Vertically zoomed xy plot not quite right.

[enelson]

Hi,

  while measuring the I-V characteristic of a two-terminal current source with my Analog Discovery 2, I noticed the vertical position of the trace in the x-y plot is not quite correct.  In the following screen capture, CH1 (yellow) is the voltage across the source plus a current measurement (shunt) resistor, and CH2 (blue) is the voltage across the shunt resistor.  The x-y plot on the right looks fine on this vertical scale.  For example, the CH2 peak is 300 mV in both plots.

When I zoom CH2's vertical scale to more closely examine the difference between the rising and falling halves of the triangle wave, the trace in the x-y plot appears to be too low relative to the x-y plot's graticule.  For example, CH2 peaks around 299 mV in the normal y-t trace on the left, but in the x-y plot on the right that peak is low by about 0.3 mV.

Exported data and the cursor suggest the zoomed y-t plot is good, so I'm thinking the calculation for where points go on a vertically zoomed x-y plot is not quite right.  Is this something you can look into?

Note this concern is not about the two vertical graticules on the left and right being slightly misaligned.  While it would be nice if the two graticules were aligned, my concern is that reading CH2 values off the x-y plot on the right gives me some (slightly) wrong numbers.

Thanks!  I'm still having a blast with the AD2.  Have a great day.

 

Edited July 13, 2021 by enelson
Added appropriate tags.

[attila]

Hi @enelson

I see the problem now in XY at high offset/range ratios.
It is fixed for the next version.
Thank you for the observation.

Edited July 13, 2021 by attila

[attila]

Hi @enelson

Could it be a ground loop issue ?
Try powering the AD2 from 5VDC supply to minimize such.

[enelson]

Hi @attila,

Correct me if I'm wrong, but the x-y plot on the right should present the same measurement(s) shown in (or by) the two y-t traces on the left.  So even if there was a ground loop issue, the ground loop should affect both plots equally, meaning for example the CH2 max on the left should match the y (CH2) max on the right.  So I think this is a subtle issue with the waveforms software, how it is displaying the x-y plot.

Having said that, I do have a 5VDC power supply on order to use with my AD2.  Perhaps you are psychic! :-)

the WaveForms software I'm using is version 3.16.3 64-bit for Windows 10, in case that helps.

Regarding ground loops, this two-terminal current source circuit is only attached to the AD2.  W1 and 1+ connect to the positive terminal of the current source, 2+ connects to the positive end of the shunt resistor = negative terminal of the current source, and 1-, 2- and GND connect to the negative end of the shunt resistor.  There are no other connections to any other power or any other device.  So that means no ground loop, yes?  Let me know if I've misunderstood.

Perhaps I can reproduce this behavior using 1+ to measure W1 and 2+ to measure W2.  No circuit involved.

Thanks for engaging me on this topic!  And have a great day.

 

Edited July 13, 2021 by enelson
Clarified response.

[attila]

Hi @enelson

 

 

[enelson]

Thanks @attila!