For some reasons the big offset that had been observed at ASC_ADC_1_7 [1][2] disappeared at around 11:24 AM this morning and never came back. I think this corresponds to the time when I was unplugging the SCSI cable of the AA board. I was trying to look at the raw voltage coming out from the AA board and so for the reason I was attaching a SCSI break out board. I didn't see any raw offset at the time, put the cable back to place and realized that the offset had been magically gone. This is strange because yesterday it came back after I unplugged and plugged the SCSI. This doesn't mean the issue is solved, but anyway we will keep our eyes on this particular ADC channel and AA board.

After the offset was gone, I checked the signal chain from the inputs of the QPD interface box through the whitening filter and ADC to the digitized numbers by injecting analog excitation signal (0.01 Vpp at 3 Hz with DS345). This was a part of the QPD check. The digitized value agreed with an expected count of +/-550 counts with the whitening off. So I conclude that the ADC and AA board are healthy at this moment.

Note :
 one of the QPD interface box [3] in the ISC R4 rack doesn't have a frequency response. It is documented in [3]. The rest of two guys do have a frequency response as designed.

[1] LHO alog 5371 "offset at ASC_ADC_1_7 seem from the AA board"
[2] LHO alog 5373 "Maintenance summary"
[3] S1102815-v3 "ISC Dual QPD Transimpedance Amplifier Chassis"