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Hi,

Will this circuit be adequate for detecting a low resistance given that I've only got 5V and 40mA to do it with?

https://www.circuitlab.com/circuit/2v25v7/lowohmdetection/

The resolution of the A2D is 10 bit with a reference of 5V. I need accuracy to 1/10th of an ohm, this hopefully gives me 1/50th. I realize that the output voltage will not be linear with regard to the value of UNKNOWN, but I can compensate for that with calculations and calibrations in the microcontroller code.

Many thanks for any help/tips. Regards, Marc.

Also, just on a side note. Can I utilize an analog port on the microcontroller that is just pulled to ground to give a noise floor level to correct other inputs that also have noise?

Your post does not give enough information about what you are trying to do so here is a sanity check.

It looks like you want to:

i) measure the value of Runknown with SW1 open;

ii) have sufficent resolution to do that using a 10bit ADC with a 5V reference;

iii) not destroy the measurement circuit when the switch closes and the voltage across the Runknown rises to 15V.

Is that correct?

Sorry signality, I should have been more specific. You are correct on all points there yes. Also I should also point out that the 5V 40mA source is turned on by the microcontroller when a measurement is required (it's a digital output pin)

Never mind. I built the circuit and it works fine.