Current measurement circuit , Shunt resistor value Selection

[Akshatha Venkatesh]

We want to measure current in the range 1 to 40 uA , and a 100kohm shunt resistor is being used to convert the current into voltage to be fed into the microcontroller. I'm trying to understand why they have selected R1 to be 100kohm , since the lowest voltage reading will be 0.1 and highest 4V. If a larger value for R1 could have been selected , the full range of the microcontroller could have been utilised, but it is not being done in the above circuit , is there any specific reason for this ?

 

[BobK]

Because it makes the math easy. 1V = 10uA.

Bob

 

[Akshatha Venkatesh]

Because it makes the math easy. 1V = 10uA.

Bob

I don't think that's the reason . Could it be because of some parameter of the op-amp ?

 

[duke37]

The shunt resistor will affect the current it is measuring, depending on the circuit. If you want more output, it may be better to put some gain in the op-amp.

 

[Frankchie]

The 100k resistor is a compromise. A higher value resistor increase the voltage the microcontroller sees, but decreases the current flow. Vice-versa for lower value resistors. The 100k resistor introduces anywhere from 2% to 8% reduction in current flow depending on the actual value of R0. You could increase the 100k resistor and that would increase the measured voltage, but it would also reduce the current flow even more than 2%-8%. Although perhaps the microcontroller program could correct the associated voltage measurement error,

I'm not sure what this circuit does. It looks like it is measuring the resistance of R0 and, therefore the current reduction due to R1 is not significant. If it does something else then the current reduction of a larger R1 could be significant.

As previously mentioned reconfiguring the op amp circuit to give amplification may be a better answer.