Scope calibration inaccuracies

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13 Sep 2007

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?

23 Oct 2007
Posts: 481

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

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.
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