I recently made a low side current sense circuit to test a power supply. It works well, but I am trying to understand the strange behavior that happens when I apply the negative lead of my probe to different places on ground. When I place the positive and negative leads directly across the .1 Ohm sense resistor, I get a very precise voltage measurement that I can then use to calculate the current. But if I move the negative lead further away from the negative side of the resistor terminal, yet still connected to ground, I start to get a voltage that does not accurately correspond to the current flowing through the resistor. Can someone explain what is going on? I have been reading some Microchip articles on low-side sensing and they briefly mention that ground references can become skewed, and they use the term "ground loops" which I am not sure I completely understand.
This is a crude drawing to illustrate the problem. As you can see, when I leave the positive lead on the positive side of the resistor, but move the negative one from directly touching the negative side of the resistor to a different spot on ground, represented by the blue pen, I get discrepant results.
Thanks for your input.