I've designed pulse oximeter circuit. When one LED lights continuously, I got perfect PPG (Photoplethysmogram) signal.
PPG 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:
enter image description here

I opearte only on AC signal. My circuit: enter image description here

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 worked? Switching LEDs definitely ruined my circuit, due to operating on AC part of PPG.

Edit: Here is my LED driver:
LED driver

DAC in LED driver

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

  • \$\begingroup\$ You haven't shown the LED driver circuit. Don't you think that might be important? Also, showing a "similar" circuit is no use for trouble-shooting. You need to show your actual circuit. \$\endgroup\$
    – Andy aka
    Commented Dec 16, 2022 at 12:10
  • \$\begingroup\$ You can't time-domain multiplex two signals through a single analog filter. \$\endgroup\$
    – Dave Tweed
    Commented Dec 16, 2022 at 12:30
  • \$\begingroup\$ I got you. Thanks. It is my actuall circuit. I'll edit my post. @DaveTweed Sorry, I expressed myself wrong. I meant to get two waves by alternately sampling the signal. when two different LEDs are lighting. Check link \$\endgroup\$
    – Dom.in
    Commented Dec 16, 2022 at 13:29
  • \$\begingroup\$ I understand that. Note that the application note you linked to shows two separate filters. See Figure 6. \$\endgroup\$
    – Dave Tweed
    Commented Dec 16, 2022 at 15:59


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