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I want learn how to view the CM and DM components of a signal on a usual two channel scope. The reason for this to be able to verify whether a noise superimposed on a signal is CM orDM.

Regarding my below drawing, I thought if I can measure Va and Vb with two seperate channels I might use the MATH function to see the CM and DM components. Imagine Test Device in the figure is a power supply with its plus and negative terminals shown and you observed noise/interference at some point by a scope.

And now you want to check whether what you have seen as noise is due to CM or DM interference.

Would the following setup work:

enter image description here In this case about the figure above I’m stuck at two points:

Where should be the probe ground leads be connected?

What would plot the CM and DM if MATH function is used? Would Ch1+Ch2 plot the common mode part of the signal?

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    \$\begingroup\$ (A + X) - (B + X) = A - B = differential \$\endgroup\$ – Andy aka Dec 11 '18 at 17:57
  • \$\begingroup\$ How about the CM? And where should be the scope groundleeds be grouded? \$\endgroup\$ – atmnt Dec 11 '18 at 17:59
  • \$\begingroup\$ DM = differential. You can try it without grounds connected for differential measurements. \$\endgroup\$ – Andy aka Dec 11 '18 at 18:00
  • \$\begingroup\$ Sorry I would write how about CM \$\endgroup\$ – atmnt Dec 11 '18 at 18:01
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    \$\begingroup\$ Use two resistors forming a node between A and B and measure at that node. If A and B are balanced differential signals then that measurement gives common mode signal. \$\endgroup\$ – Andy aka Dec 11 '18 at 18:04

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