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

Question

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

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) :

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?

Answer 1

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.

Answer 2

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