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Message ID: 15184     Entry time: Mon Feb 3 15:22:39 2020
Author: Jon 
Type: Update 
Category: PSL 
Subject: c1psl progress/Acromag ADC grounding 

I tested the c1psl AO channels on the electronics bench on Friday. While I found all the wiring to be correct, some of the channels exhibited excess noise with all appearances of a grounding problem.

Today Jordan, Gautam, and I investigated this further. It is indeed a grounding problem, but actually with the Acromag ADCs. The Acromag DAC outputs are single-ended (return is grounded), so (for the purpose of a loopback test) I would expect to leave the ADC inputs ungrounded. This is the configuration I tested Friday. Today we also tested driving the ADC with a floating source. The ADC noise behavior is exactly the same, whether the source end is grounded or not.

However, grounding the minus pin of the ADC channel eliminates the noise. We don't understand why this seems to be required irrespective of the driving source, so there something we're missing about the ADC design. As it turns out, this same fix was made to the AI channels of the previously-upgraded Acromag machines. I know Chub and I had to do this for the AI channels of c1vac, but at the time we thought the source grounding was causing the issue. However, today Jordan and I looked inside c1iscaux, which Chub wired, and confirmed that its AI channels are wired in the same way.

So in any case, Jordan is grounding the c1psl AI channels in the same way as c1iscaux. Once this is done, we'll continue with the bench testing tomorrow.

gautam: here are my notes about this issue when i was doing the c1iscaux testing. As I note there, "previously-upgraded Acromag machines" in the plural may be a bit of a stretch - I have no idea what the grounding situation is in c1susaux / c1auxex for example.

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