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?
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?