- Reference resistor R2 should be 20 times the maximum DUT to be measured, because the 20x gain used when measuring voltage across DUT.
No need to be 20x. The key to maximizing the resolution: to make sure that the voltage drop over the reference resistor is as close to the Vref selected for the ADC as possible - maybe 80 - 90% of the Vref. This approach produces the largest digit.
In my case, the current is about 10ma so something close to 100R would be ideal.
A sub-optimal approach, which I utilized, is to oversample. Fairly effective as well.
Hi dannyf,
You may want to get the voltage drop across the DUT also as large as possible in order to maximize the numeric value from the ADC when measuring DUT, too.
For example, if you choose the reference resistor R2 to be 100 ohms, short-circuit the DUT and adjust the R1 so that the voltage across R2 is 80% - 90% of VREF, you have maximized the measuring range for the reference resistor R2. The numerical values you are getting from the 10-bit ADC is in range 800 - 900.
But you may also want to obtain as high measuring values for the DUT too in order to maximize the dynamic range of the DUT as well. You are taking advantage of the 20x gain of the ADC when measuring the differential voltage across the DUT, so the maximum voltage across the DUT can be 1/20 of the VREF. That means also that the maximum voltage across the DUT can be 1/20 of the reference resistor R2. Thus, the maximum value for the DUT is approximately 1/20 of the reference resistor R2.
With the 100 ohm reference resistor the maximum DUT is approximately 5 ohms, and you should get almost 5 milliohm resolution using 10-bit ADC. Oversampling by factor of 100, you should gain one extra digit ie. you should obtain resolution of 0.5 milliohms which is pretty impressive.
I haven't tested this though, so I may have missed or overlooked something.
Br,
Kalvin