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!

 

Hello,

Without any information on the parts used and schematic, we can not give you a solution.
Please post a schematic here using the "Upload a File" button.

Bertus

 

Sorry, I've included a diagram of the transimpedance amplifier:



The op amp I'm using is a TL081CP, the feedback resistor was originally 15.6 M ohms, but I have tried various other values. The photodiode is this one.

 

Hello,

You might want to have a look at the attached PDF.

Bertus

 

Thanks bertus, I'll take a look

 

Sounds like you viewing the output with an oscilloscope set to AC input coupling.
It should be set to DC.

 

Sounds like you viewing the output with an oscilloscope set to AC input coupling.
It should be set to DC.

Hmm I was using a different oscilloscope, so it could be that! I'll check the settings when I'm back in next week.