| Electronics > Projects, Designs, and Technical Stuff |
| Measuring resistance with high accuracy |
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| mzzj:
--- Quote from: splin on September 17, 2019, 11:19:00 pm --- 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. --- End quote --- I was thinking of balancing the range with ~450ohm reference resistor. Current and self-heating "felt" bit too high but that's probably because I'm accustomed to platinum resistance thermometers (like Pt-100) where you have "crazy" tempco and self-heating effect. |
| madwolfe:
I have achieved an accuracy that I'm pretty happy with (0.06%), just using the 'voltage divider' with a 10k resistor and measuring either side of the resistor. I will probably stick with that as a measurement strategy for now. I will have calibration in firmware. Since I already have bought a couple of the ADS1115, I would prefer to stick with it and just read the two voltages and calculate from there. It doesn't have an external voltage reference however, is there a significant advantage of referencing the ADC to the voltage divider's voltage? Besides not having to calculate as much. |
| madwolfe:
Also, I want to buffer the input of the ADC however I cannot determine what opamp characteristics are desirable here. Eg. what is the advantage of using the LTC1050 over the LM358? I don't need a high slew rate so is it mainly higher stability, lower drift? There's just way too many opamps to choose from. |
| madwolfe:
--- Quote from: mzzj on September 18, 2019, 05:44:27 am --- --- Quote from: splin on September 17, 2019, 11:19:00 pm --- 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. --- End quote --- I was thinking of balancing the range with ~450ohm reference resistor. Current and self-heating "felt" bit too high but that's probably because I'm accustomed to platinum resistance thermometers (like Pt-100) where you have "crazy" tempco and self-heating effect. --- End quote --- What do you mean by "balancing the range"? With my calculations, I max out of the range of the ADC at around 4000k on my sensor with a 450ohm resistor. So 450ohms isn't suitable, or do you mean that it's applied in another way? |
| Kleinstein:
Having the same reference for the current divider and the ADC helps with noise. 2 separate references adds some extra noise. At the 16 bit level and with some repeats this may not be so bad. For buffering the requirements for the OP depends on the impedance range and the required accuracy. For the OP the important parameters are input current, noise, offset drift and a few others. In this case it depends, wether a single buffer and external MUX is used, or a buffer for every input to the ADC. With just 1 buffer one could compensate much of the offset error in software. |
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