30 - Thermocouple measurement

Abstract

Obtaining good accuracy in thermocouple systems mandates care. The small thermocouple signal voltages require careful consideration to avoid error terms in signal processing. In general, thermocouple system accuracy better than 0.5 °C is difficult to achieve. Major error sources include connection wires, cold junction uncertainties, amplifier error, and sensor placement. Connecting wires between the thermocouple and conditioning circuitry introduce undesired junctions. These junctions form unintended thermocouples. Thus utmost care should be taken while considering an amplifier. Thermocouple amplifiers need very low offset voltage and drift, and fairly low bias current if an input filter is used. The best precision bipolar amplifiers should be used for type J, K, E and T thermocouples which have Seebeck coefficients of 40 V/°C to 60 μV/°C. In particularly critical applications, or for R and S thermocouples, a chopper-stabilized amplifier is required. Linear Technology offers two amplifiers specifically tailored for thermocouple applications. The LTKA0x is a bipolar design with an extremely low offset, low drift, very low bias current, and almost negligible warm-up drift. For the most demanding applications, the LTC® 1052 complementary metal–oxide–semiconductor chopper-stabilized amplifier offers 5 mV offset and 0.05 μV/°C drift. Input bias current is 30 pA, and gain is typically 30 million. This amplifier should be used for R and S thermocouples, especially if no offset adjustments can be tolerated, or where a large ambient temperature swing is expected. Other circuit considerations involve protection and common mode voltage and noise.