Scope calibration inaccuracies

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13 Sep 2007
Posts:

I performed scope (CS328A) calibration and generator calibration and I have some questions.
After calibration, when I connect the generator output to the input, the waveform is not symetrical around 0 VDC (some DC Offset is read) and the difference between channels is about 60 mV for 0.5 Vpk-pk signal.
Because I used the internal refernce, I would expect the absolute accuracy to be within ~2% but the relative accuracy should be much better (for 10-bit acq. ~0.1%).
However, the DC offset error is ~5% and the difference between channels is also ~5%.
Can someone explain it to me or the calibration algorithm/hardware is at fault?
Martin

23 Oct 2007
Posts: 398

Hello Martin,
You are right - there is a fault in the calibration routine. What we found is that if you calibrate the sig gen on it's own (not preceded by a standard cal) then it calibrates correctly. So the sequence if you want a full cal is to do standard cal, close the calibration dialog, open it again, and then do the sig gen cal. The next version will correct this.

26 Oct 2007
Posts:

Bart,
I recalibrated sig gen again and this time the results are much more accurate.
The DC offset readings for each channel differ by about 3 mV (for 1 Vpk-pk signal) which causes V min and V max and other related readings to be different by about the same amount but they are within accuracy I expected.
This problem and USB problem (v6420 works fine for now) were my major issues that seem to be handled for now.
Thanks,
Martin
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