Linearized Platinum RTD Signal Conditioner

The best choice for stability and high accuracy in temperature measurements is Platinum resistance temperature detectors (RTD). However, the signal conditioning is complicated by the RTD which exhibit a non-linear temperature versus resistance characteristic. The non-linearity amount of 0 C to 100 C is up to 0.4 C. Here is the schematic diagram of linearized platinum RTD signal conditioner :

This circuit use A1 as a negative gain inverter to drive a constant current across the platinum sensor. The current reference is provided by the 1oK resistor and LT1009. The self heating induced errors reduced and the voltage through the RTD is low because A1 operates at negative gain. The  platinum sensor’s temperature varies A1’s output potential and fed to A2.

The offsetting and scaled gain are provided by A2. Therefore, the A2’s output will swing 0.00V to 10.000V for 0.00 C to 100.00 C temperature swing at the RTD. The noise pick-up is limited by the 1uF capacitor. Returning a small portion of the circuit’s output to A1 will correct the 0.4 C non linearity error due to the RTD’s imperfect response. This circuit is calibrated by substituting the sensor with a precision decade box. The deviation of the each sensor is different depends on the manufacture that produced it. The purity of the material fix the gain slope of the platinum. [Source: Linear Technology Application Note]