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.
(Additionally, discharge/charge thru a known good capacitor sometimes for reference.)
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.
I was thinking of balancing the range with ~450ohm reference resistor.
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.
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?I was thinking of balancing the range with ~450ohm reference resistor.
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.
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.
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.
Rsensor (ohms) | Vsensor (V) | Change in Vsensor for 0.1% change in Rsensor (uV) |
20000 | 2.731 | 909.6 |
10000 | 2.048 | 1023.5 |
10 | 0.0041 | 4.1 |
Rsensor (ohms) | Vsensor (V) | Change in Vsensor for 0.1% change in Rsensor (uV) |
20000 | 4.006 | 87.5 |
447 | 2.048 | 1023.5 |
10 | 0.0897 | 87.7 |
Ah ok, I understand. That does make a lot more sense for the lower end of the measurements.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.
The ADS1115 has a PGA so the possible ...
I just had a look at this ADC, it looks pretty good! It also has an internal programmable current source that I could also use? Is there any reason not to go with that instead?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.Personally I'd use a 24bit lower noise ADC - AD7793 modules are around $5 on ebay and very suitable for your application.
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.
With a relatively low reference resistor, the worst case power consumption for the DUT and reference resistor can be quite high. With 500 Ohms reference and sensor this could be something like 5 mA * 2.5 V = 12.5 mW. This won't burn the resistor, but self heating could change this quite a bit, especially for the sensor.Very true - I'll keep that in mind
The ADC internal PGA actually is only faster input sampling. So it will increase the input current at high gain. One the positive side, the internal PGA gain is quite accurate and stable.
With readings to compare the voltage over the reference resistor and the external resistor, most of the reference errors and also ADC gain drift are not relevant.