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I'm trying to design a circuit which can measure small resistances down to 0.1 Ohm and a max. of 10 Ohms. I won't be measuring actual resistors but rather large coil of wires, upto 500 m (as you can imagine, these wires are quite thick).

Here's the circuit I came up with:


The circuit works by maintaining a constant current through the device under test, R2. With a current of 100 mA, R2 would develop a voltage between 10 mV to 50 mV.

I think in an ideal world this would work but in practice I may have a hard time measuring 0.1 Ohms with this - mainly due to the ADC. Let's assume the ADC is 10-bit with VREF of 5V. This translates to 5mV per step. If R2 = 0.1 and Iout = 100 mA, then the voltage present at the ADC would be 50 mV - but I'm not sure how buried under noise this would be.

My question is, should I increase the gain to, say, 50. If the gain is 50, then the voltage present at the ADC would be 500 mV - but the max. measurable resistance would be 1 Ohms. To measure 10 Ohms, I would need to lower the current to 10 mA instead of 100 mA. A way to do that would be use an FET to switch out R1 and connect a 20 Ohm resistor at Iout.

I don't need the circuit to measure the resistance precisely - a tolerance of +/- 10% is fine.
