Analog Linearization of Resistance Temperature Detectors (RTD) Using the Intrinsic Curvature of BandGap Voltage References

Although resistance temperature detectors (RTD) are more linear than thermocouples, they present second and third order non-linearities, and a linear signal processing circuit which converts the voltage on a Pt-100 RTD to an output voltage with 10 mV/degrees C presents a maximum non-linearity error of 1.07 degrees C (10.7 mV) in the 0 to 85 degrees C temperature range. Although these non-linearities can be corrected digitally, there are cases where a simple analog linearization can be used with advantages. In this work we present an analog linearization technique that uses the output of a conventional Brokaw bandgap cell as the reference voltage of a differential instrumentation amplifier. The intrinsic curvature of the bandgap voltage reference, caused by the non-linear variation with temperature of the VBE of a transistor, creates a compensation voltage that can reduce the non-linearity of the signal processing circuit by one order of magnitude (down to approximately 0.14 degrees C) in the same temperature range.