Summary: It seems like noise from the coil driver and DACs cannot explain the excess noise seen on the BBSS QOSEMs. The old 18Bit DACs would be limiting in R,P,Y DOFs below 10Hz, The new 32Bit DACs and coil driver are subdominant to ISI noise.
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.
Attached in Figure 1, is a noise budget for the BBSS M1, showing contributions from ISI motion as measured by the stage 2 GS13's (more details in LHO alog 91744), coil driver output noise, and DAC output noise (both 18 and 32 bit variants). Here we see that the DAC and Coil driver noise should not be limiting the QOSEM noisefloor, particularly above 10Hz. The 18bit DAC may be dominant below 10Hz in the R, P, Y DOFs. It's my understanding that LHO is using the 32Bit DACs, while LLO is using 18Bit DACs for the BBSS M1.
The conclusions here is more evidence that it is something in the QOSEM itself limiting the noisefloor of the BBSS M1. The leading theory is that its due to excess intensity noise from the QOSEM LED. We are currently investigating solutions, like upping the gain on the sat amps LED intensity feedback loop. More details can be found in the follow ticket: 39047
I added a comment to the FRS analyzing the Transimpedance OpAmp circuit, go have a look. Good news there is hope!
TLDR The OpAmp used for the TIA cirucit is not optimal for that application. The rest of the use of that OpAmp should be fine, but for current sensiitve measurements, the TIA should be low input bias current, i.e. JFET.
In this application, the chopper is better on paper due to the elimination of 1/f noise at low frequencies, and choppers arent as bad as they used to be, so I stand corrected. If only they were synchronized!