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I have built a photodiode circuit using AD712 (precision bifet) and the circuit is working as expected. I have added a programmable gain using a multiplexer which lets me choose a gain from 1 to 100 in 8 steps. I noticed that when my gain is set to maximum, I am picking up some signal which is insync with the frequency of ambient lights (however, the sensor is enclosed in a box).

To make sure that the signal is due to optical noise, I shorted the inverting and non inverting inputs of my OP AMP which leads to no signal (0 value) for all gain setting.

Detail about circuit: Photo-diode to a trans-impedance circuit -> 2nd stage gain -> RC -> Buffer -> ADC.

While reading about other photodiode circuits, I saw various implementation using OP07C with offset nulling techniques.

I am not sure that if I do need to do some offset as there is 0 signal when photodiode is shorted - which makes me believe that I should focus on optical noise.

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  • \$\begingroup\$ Did you try placing some thick heavy object in front of the "enclosure" just to see if you could reduce the signal a bit more - this allows you to positively distinguish between maybe an IR effect or some electrical pick-up? Shorting the inputs tells you nothing. \$\endgroup\$ – Andy aka Mar 20 '15 at 8:54
  • \$\begingroup\$ @Andyaka I tried my best to but in a box and then in another wooden box. The signal went down by a bit , hinting that it has to do with optical noise. Any way to make sure that it is not electrical noise? I do have an oscciloscope, if it is needed. \$\endgroup\$ – Anuj Purohit Mar 20 '15 at 10:01
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The offset voltage is the voltage you'd need to apply at the input to produce 0V at the op amp's output. It is DC and has nothing to do with your problem. Either light is somehow getting in, or there is electrical noise coupling to your circuit; hard to say without knowing how it's powered.

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