Scope channels have offsets of -5 mV when connected to W1, W2

[ganesh]

Hello!

Due to the Covid-19 crisis and inaccessibility to labs, I have been given an Analog Discovery kit (legacy version) by my university to use as a waveform generator and oscilloscope. While using the scope functionality, I find that when I connect the W1 and W2 terminals to scope channels I get a  -6.3 mV offset (average) in CH1 and a -4.5 mV offset (average) in CH2 even when no waveforms has been generated by the Wavegen tab. When the scope channels are left floating or shorted, the offset voltage reduces to around +500 uV . I have attached screenshots of my Scope window for all three situations.

I do not have a multimeter to calibrate the kit and I have also tried the solution mentioned in this forum post (https://forum.digilentinc.com/topic/15432-auto-calibrate-for-analog-discovery-2/) but it didn't seem to make a difference. Any help would be greatly appreciated.

Thanks in advance!

[attila]

Hi @ganesh

As simple solution:
You can use the "Zero offset" option to reduce/eliminate the offset error in Scope.

If you want high precision let the device to heat up.
See the minor temperature drift in the following post:

 

To manually calibrate the Scope offset without DMM:
- have inputs to ground (or leave floating) 
- note the measured average value
- open the calibration window, parameters tab and subtract the measured from additive value
  Scope HG: when range <= 500mV/div, LG > 500mV/div
Like here I measured -1.5mV and had the value of -0.0635. To calibrate it I had to change it to -0.062
Note: the Scope resolution at HG is ~0.3mV, at LG is ~3mV

 

To manually calibrate the Wavegen offset without DMM:
- connect Wavegen to Scope positive input, negative to ground
- note the measured average value
- open the calibration window, parameters tab and add the measured to additive value
  Wavege LG: when amplitude <= 1V, HG > 1V
Like here I measured +2.2mV and had the value of -0.018. To calibrate it I had to change it to -0.0158
Note: the Wavegen offset resolution is about 0.7mV