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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.

enter image description here

Thanks for your input.

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  • \$\begingroup\$ Which frequencies do you expect on your board? How big is Rsense? How much resistance do you add, when moving the ground probe away? \$\endgroup\$ – Christian Jun 30 '20 at 18:27
  • \$\begingroup\$ You are only supposed to measure across Rsense and not anywhere else. \$\endgroup\$ – user105652 Jun 30 '20 at 18:39
  • \$\begingroup\$ @jm567 Obviously in the sketch there is no difference. So more information is needed. Perhaps you would take a photo showing both probes in the first case and a second photo showing the second case. That might be a good start. \$\endgroup\$ – scorpdaddy Jun 30 '20 at 18:40
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    \$\begingroup\$ This makes perfect sense. To layout this board properly, lay down +sense and -sense as dedicated tracks. Using GND instead of a -sense track means that all currents flowing on GND inbetween the resistor and your sense input will alter that signal. GND is not 0Ω - all traces have a small resistance - so (big) currents will always skew (tiny) analog signals, even on ground tracks. \$\endgroup\$ – rdtsc Jun 30 '20 at 18:56
  • \$\begingroup\$ Does the measured voltage increase or decrease when you measure farther from the resistor? \$\endgroup\$ – Cristobol Polychronopolis Jun 30 '20 at 19:05
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Your wires or PCB is not made of superconductor.

Real world conductors have resistance.

So when the currents are flowing in conductor with some resistance, there will be voltage difference.

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