Here's a sketch for a pt100/RTD frontend circuit.
There are a couple of ideas here which should improve precision:
- 4-wire connection, to eliminate lead-resistance effects
- Ratiometric measurement (both a reference and the signal go to the ADC). This should minimize effects from fluctuations in the sensing current.
- AC-excitation. The sensing current can be reversed with at TTL logic signal. Some ADC chips have an output for this, and they average the measurement done with current flowing in both directions. This eliminates effects from DC-offsets (thermovoltages etc).
- The circuit is centered around a particular temperature (here +32C) and the signals amplified so a twenty degree span of +22C to +42C should give about +/- 4 V output.
NI Multisim file for this: pt100_sensor_circuit_v3