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AD2 scope voltage inaccuracy


Could someone please tell me whether it is normal for a 4.5Vpp signal to show on the AD2 scope (with daughter board and real probe) as a 4.0Vpp signal when at 2V/div? If not, any advice for troubleshooting?

Additional information:

The ground clip is grounded properly. It doesn't change the signal when I connect the ground wire to my circuit ground as well.

20Vpp shows as 17.5Vpp at 5V/div.

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Thanks, jpeyron.

Update: I tried calibration, but it did not help. Despite having set my 10x probes to '10x' in each channel in the software, during calibration there was an error message indicating that the measured voltages were about half of what they actually were, causing me to wonder whether the '10x' setting did not register properly with the calibration routine.

With 10x probes (and 10x Ch settings), both probes on both scope channels have the same problem as stated in my original post: about 10% too-small peak-to-peak voltage relative to my highest-quality DMMs and my fancy new calibrated bench scope (MSOX4054A).

The attached scope shots have descriptive file names, where 're-calib' means "re-calibrated with 1x probes (and 1x Ch settings)".

So far, my conclusions are:

-- Peak-to-peak voltages (at least in the 4V to 20V range) are 8-14% inaccurate with 10x probes.

-- Peak-to-peak voltages (at least in the 4V to 20V range) are 2-5% inaccurate with 1x probes.

-- Calibration must be performed with 1x probes and 1x Ch settings to avoid an error message.

-- Calibration, whether with 1x or 10x probes, does not help with peak-to-peak voltage inaccuracy.


10Vi SW w 1x probes - w re-calib.png

10Vi SW w 10x probes - w re-calib.png

10Vi SW w 10x probes - MSOX4054A.png

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I suspect you are observing the so-called "Gibbs Phenomenon".
If I throw in more bandwidth to represent a signal, the overshoot actually gets worse. But, the duration of the peaks shortens at the same time, so the overall approximation still becomes more accurate in terms of energy / power.
Nature is playing tricks at us and this guy


is laughing from his grave.

In other words, the result depends strongly on the characteristics of the instrument, not the signal itself.

Your 10 % PWM signal is nasty because its spectrum is so wide.
Here is how it looks in calculation: I've drawn an arbitrary line at -20 dB, which isn't particularly accurate but rougly corresponds to the ~10 % voltage error you're observing.
Only the 19th harmonic drops below that line.


Here is the same again, with the pulse width at 50 % - a regular square wave.The spectrum is much friendlier, with most of the energy in the fundamental:



To sum this up, the "peaks" of your test signal are not well defined. Try using the calibration output from the KS scope as a test signal, it should have adequate filtering (at least suitable for the KS instrument).
A sine wave avoids the problem altogether.


Calculation, if anybody is interested (Octave / Matlab):
close all; sig = zeros(1, 1000); sig(1:500/10)=1; sig = repmat(sig, 1, 1234); figure(); q=20*log10(eps+abs(fft(sig))); h=plot((0:numel(q)-1)/1234, q-(q(1+1*1234))); xlabel('harmonics #'); ylabel('dB'); grid on

Edited by xc6lx45
stray bracket filtered out...

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attila, calibrating the 10x scope probes using their screw adjustments fixed the issue. Now the peak-to-peak voltage is correct.

xc6Ix45, thank you for the science behind the issue.

You both made my day. Thank you!

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