Noise issues with CS328A

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16 Jul 2007

While performing the first measurement with my new CS328A, i have experienced severe noise issues. Circuit under test is floating (i.e. powered by isolating transformer). Scope graph shows some high frequency noise and huge amounts of mains noise (at 50 Hz). Doing same measurement with my old analog scope does not show any noise at all, either keeping scope chasis grounded or not. Therefore, it seems that there is a ground loop problem between CS328A ground and USB ground. When connecting together probes from CS328 and my analog scope, both show the same noise, so it looks like probe from CS328 is actually the source of noise. Even shorting tip and ground clip of CS328 probe, scope graph still shows a significant amount of noise. What can i do? Best Regards.

24 Jul 2007
Posts: 395

Hello Tiger,
please can you send an example .apc file (do a Save as..) of what you are capturing to support@cleverscope. We use the scope all the time looking at sig gens, and other cleverscopes, and do not experience noise issues. We have in fact just published a document on the digitizer performance in our resources page on the website. So, given that we do not expect it, here are some pointers:
1. There is no source of 50 Hz in the Cleverscope or it's power supply. This suggests that the earth is in fact floating, and you are seeing the mains frequency coupled through your isolating transformer. Please verify that the ground clip on the scope probe is conductive from the ground clip through to the BNC shield (use a multimeter), and that the connection between the probes cable socket and the probe body is nice and snug (the probe includes a disconnect facility if you trip over it - the probe body will seperate from the probe cable socket).
2. It would be very useful to see the noise you are talking about, so please send us an .apc file. That will also show us your settings, which may be helpful.

If the performance is not the same as in the digitizer performance document, we would like to find the reason.

Thanks. Sorry about the delay.

27 Jul 2007

Hello BartSchroder,

Thank you for your response. I was investigating the problem a little further. First, the problem is not exactily as described in first post, since i plugged inadvertently the analog scope into a socket without ground connection. When plugged into a socket with ground connection, my analog scope captures exactily the same noise than cleverscope, therefore, cleverscope has no problem at all :). So, the first conclusion is that isolating also the scope resolves the noise issue. However it is not a good solution. It seems to be a magnetic coupling between transformer and ground loop consisting of ground wires between sockets, scope and PC. Shielding transformer and connecting shield to ground removes most of the noise at mains frequency :rolleyes:. However, HF noise remains and the only way to remove it, is full isolating from ground all the system (circuit, scope and PC). This HF noise is maybe induced in ground loop from normal electrogmentic noise wherever you are. The level of this HF noise is about 20 or 30 mV p-p. I will save some captures of the scope to show you the captured noise.

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