Oscilloscope Noise Issue with Different Resistor Values

[SteveLTN]

Hello everyone,

I'm encountering an issue while using an oscilloscope. When I connect the probe tip and ground across a resistor, everything seems fine. However, when I place multimeter leads across the same resistor simultaneously, my oscilloscope begins to pick up a 100 Hz noise, which I suspect is related to mains interference.

Interestingly, I've noticed that the amplitude of this noise increases with the value of the resistor. I initially thought that according to Faraday's Law of electromagnetic induction, the noise level should remain constant since the loop area created by the oscilloscope's probe and ground lead remains unchanged.

Could someone help clarify why the noise amplitude increases with resistor value? Is there another principle at play here that I might be overlooking? Thanks in advance!

[MarkT]

Its capacitive coupling to the long leads of the multimeter from the mains wiring in the room and probably from you.

Effectively the noise is constant-current as the source voltage is high and the capacitive reactance large (perhaps a few pF or so).

Given constant current pushing into the resistor, the larger its value, the larger the voltage.