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Measuring resistance with high accuracy
madwolfe:
I am attempting to read a sensor that changes resistance (from 10 ohms to 20k), to a high degree of accuracy, aiming for +-0.1% (ignoring the accuracy of the sensor itself). I feel like this is a frequently asked thing, but after searching on the forum, I couldn't find anything that quite fits my requirements.
My current strategy is a resistor divider, with a fixed resistor between the ADC input (ADS1115) and a fixed 3.3v, and the sensor between the ADC and ground. Unfortunately, it is not accurate enough for my needs.
My replacement idea is to have a constant current source (Looking at LT3092) driving the sensor and the ADC reading the voltage across the sensor. The sensor resistance will be R_SENS=V_ADC/I. The current will either be fixed, using a measured resistor or voltage reference (to reduce temperature variance), or I will use a DAC to program the current source so that the current is in the ideal current range for the current resistance. The DAC will maybe be the DAC8571 or similar.
Is there any thoughts on the best strategy? Before I buy components, PCBs etc. While it would be good to keep the price low, I would prefer to have a more accurate scheme.
coromonadalix:
An dmm like the hp34401a in 4wire mode could do the job, 0.035% precision ? but costly
How long do you need to take theses measurements, what is the sensor you need to probe ?
a wheatstone bridge maybe ?
https://www.omega.ca/en/resources/wheatstone-bridge
https://www.analogictips.com/wheatstone-bridge-part-1-principles-and-basic-applications/
App note ads1232
http://www.ti.com/lit/ml/slyp163/slyp163.pdf
madwolfe:
Thanks for that. It will be logging indefinitely, and packaged on a small PCB so a DMM will not be an option.
A Wheatstone bridge definitely looks like a possibility, I had completely forgotten about then. It looks like it linearises the relationship between the resistance and the output voltage a bit better over the range I'm looking at. That app note is very handy!
jaromir:
--- Quote from: madwolfe on September 16, 2019, 10:04:59 am ---My current strategy is a resistor divider, with a fixed resistor between the ADC input (ADS1115) and a fixed 3.3v, and the sensor between the ADC and ground. Unfortunately, it is not accurate enough for my needs.
--- End quote ---
This is good start. Measure voltage on sensor (use analog switch or multichannel ADC), then voltage on resistor, divide the two voltages and multiply by resistor value. Result will not depend on 3,3V precision.
Google for ratiometric measurement
http://lpvo.fe.uni-lj.si/fileadmin/files/Izobrazevanje/OME/sbaa110_Understanding_Ratiometric_Conversions.pdf
capt bullshot:
Ratiometric measurement is the way for least cost here IMO.
10R to 20k is quite a dynamic range, as you said, using a single resistor might not be sufficient.
I don't know what you're using the ADS1115 inputs and PGA for besides your sensor.
Anyway, I'd suppose to use two of the ADS1115 inputs (one to measure the voltage across the sensor, the other one to measure the voltage across the reference resistor) in differential mode. Maybe the PGA comes in handy to extend the ranges. Another way would be to use multiple resistors to cover the range, and switching them by an external multiplexer using some clever circuitry to take the ON-State resistance of the MUX out of the equation. Something like an 'HC4052 or 'HC4053 could do this job.
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