Summary: ADC noise is very close to, if not limiting the QOSEM signals. It doesn't seem to be enough to explain the elevated QOSEM noisefloor seen, but whitening gain should not be lowered any futher than it already is.

This is one in a series of posts investigating the excess noise seen on the BBSS M1 QOSEMs. In short, at both LHO and LLO, the noisefloor of the QOSEMs appears to be around 8pm/rtHz at 100Hz, quickly increasing to 50pm/rtHz by 1Hz; where we expect the noisefloor to be flat at ~5pm/rtHz across this bandwidth. See more details in LHO alog 91466. 

At both LHO and LLO, changes have been made to the QOSEM sat amp to reduce the whitening gain (see LHO:91853 and LLO:82137), in response to long saturation periods when the BBSS is rung up. In the figures below we plot noise budgets of the LHO BBSS, now including the new whitened ADC noise floor. The expected noisefloor of the GS 16 Bit ADC was taken from T1000322. Keep in mind that the true ADC noise floor often varies from channel to channel and unit to unit. Informally, I've heard they can vary by as much as factors or 2-3, especially towards the lower frequencies, however I dont have any solid reference for this.

Figure 1 shows the raw voltage noise of the QOSEM signals (Sum, XDiff, YDiff) after dewhitening, along with expected shot noise level and ADC noise. During this section of data, the BBSS damping loop was off, and the QOSEM LED sum feedback was turned off (see LHO:92011 for details). Here we see that the ADC noise is very close, if not limiting the QOSEM XDiff and YDiff signals, particularly if we consider true ADC could vary higher than shown. Certainly, whitening gain should not be reduced any futher. 

Figure 2 shows an open loop noise budget of the BBSS M1, now with the whitened ADC noise propagated in. Again we see that the ADC noise is very close to, if not limiting, particularly at the high frequencies. 

With this said, I'm not convinced we can blame ADC noise for this excess QOSEM noise we've been seeing. In prior budgets (such as LHO:91871), LHO QOSEMs were running with about 10x more whitening gain and the QOSEM noise floor was not sigificantly different. With that said whitening should certainly not be lowered any further.