Hi everyone,
I'm creating a circuit to measure PPG (blood oxygen saturation) readings using and LED and photodiode, and I'm having some issues with my transimpedance amplifier in the photodiode part. A couple of weeks ago my circuit worked fine, showing a clear difference between the signal voltage from the amplifier when the photodiode was covered and uncovered.
However since last week, when I cover the photodiode, the signal peaks and then quickly returns to the uncovered baseline level. Then, when I uncover it, the opposite happens: the signal peaks briefly in the inverse direction and returns to baseline. I have tried swapping out all the components and wires (except for the photodiode), moving the circuit to a different part of the breadboard, putting a capacitor across the op amp +/- voltage, altering the feedback resistor value, nothing seems to make any difference.
Can anyone offer any advice, or anything I can try? I can post pictures or even a video of the signal changes on an oscilloscope if anyone wants to see it.
Thanks!
I'm creating a circuit to measure PPG (blood oxygen saturation) readings using and LED and photodiode, and I'm having some issues with my transimpedance amplifier in the photodiode part. A couple of weeks ago my circuit worked fine, showing a clear difference between the signal voltage from the amplifier when the photodiode was covered and uncovered.
However since last week, when I cover the photodiode, the signal peaks and then quickly returns to the uncovered baseline level. Then, when I uncover it, the opposite happens: the signal peaks briefly in the inverse direction and returns to baseline. I have tried swapping out all the components and wires (except for the photodiode), moving the circuit to a different part of the breadboard, putting a capacitor across the op amp +/- voltage, altering the feedback resistor value, nothing seems to make any difference.
Can anyone offer any advice, or anything I can try? I can post pictures or even a video of the signal changes on an oscilloscope if anyone wants to see it.
Thanks!