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Measuring resistance with high accuracy

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ejeffrey:
RC timing is fine if you don't need much accuracy and only have digital pins and comparators to work with.  It isn't a good way for accurate resistance measurement (OP specified 0.1%).  You are using a capacitor as a reference instead of a resistor and capacitors generally have orders of magnitude worse accuracy, drift, and linearity than even inexpensive resistors.  Furthermore, you can't do a 4-terminal measurement and the GPIO pin on resistance is in series with the DUT and can generate a large error.

For exactly these reasons RC time is a good and common way to measure capacitors.  Due to their generally poor tolerances it is rarely needed to measure capacitors much better than 1% and resistors of that accuracy are easily available.

babysitter:

--- Quote from: babysitter on September 16, 2019, 05:58:23 pm ---(Additionally, discharge/charge thru a known good capacitor sometimes for reference.)

--- End quote ---

This should have been resistor.

Kleinstein:
The measurement of the resistance via a delay gets tricky due to the switch resistance. So it may be practical for large values, but not for small ones like 1 K or less. In addition at high resistance parasitic capacitance can change things.

mzzj:
Layout? external wires? thermovoltages?

Tie 3.3v to ADC ref input and 3.3k precision resistor, 1mA to 10 ohms  gives you 10uV per 0.1%, easy peasy for something like LTC2484. Upper end of the range is easy after that.

splin:

--- Quote from: mzzj on September 17, 2019, 04:40:26 pm ---Layout? external wires? thermovoltages?

Tie 3.3v to ADC ref input and 3.3k precision resistor, 1mA to 10 ohms  gives you 10uV per 0.1%, easy peasy for something like LTC2484. Upper end of the range is easy after that.

--- End quote ---

Better still, use a 447ohm precision resistor to give 70.6uV (21.4ppm) per 0.1% change in sensor resistance at both 10 ohms and 20k, and 500+uV/0.1% from 100 to 2000 ohms with the best sensitivity at 447 ohms. Change the reference to get the best sensitivity to match your application needs.

A higher value reference resistor is likely to be preffered given that:

a) Most ADCs have different errors at zero compared to full scale. Eg, the LTC2484 has 2.5uV offset error but 82.5uV max gain error before calibration + .33uV/C gain drift (with 3.3V reference).

b) At 10 ohms, the reference resistor will be dissipating 24mW, compared to .5mW at 20k ohms, causing it to increase in temperature by up to 10C depending on its type and heatsinking. The reference voltage could be reduced, but that would increase the ADC noise relative to the signal level.

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