Make With Mouser IoT Design Contest - Baby Vital Monitor_02 SPO2 [PPG]

SpO2:

SpO2 is the measurement of percentage of oxygen saturation in blood. The basic principal is Photoplethysmography [PPG].

In this blog, I’ll share my experience on PPG experiment following traditional method. And then I’ll share how we can use the ADPD4100 for this purpose.

I worked with PPG before receiving the Evaluation board of ADPD4100. That time, I used RED / IR LEDs and Photodiodes for reflectance PPG and constructed the Analog Front End using general purpose op-amp as transimpedance amplifier and filter circuitry.

Before getting into the experiment, I would like to share some background study [theories] for those who wants to work on PPG from scratch.
**the original project was also shared on another blog of mine in ELEMENT14 forum.

Photoplethysmography:

"Photoplethysmography (PPG) is a simple and low-cost optical technique that can be used to detect blood volume changes in the microvascular bed of tissue. It is often used non-invasively to make measurements at the skin surface. The PPG waveform comprises a pulsatile ('AC') physiological waveform attributed to cardiac synchronous changes in the blood volume with each heart beat, and is superimposed on a slowly varying ('DC') baseline with various lower frequency components attributed to respiration, sympathetic nervous system activity and thermoregulation. Although the origins of the components of the PPG signal are not fully understood, it is generally accepted that they can provide valuable information about the cardiovascular system."

-source: https://www.ncbi.nlm.nih.gov/pubmed/17322588

reflective.png
sensors-17-00130-g007.png


The working principle is pretty simple. A red LED is used to pass light through finger tip and a photodiode is used to receive the reflected light. This contains both the DC and pulse AC components in terms of photo-current. A transimpedance amplifier converts the signal into voltage and amplifies it. Filters are used to get rid of the DC component and the remaining AC component is further amplified to observe using oscilloscope or send to PC using a DAQ system. The PPG signal represents the heart beat, which can be captured at finger tip and takes a small time gap. A simultaneous comparison of ECG and PPG explains the relation and time gap between arrival.

PPG_Signal.png
ecg and ppg.png

**images are collected from internet [https://www.comm.utoronto.ca/~biometrics/PPG_Dataset/ ]


Block Diagram -
BLOCK DIAGRAM 1.jpg



Key Analog Modules -
  • Transimpedance amplifier
  • Basic High and Low Pass filters
  • Optical Isolation


Sensor-

The sensor module consists of a RED LED and photodiode. To drive the LED at constant ~22mA, a LM317 based simple driver ckt is used.
However, the intensity can be varied using an onboard potentiometer.

sensor1.JPG
isolator.jpg

Figure: sensor module with AFE Opto-isolator to check the output in oscilloscope without adding mains (50/60 Hz)noise


Transimpedance Amplifier and signal conditioning-

The TIA converts the photodiode’s current output signal to a usable voltage level. The implementation of this current-to-voltage conversion consists of a photodiode, an amplifier and a resistor/capacitor feedback pair.
The output signal is passed through a High Pass filter [0.03Hz] and then to a Low Pass filter [cut off ~15Hz] to produce a clean PPG signal which can be observed using oscilloscope or DAQ. But an opto-isolator can be very useful to get a noise free output. Without an opto-isolator, oscilloscope / DAQ ground connects to the AFE supply and produces distorted signal. I have used batteries to power the AFE, so that i can avoid noise at the input side.
article-2017july-design-transimpedance-amplifiers-fig1.jpg
I2V.gif


Output:

using oscilloscope-
output.jpg


using a DAQ system and software developed by Department of Biomedical Physics and Technology, University of dhaka -
DAQ PC output.jpg



ADPD4100 Pulse oximetry [SpO2 measurement] configuration:

ADPD4100 evaluation board has two LEDs of red - 660 nm wavelength and infrared (IR)- 940 nm wavelength. Deoxygenated hemoglobin absorbs more of the 660 nm wavelength light, and oxygenated hemoglobin absorbs more of the 940 nm wavelength. The photodiode perceives the non-absorbed light.

The perceived signals are then divided into dc and ac components. The dc component represents the absorption of light due to the tissue, venous blood, and non pulsatile arterial blood. The ac component represents the pulsatile arterial blood. The SpO2 percentage is then calculated by:

%SpO2 = (ACred/DCred)/(ACIR/DCIR)

ADPD4100 evaluation board has green LEDs with wavelength around 540 nm which is generally preferred in heart rate monitoring. It yields the best PPG signal as it has a higher modulation index than red or IR LEDs. It also provides a decent CTR level; therefore, power consumption would not be too high.

source: Multiparameter Vital Signs Monitoring Is Easier Than Ever Before by Electronics Maker November 21, 2020

spo2 configuration layout.jpg
spo2 signal.png

Above figures represent the ADPD 4100 typical PPG circuit, we need couple of it for SpO2. The right side figure shows sample PPG signals for IR,Red and Green LEDs.

I am working on the operation modes and settings of the registers to configure the ADPD4100 evaluation board to physically check the output. Once I get the desired output, I'll share all the details in another blog.

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Sunnyiut
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