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.