The signal path for the ASDC signal is AS55 PD --> D990543 (interface board) --> D990694 (whitening board) --> D000076 (AA board) --> ADC Ch 31. Everything in this signal chain should be able to handle signals in the range +/- 10V, which should correspond to the full range of our +/-10V, 16bit ADCs. But the ASDC signal seems to saturate at ~2000 counts (i.e. turning up the analog whitening gain doesn't make the signal get any bigger than this). I investigated this a little more today.
So the problem lies somewhere downstream of the D990694. There are other anomalous behaviours of this channel - e.g. engaging the analog whitening filters changes the DC offset of the signal. I am going to pull out this board to check it out.
Why does this matter? I want to calibrate the ASDC level (and eventually the other PD DC signals as well) into Watts. This is useful for IFO diagnostics, noise budgeting the shot noise level etc.
According to the AS55 schematic, the DC transimpedance is 66.7 ohms. I claim that the DC power on the AS55 photodiode during a DRMI (no arms) lock is ~1mW. The C30642 photodiode (InGaAs) responsivity is ~0.8 A/W. So I'd expect ~50mV to be the signal level into the ADC (assuming gain of all the other electronics in the signal chain at the start of this elog is unity). This corresponds to ~163 counts (since the ADC conversion factor is 2^16 counts over 20volts). The DC signal level I observed is ~200 counts. So things seem roughly consistent.
*Note: Despite my above statement, I don't think it is true that the AS110 PD has more light on it - the BS splitting the light between
AS55 and AS110 PDs is a 50-50 BS, and using the crude method of putting an Ophir power meter in front of both PDs and
monitoring the power while the Michelson was swinging around freely showed roughly the same maximum value.