If using a scope with isolated inputs, can I attach the ground clip anywhere in a circuit (below max V) ?

Question

As long as I am below the scope's voltage ratings, if it has isolated inputs does that mean that I can attach the ground clip anywhere in a circuit, in a manner similar to how a volt meter would be attached?

Answer 1

You can get truly isolated scope amplifiers, but they are rare and have limited bandwidth. They are sometimes called "true differential" amplifiers. To see if you have one, measure the resistance between the two ground clips using an analog ohmmeter. It should read infinity.

http://www.tek.com/Measurement/cgi-bin/f...

Answer 2

Not if you want meaningful readings.

If your reference point is not at ground (or common) then your readings are going to be off.

Answer 3

IF, by "isolated" you mean a differential probe pair, then you are correct that you can place the negative lead of the pair to any point in the circuit, so long as the voltage at that point relative to earth ground is less than the common mode voltage of the differential amplifier or differential setup on the 'scope.  The ground leads of the differential pair of 'scope probes must either float, or be tied to earth ground.

Other that that, there is really no such thing as an isolated single-ended 'scope probe or amplifer for such.  The ground lead must be tied to earth.  If your ground lead is not connected, the amplifier is still tied to back earth ground inside the 'scope.

I've seen a few charred-black burned-out 'scope probes and blown amplifiers in my day when someone thought they could do a 'differential' measurement with a normal 'scope probe.

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