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Suppose I need to view voltage between two arbitrary points of a circuit with an oscilloscope. For that I can use two probes and A-B math, but should I connect the ground lead to the circuit? How to maximize CMRR?

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  • \$\begingroup\$ Yes, connect the ground lead to get best results providing it's safe to do so. \$\endgroup\$ – Andy aka Apr 27 '16 at 16:39
  • \$\begingroup\$ Several years ago one man say me just tie ground leads of two probes and don't connect it to the circuit under test. But he couldn't explain me why. Maybe it's wrong. \$\endgroup\$ – user54579 Apr 27 '16 at 16:45
  • \$\begingroup\$ @Andyaka At least one ground probe must be connected to the circuit for this to work, no? \$\endgroup\$ – KnightsValour Apr 27 '16 at 16:47
  • \$\begingroup\$ @KnightsValour for "what to work"? \$\endgroup\$ – Andy aka Apr 27 '16 at 16:48
  • \$\begingroup\$ @Andyaka For the oscilloscope measurement. Why would it be unsafe to connect a ground lead? \$\endgroup\$ – KnightsValour Apr 27 '16 at 16:51
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Yes, you need to still connect it to ground of the device under test because you are doing two single-ended measurements, not a true differential measurement.

A true differential probe has no ground and uses the ground of the test equipment.

Without a ground reference, there is no guarantee that ground reference of the two single-ended probes will match. So then you might say, OK, I can connect the two probes grounds together. But then you might hit the max voltage limit of your single-ended measurement. For example, what if the scope probes ground reference floats up to 8V from external charges and your scope clips at 10 V (at 1V per division setting), now you can only measure up to 2V before the signal clips.

So basically, because you are actually doing two single-ended measurements, you still need to ground your device to your scope or else you'll pick up all sorts of noise and weirdness (AC line, etc).

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  • \$\begingroup\$ As you say the common mode voltage range is one thing and then the non-linear common mode gain characteristics that will cause unpredictable (usually minor on a modern scope but high in an old valve scope I had) distortion if the commons are left floating. If the inputs are truly isolated from each other including the grounds then the grounds HAVE to be connected or you are measuring static charges and bias currents etc that may be much larger than your differential voltage. \$\endgroup\$ – KalleMP Apr 27 '16 at 18:22

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