I have a simple setup described here :

enter image description here

Laptop is battery powered. It communicates with DUT over USB.

DUT is powered from switch power suppy. DUT earth / GND resistor = 1M

Differential probe resistor between measurement GND and output (connected to scope) is 4M. Differential probe is set to 100:1. It measures a +-100 voltage on A point.

When running : DUT periodically sinks some current during very short duration < 10ms. It's normal behavior, in this case I can get +-100 voltage on A point. Otherwise I will have a 0 voltage on A point.

I have some issue with this measurement :

  • First the less important : I can see mains voltage on differential probe (when I should have a 0 voltage measurement)
  • Most important : USB communication only works for a short time. After some seconds, I lost USB communication with DUT (I have not yet determined if DUT resets)

Of course, when differential probe is not connected to DUT, I do not have any issues.


1 Answer 1

  • poor CMRR on Diff Probes
  • connect 10nF caps from Laptop and DUT to Earth Gnd

If using 2 probes to make Diff probe inductance , the probe inductance is mismatched x% and thus poor CMRR=1/x%

  • 1) Connect scope probe ground clips (earth gnded) to 10nF cap to DUT Gnd
  • 2) twist both probes around each other for better mutual couple A-B cancellation
  • \$\begingroup\$ Thank you very much for your help. You gave me some solutions. I will test it. Could you give me some precise explanations about these issues? I would like to understand root causes of theses two problems. \$\endgroup\$
    – rem
    Commented Jan 24, 2019 at 19:19
  • \$\begingroup\$ Look up probe + common mode (CM) noise rejection and CMRR, SMPS leakage current, radiated noise, probe gnd inductance \$\endgroup\$ Commented Jan 24, 2019 at 19:50

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