Pulse Oximeter Design Using Microchip’s Analog Devices and ...Pulse oximeter is a non-invasive...

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

2013-2015 Microchip Technology Inc. DS00001525B-page 1

AN1525

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---------------------------------------------------------------=

DS00001525B-page 2 2013-2015 Microchip Technology Inc.

AN1525
FIGURE 3: SAMPLE CALIBRATION CURVE
CIRCUIT 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


AN1525
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


AN1525
FIGURE 6: PROGRAM FLOWCHART
Initialization

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


AN1525
NOTES:

AN1525

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


AN1525
SHEET 2: MICROCHIP PULSE OXIMETER DEMO BOARD SCHEMATIC 2
Q1

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


AN1525

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.


AN1525
NOTES:

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• 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.

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• Microchip is willing to work with the customer who is concerned about the integrity of their code.

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Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of ourproducts. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such actsallow 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 deviceapplications and the like is provided only for your convenienceand may be superseded by updates. It is your responsibility toensure that your application meets with your specifications.MICROCHIP MAKES NO REPRESENTATIONS ORWARRANTIES OF ANY KIND WHETHER EXPRESS ORIMPLIED, WRITTEN OR ORAL, STATUTORY OROTHERWISE, RELATED TO THE INFORMATION,INCLUDING BUT NOT LIMITED TO ITS CONDITION,QUALITY, PERFORMANCE, MERCHANTABILITY ORFITNESS FOR PURPOSE. Microchip disclaims all liabilityarising from this information and its use. Use of Microchipdevices in life support and/or safety applications is entirely atthe buyer’s risk, and the buyer agrees to defend, indemnify andhold harmless Microchip from any and all damages, claims,suits, or expenses resulting from such use. No licenses areconveyed, implicitly or otherwise, under any Microchipintellectual property rights.

2013-2015 Microchip Technology Inc.

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DS00001525B-page 11

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Pulse Oximeter Design Using Microchip’s Analog Devices and ...Pulse oximeter is a non-invasive...

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