Vern, Hugo,

During the chamber side testing HAM-ISIs are locked. Part of the testing consists in measuring the DC voltages outputted by the CPSs to make sure that the probes and the electronics are still functional. This measurement is taken at the BNC output of the ADE boxes. 

We recently observed that the voltages we measured depended on the number of probes that were connected to the ADE Boxes. We also noticed that the voltages measured with only one probe connected to the ADE boxes were high in comparison to what we expect on a locked ISI.

Vern and I investigated it this afternoon: 

• The CPSs have a driven shield. This driven shield is exposed on the BNC connector of the in-air CPS cable (feedthrough side). Accidentally grounding this shield to the test stand would significantly affect measurements. To avoid such situation, we adjusted the position of the feedthrough to make sure that this driven-shield could not get in contact with the test stand. 
• We bolted the ADE boxes together to make sure that they share the same ground.
• We grounded the ADE boxes to the test stand.

The voltage measured at the BNC output of the ADE boxes would then match our expectations (0V +/- 4V), without varying with the number of probes that were connected.

We tried disconnecting the grounding cable between the test stand and the ADE boxes. The DC voltage read on the ADE boxes increased and became dependent on the number of probes that were connected, again. This latest observation confirms that the issues we experienced on the CPSs were related to the grounding of the ADE boxes.

conclusion:

• ADE boxes need to share the same ground. 
• ADE boxes must be grounded to the test stand of the ISI. 
• Not having the ADE boxes grounded to the test stand cause the CPS readouts to vary with the number of probes that are connected to the ADE boxes.

Voltages and noise spectra were re-measured for each CPS and HAM-ISI Unit #4 (HAM2) chamber side testing report is being updated.

Pictures are attached