All, I have created a photo-diode amplifier(2 stage) which allows me to set a gain from 2 to 551 (v/v) [4.6 to ~55dB] in 64 steps.
The gain is achieved using pre-calculated resistance to provide non-linear gain (as per the application).
The gain's theoretical chart is provided below:
Now when I read Signal and plot it with respect to Noise (Noise: when no light it exposed to the detector), I get the following data (12 bit adc)
I think that I am happy with this as the noise is pretty much low and signal rises as expected!
Whats concerns me is the analysis of noise in absence of signal. In this case, noise seems to be bouncing around for higher gains. Note that ADC, OPAMP all have decouplings caps right next to the chips (.1 uF, 1 uF, 10 uF).
I may live with the noise and do digital averaging to get a nice curve and use it, but is this noise behavior OK and expected?
To explain better, I am concerned that why is the noise not like this?