I'm poking at the inside of a USB oscilloscope, with the hope of modifying it so that the scope probe commons can be referenced to different voltages.
By my figuring, this 10x probe is a 9 MOhm resistor, in series with the scope's 1 MOhm internal resistance. Thus, a 10:1 divider.
By adding 10 MOhm to the probe, I should turn it into a 20:1 divider. When I insert 10 MOhm into the positive signal path I do get a rough 20:1 effect, but I start seeing AC 60 Hz noise.
When I instead put the resistance in the common path, between the signal common and the scope probe common, I see vastly more AC noise.
My USB scope is isolated from my PC with a commercial USB isolator. Can anyone tell me what's going wrong, and how to fix it? I thought this might just be a consequence of having all this strewn about my desk instead of pleasantly soldered together with minimum signal paths. But I would have expected the noise to be much lesser magnitude, and much more responsive to physical rearrangement. Moving leads and parts has zero observable effect. And it still wouldn't explain the difference between inserting the resistance in the positive and negative paths.
Why is this happening, and how can I fix it?