What is a Thermocouple?
A thermocouple can quantify temperature as it relates to a reference temperature. This reference temperature is usually sensed using a thermistor, Resistance Temperature Detectors (RTDs) or integrated silicon sensor. The wide temperature range of the thermocouple makes it appropriate for many hostile sensing environments. A thermocouple consists of two dissimilar metallic wires that are connected at two different junctions, one for temperature measurement and the other for reference. The temperature difference between the two junctions is determined by measuring the change in voltage across the dissimilar metals at the temperature measurement junction. The International Society of Automation defines a number of commercially available thermocouple types in terms of performance. Type E, J, K, and T are base-metal thermocouples and can be used to measure temperatures from about -200°C to +1000°C. Type S, R, and B are noble-metal thermocouples and can be used to measure temperatures from about -50°C to +2000°C.
Thermocouple Amplifier Circuit
The circuit below can be used for remote thermocouple sensing applications. The thermocouple is connected to the circuitry via a shielded cable and Electromagnetic Interference (EMI) filters. The thermocouple is tied to a positive and negative supply via large resistors so that the circuit can detect a failed open-circuit thermocouple.
The TC913A auto-zeroed op amp has been selected because of its low offset voltage of 15 μV (max) and high Common Mode Rejection Ratio (CMRR) of 116 dB (typical). Auto-zero and chopper amplifiers are good thermocouple amplifiers due to their low offset voltage and CMRR specifications. The cold junction compensation circuit is implemented with a TC1047A silicon Integrated Circuit (IC) temperature sensor located on the Printed Circuit Board (PCB).