DC current consumption with sense resistor, different measure on oscilloscope and multimeter

by rem   Last Updated April 12, 2018 14:25 PM

I'm doing low side current measurement over a 0.1ohm resistance. Load is a DC motor powered with 12V.

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Cable length between resitor and power supply ground ~60cm

I get a voltage of :

  • 75mV => 750mA using power supply current info
  • 75 mV => 750mA using multimeter measuring resistance voltage drop
  • 100 mV => 1A using multimeter measuring voltage drop between resistance and ground on power supply (60cm of cable between resistor terminal and power supply GND)
  • 100 mV => 1A using DSO-X 2014A oscilloscope and doing single ended measurement
  • 75 mV => 750mA using DSO-X 2014A oscilloscope and doing differential measurement (using 2 probes and A-B math function)

Here is a scope capture with single ended measurement (average down with lower horizontal sensitivity) :

enter image description here

How can we explain this difference? I think it is because of resistance terminal measure point that is 60cm from ground. But how can we precisely explain and quantify it?

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