Opening up the AA chassis (S/N S1102788) we confirmed that the AA boards D070081 (chn 16-23 S/N S1202203; chn 24-31 S/N S1202202) have AD8622 stuffed. On channels 20-27 all of them (input and output) were replaced by ADA4075-2ARZ. This incudes the channels for the LSC_REFL_SERVO readbacks, the IMC-REFL_SERVO readbacks and the SQZ-CLF_REFL_RF6 readbacks. Attached are some measured transfer functions for a channel with the ADA4075-2 (reference trace with notch) and for a channel with the AD8622 (no notch). Varying the drive voltage from 1mV to 5V made no difference for the ADA4075-2, For the AD8622, one can clearly see that it cannot drive fast enough and at frequencies around the notch it looks anything but a notch.
Plots: AD8622 transfer function for 5V, 1V, 100mV, 10mV and 1mV drive compared against the ADA4075-2.
Repeating the measurements done in alog 47345, we find significant improvements. We now have good coherence between CTRL and SLOW for most of the frequency range. Due to the pole at 500 Hz the SLOW readback runs into quantization noise above 15 kHz. The ERR signal readback also has much better coherence. Between 10 and 100 Hz it seems limited by the voltage noise of the first boost stage.
The line at ~14 kHz dominates the rms.
And here is the low frequency version.
Here are the plots for the IMC servo. The faster OpAmps also improved the IMC-F and IMC-L readbacks. The IMC_I readback shows flat noise at a relatively high voltage level. It may be limited by out-of-band noise.
First plot show current readbacks compared to the old ones.
Second plot shows the raw input signals in counts. There seems to be some excess noise between 10 and 50 Hz in IMC-F. Maybe a glitch here since the next plot looks different.
Third plot shows a full bandwidth spectrum. MC-F approaches the quantization noise between 1 and 20 Hz.