Clamp on cable ferrite for scope lead

according to your diagram, the circuit under test has a floating ground and when that is connected to the scope ground anything can happen

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Thanks, but as you know, it is isolated and has a floating ground precisely because we need to attached the scope ground (earth) to any of its nodes.

treez said:

...it is isolated and has a floating ground precisely because we need to attached the scope ground (earth) ...

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The isolation is nominal and notional. There is considerable capacitive and inductive coupling within the transformer. The isolation is designed to prevent shock hazard and it passes a DC test (for isolation).

My personal opinion (I may be wrong as usual) is that it is better to isolate the scope using a transformer because a good scope has a decent ground and that is connected to earth only at one point (chassis).

If you have 1:100 probes, please use them because they will be better to reduce the coupling noise too.

Your statement is correct only in theory.

My personal opinion (I may be wrong as usual) is that it is better to isolate the scope using a transformer because a good scope has a decent ground and that is connected to earth only at one point (chassis).

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Thanks, yes we tried that but it didnt stop the problem