I was given an ISS box which was made in 2013. It is numbered D1300694-v1 and the schematics are present in DCC. I tried to go through the schematic to get an understanding of it and mark the differences of the real circuit on the schematic. PFA a commented schematic which marks the measured component values and shows what is not present on this circuit.
Breif Description of this circuit and what it intended to do:
The circuit is DC coupled and the PD signal first goes through an instrumentation amplifier with about 1.33 gain. Then a 5V reference is subtracted from this signal (so the servo do DC stabalization to keep PD signal at 5V).
Then the signal goes through 3 stages of filtering and amplification.
Stage 1: (Always On) Is a simple gain of 6.36
Stage 2: Is a 554 Hz Zero and 10kHz pole with DC gain of 18 (Called Boost 1)
Stage 3: Is a 0 Hz Pole and 4.57MHz zero with the open loop gain of AD829 at DC. (Called Boost 2)
The Boost 1 and 2 are supposed to start only when rms value of PD signal is below a certain threshold. Currently, this functionality is not working due to a problem in RMS-to-DC converter as specified in elog 40m/9332.
I will see if I can find a way to hack this circuit to do what we want to do without making any irreversible changes.