After reading many posts with difficulties reading a Resistive Temperature Detector (RTD) such as a PT-1000, NTC with e.g. an Arduino, I am wondering if a time based approach could be used:
If I take a 1000uF capacitor and charge it via the RTD to a certain voltage (measured by an analog input), this takes time that varies depending on the R value of the RTC. One digital output of the Controller must be used to charge and discharge all connected R-Cs and to let the Controller determine the exact starting time of charging. Once the analog input reaches the threshold of e.g. 90% V+ the time is stopped and R & T could be determined.
- The obvious disadvantage is that reading T takes time and limits the sampling rate, but seems acceptable for most applications.
- The obvious advantage is the simplicity of the circuit similar to a voltage divider.
- Is this idea reasonable at all?
- What could be the accuracy of the reading for a PT-1000?
- What are the tradeoffs in capacitance vs accuracy.
EDIT: Notes: This solution is intended for Arduino projects and not for a high volume products. The 0..1023 resolution 10 bit A/D converter of Arduino UNO's has a poor resolution for resistive devices with voltage divider yielding about 14 steps for 18 Kelvin. This is a theoretical idea and that is why it is posted here.
Update: Andy pointed out that capacitance change of the circuit could be a problem. I looked up that typically "Capacitance increases less than 5% from 25 ºC to the high temperature limit." If the capacitor is kept at room temperature, the problem could be controlled. Initial calibration due to tolerance of the capacitors could be problematic.