Total
Endurance, TSHARC, USBCheck, VariSense, ViewSpan,
WiperLock, Wireless DNA, and ZENA are trademarks of
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SQTP is a service mark of Microchip Technology Incorporated
in the U.S.A.
Silicon Storage Technology is a registered trademark of
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GestIC is a registered trademarks of Microchip Technology
Germany II GmbH & Co. KG, a subsidiary of Microchip
Technology Inc., in other countries.
All other trademarks mentioned herein are property of their
respective companies.
© 2013-2015, Microchip Technology Incorporated, Printed in
the U.S.A., All Rights Reserved.
ISBN: 978-1-63277-317-3
QUALITY MANAGEMENT SYSTEM
CERTIFIED BY DNV
== ISO/TS 16949 ==
 2013-2015 Microchip Technology Inc.
Microchip received ISO/TS-16949:2009 certification for its worldwide
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.
DS00001525B-page 11
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DS00001525B-page 12
 2013-2015 Microchip Technology Inc.
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