Circuit provides accurate RTD measurements

Tito Smailagich, ENIC, Belgrade, Yugoslavia -- EDN, 4/12/2001

The circuit in Figure 1 is an efficient measuring circuit for PT100 RTD elements. IC₁ provides an accurate 2.5V output and, together with P₁ and R₁ , also provides a stable 1-mA current to the RTD element. The output of IC₂ is –0.1V. P₂ provides a zero adjustment (at 0°C) for IC₃ , an amplifier with a gain of 25. P₃ provides a gain adjustment. If, for example, you replace the RTD element by a fixed resistance of 124.78W (the RTD's resistance in Table 1 at 64°C), you would trim P₃ to obtain 0.64V output. Tolerance values for Class A and B elements are ±0.35 and ±0.8°C, respectively. You can use the standard values from Table 1 or, if you need more accuracy, you can calibrate the PT100 element in a controlled-temperature environment.