Measuring current using a shunt resistor and a DAQ

[JarkonNIAlias]

Hey,

I have been doing a current measurement system, which uses one or more DAQs to measure current using shunt resistors.

I wanted to calculate (simplified) the resolution of the system and ran into the following sentence.

Current measurement guide says: "you should use the smallest value resistor possible because this creates the smallest interference with the existing circuit. However, smaller resistances create smaller voltage drops, so you must make a compromise between resolution and circuit interference."

Question is how does resistor size (value) affect resolution?

Because if you have a DAQ e.g. NI 9238 that has 24 bit ADC and +-500 mV range, isn't the resolution always the same: 1/2^24 = ~60nV, as long as the voltage drop doesn't go below 60 nV?

[Buechsenschuetz · Accepted solution]

I think there is some confusion concerning the point of view. 

Of course the resolution capability of the DAQ will always be the same. 

But you get full resolution of the value to be measured only if you drive the DAQ input with full scale (500mV in this case). Then you get a resolution of 60nV per step and 24**2 steps. 

If you drive the DAQ input with 100mV only, of course the DAQ resolution will stay the same (60nV). BUT the resolution of the value to be measured drops to 1/5 since you only use 1/5 of full range. 

So you always have to compromise between using the voltage drop for driving the full input range and the allowable voltage drop. If you can live with a voltage drop of 500mV in your setup, use a shunt resistor for 500mV voltage drop at full current, and use full resolution of 24**2 steps. If this is too much, the resolution of the measured value will drop proportionally. 

Of course you can use a pre-amplifier to "multiply" the voltage drop across the shunt resistor. Using a preamp circuit is easy when one side of the shunt is connected to GND of the system. If the shunt is on the high side, or if the circuit to be measured is "floating", i.e.without reference to GND of the DAQ, you will need a preamp for a high common-mode voltage. 

[JarkonNIAlias]

Thanks!

That's what I thought was happening, but couldn't find much on this subject.

"BUT the resolution of the value to be measured drops to 1/5 since you only use 1/5 of full range." +1 for this.