An analog signal conditioning circuit for thermocouple temperature sensor employing thermistor for cold junction compensation

Thermocouples are widely used as temperature sensors in industrial as well as laboratory settings owing to their low cost, usability and stability over a wide range of temperatures. However limitations arise in the form of cold junction compensation and non-linearity of the thermocouple emf. The present paper proposes an analog signal conditioning circuit that performs the dual task of cold junction compensation and linearization. Cold junction compensation is done using a thermistor based circuit to emulate the output voltages of the thermocouple at different ambient temperatures with respect to 0°C, and linearization is performed by employing a temperature-compensated dual-input logarithmic amplifier. Optimum circuit parameters have been found by standard optimizations methods, and simulations have been performed using a circuit simulation software `PSpice´ by considering the characteristics of a 5kΩ thermistor and a T-type Copper Constantan Thermocouple. Nonlinearity errors and variation of output voltage of the signal conditioning circuit with ambient temperature are investigated from the simulation results.