Difficulties with multiplexing PPG signal in pulse oximeter.

[Dlibip6]

Hi,
I've designed pulse oximeter circuit. When one LED lights continuously, I got perfect PPG (Photoplethysmogram) signal.

Then I measure PPG while blinking LEDS, like that:

I got stuck. There is no signal or just noise. When I removed low pass filters, I easily got my opamp saturated. I lowered LED current (I have current source for LEDs) and opamp gain, but there were little information in output signal.

I went back to circuit with capacitors (low pass filter) and changed algoruithm to measure more than one sample after turning on/off LED:

I opearte only on AC signal. My circuit is similar to found on web, e.g. Digikey, NXP:

Yes, I have low speed ADC (MCP3426). I set 240SPS on 12-bit to measure faster.
I have changed a lot of things, even software delay, but I have difficulties to get two signals from one photodiode. How to people make those pulse oximeters?

 

[danadak]

I think your output amp configuration not quite what you want. I dropped
the 1V thru 56K as I dont think you need that for biasing. Also your load
R5 and C5 I think need recomputing per, and its just too big a load to get
the RRIO behavior I think you need in signal chain.

Regards, Dana.

 

[danadak]

Maybe also C5 should be .1 uF, or consider doing an active filter stage :

I did look at phase margin, I think you are fine there.

Some useful ref material - https://www.analog.com/en/technical-articles/how-to-design-a-better-pulse-oximeter.html


Regards, Dana.

 

[Dlibip6]

Thanks for your feedback.
Do you mean to disconnect R4?

I should tell, that I removed C5 on output. Now, there is only R5.
Anyway, I got that characteristic:

 

[danadak]

Yes, I believe you do not need R4. The bias you have going to NI input of
OA2 I feel is adequate.

Do you have scope shots of the output of OA1 and OA2 ?


Regards, Dana.

 

[Dlibip6]

The bias you have going to NI input of
OA2 I feel is adequate.

I put 1V there from DAC (VOFFSET).

At this point, I don't have scope shot of OA1 and OA2. Due to deadline of project I'm going to leave it after monday. Still, I'm willing to get help and suggestions.


Here is my LED driver circuit:

I turn off LEDs by programmatically setting power-down mode: 1k to GND in DAC.