Resistance Temperature Detectors or RTD’s are metals with a temperature coefficient highly predictable. Commonly used is Pt-100 or Platinum with 100ohms ohms at 0°C. Can’t measure resistance directly – so need to convert it to voltage using a constant current source.
Now we have a multitude of problems – first a stable current source temperature independent, less than 1 mA preferably to avoid self-heating. Next RTD’s are not always 100.00 ohms at 0°C.
So a mechanism of calibrating the inputs are required. Also resistance is a second order function of temperature for positive temperatures. Use of a look-up table is advisable.
Use a temperature-compensated current source using LM334. Use an op-amp as in Microchip’s AN687 to get a 3rd wire compensated Voltage equivalent to Resistance.
Use a dual ADC to measure voltage at current source and voltage across 2nd wire. Do the math and you get the RTD resistance minus wire resistance.