Miriam, Evan G, Evan H

*SUMMARY*

We investigate the possibility that there occurs a saturation or a slew rate problem somewhere in the analog electronics of the sensing chain causing (or resulting in) a blip glitch. This investigation finds no issue with either of these for a sample of the four loudest blip glitches in O1 data.

*DETAILS*

We investigate four different times: 1128085613.28, 1128221842.47, 1128264648.20, 1130156793.54

1. Get the data from OMC-DCPD_A_OUT_DQ and OMC-DCPD_B_OUT_DQ. These data have undergone some digital filtering to undo what the analog filtering did before the ADC. It is our best estimate of the photocurrent from the OMC DCPD. The units are in mA.

2. Convert the data into Volts (V=IR), where R=400 Ohms (see DCC:D060572) 

3. Following the schematic of the in-vacuum OMC DCPD, there are two zpk filters, each with one zero at 8Hz and one pole at 80Hz (we ignore the 15.9kHz pole, as it is far out of the frequency band of interest), and there is a gain of 2 at the differential output. We apply this filtering to the DCPD data (in V). The resulting time-series has to be within +/-15 V to not saturate. In the selected times of blip glitches, we obtain voltages that are well within this bound (see attached document, "preamp" plots) 

4. Continuing with the signal chain, the whitening chassis (DCC:D1001530). The whitening gain setting has a nominal value of 0 dB. In addition, only a single whitening stage is applied with one zero at 1Hz and one pole at 10Hz. The resulting time-series has to be within +/-15 V to not saturate the whitening op-amp, and within +/-20 V to not saturate the ADC. In the selected times of blip glitches, we obtain voltages that are well within these bounds (see attached document, "whitch" plots).

5. Typical slew rates for the op-amps used in these circuits are 2.5 or 5 V/us. None of the times selected show such high slew rates.

6. Note that the plots in the attachment are dominated by the low frequency content of the signal, that has an amplitude much greater than blip glitches. Only after whitening the time-series, the blip glitches become dominant.

*CONCLUSION*

The analog electronics in the sensing chain do not appear to be responsible for blip glitches.