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

enter image description here

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

Measures done :

  • in single ended measurement (with oscilloscope ground connected to power supply)
  • with multimeter when it measures on COM connected to power supply ground

shows influence of cable length. Cable is classic cable of 3mm diameter, 60cm length?

How can we precisely explain this difference?

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  • \$\begingroup\$ You are trying to use a DC measuring multimeter to measure an AC signal... \$\endgroup\$ – PlasmaHH Apr 12 '18 at 13:52
  • \$\begingroup\$ I want to measure DC component of an AC signal. \$\endgroup\$ – rem Apr 12 '18 at 13:58
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    \$\begingroup\$ The average multimeter doesn't just magically ignore the AC part \$\endgroup\$ – PlasmaHH Apr 12 '18 at 13:59
  • \$\begingroup\$ oups... I updated my edit, it was a copy paste mistake. Sorry. From what I see, cause is wire length, but I want to know how this 250mV difference can be estimated! \$\endgroup\$ – rem Apr 12 '18 at 14:13
  • \$\begingroup\$ What is it about these scenarios that surprise you? \$\endgroup\$ – Andy aka Apr 12 '18 at 14:25
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There's only one explanation for the 25 mV difference between the resistor terminal and ground - there must be resistance!

I just measured my Pomona micrograbber (banana to grabber) with my U1282A DMM and got .06 ohms. So it's definitely not 0. You could probably assume the connections are also contributing.

If you want to be certain that this is the cause, try another value shunt resistor and see if you still get the same magnitude offset.

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You have discovered that the path from ground to the resistor has itself some resistance, and will add to the measured voltage

For this reason there is something called kelvin sense, that is, measure directly at the part with known resistance, with traces that carry no significant current. https://www.allaboutcircuits.com/textbook/direct-current/chpt-8/kelvin-resistance-measurement/

You can calculate the additional resistance, the current is 750mA and there are 25mV in the ground to resistor section, so cable-connector-soldering resistance is around 0.03 ohms from power supply ground to resistor.

Additionally your load is not constant like a resistor, a motor will slightly or largely change its consumption due to commutations and torque changes, in the Oscilloscope it is seen a ~300Hz component, you want to measure the RMS value which is like the equivalent DC value of a waveform (power-wise) maybe your multimeter or scope can measure in RMS if not your multimeter will add error to the measurement on a not DC signal and probably apply some kind of filtering were you will end getting somewhat the average of the signal

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  • \$\begingroup\$ I'm not sure about reactance of 0.03ohm caused by my cable , it is this one cdn-shop.adafruit.com/product-files/2611/… but in all case cause is cable length. \$\endgroup\$ – rem Apr 12 '18 at 16:41
  • \$\begingroup\$ Well you have 100 mV when you measure from point A to point C and 75 mV between B and C that means that between A and B there is 25 mV it can be the cable, but more likely the connection point has rust, small contact area, etc... you can measure between A and B to be sure \$\endgroup\$ – LuisF Apr 12 '18 at 18:34

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