Look at the effects of the ADC voltage range on the ADC noise floor.
ADC input was terminated with 50 ohms. We then looked at the channel with DTT. This was at +/- 10 V range. We used C1:SUS-PRM_SDSEN_IN1 as the test channel.
The map.c file (in /opt/rtcds/caltech/c1/core/advLigoRTS/src/fe/ ) then had two lines added at line 766.
//JCB temporary 2.5V test, remove me
adcPtr[devNum]->BCR &= 0x84240;
This hard coded the 2.5 V range (we default to the 10 V range at the moment).
We then rebuilt the c1x02 model and reran the test.
Finally, we reverted the code change to map.c and rebuilt c1x02.
I've attached the DTT output of the two tests.
It appears the ADC is limited by 1.6 uV/rtHz. Hence the increase in noise in counts by a factor of 4 when we drop to +/- 2.5 V from +/- 10 V.