change in sampled value if signal is vertically shifted?

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20 Aug 2008
Posts: 24


after using my CS328A for a while - and I am pleased everytime I can use it :-) - I faced the following issue (and could not find anything comparable in the postings):

Sampling an ADC reference signal of ~1.2V I wanted to see small AC components on it (some 1-5mV from the ADC start / stop). The problem is that if I change the vertical shift of the signal in the Scope Graph the sampled and displayed value changes. Span was initally set to 100mV, I was using x10 standard probes (but phenomenon is there for x1, too), aquisition at 12bit, averaging is OFF.

For example a 50mV vertical shift (using the mouse in the Scope Graph or the buttons) leads to a change of 20mVdc, same for channel A and B, or if using the buttons to change the span. If I press the FIT button several times I can see a some how funny signal hopping... Math Graph and Signal Information both follow the Scope Graph, so ""wrong stays wrong"".

It seems to be related to the displayed resolution because for the x1 probe it starts to ""hopp"" (by 1mV) if the span is set to very small values (e.g. 1.1930V to 1.2130V).

All this ONLY for a ""live"" aquisition (e.g. AUTO), no changes on a stored signal, so maybe a question of the aquisition unit of the CS328A? Or did I miss something regarding the usable measurement ranges or resolution?

I already performed a self-calibration but without any effect.

It would be very nice if anybody has a suggestion on it. Thanks in advance!

Kind Regards
Gotthard Sauer-Sperling
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