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Theory of Operation

Theory of Operation www.ti.com 5.1.2 Cold Junction Compensation (CJC) A thermocouple measures the temperature difference between hot and cold junctions. Thermocouples do not measure the absolute temperature at one junction as illustrated in Figure 8. The cold junction is also sometimes referred to as the reference junction. Figure 8. Simplified View of Cold Junction Compensation 10 Temperature Sensor Interface Module for Programmable Logic Controllers TIDU271–May 2014 (PLC) Submit Documentation Feedback Copyright © 2014, Texas Instruments Incorporated

www.ti.com Theory of Operation The voltmeter measures the same voltage for all three example cases where 100°C temperature difference between hot and cold junctions always generates 4 mV. Connecting a voltmeter to measure the thermocouple output creates one more junction, J2. This junction also creates a small voltage and the difference between the two voltages is measured by the voltmeter. Figure 9 shows a unique case of a T- Type thermocouple (for simplicity, where the copper-to-copper connection does not form another junction). But when thermocouples other than T-Type are employed, two parasitic (unwanted) thermocouples are created with leads at the meter connection. In the PCB, these unwanted thermocouples are one of the biggest concerns. Each dissimilar metal connection creates a new thermocouple when proceeding from the measuring end to the wire connector, to the solder, to the copper PCB trace, to the IC pin, to the bonding wire, and to the chip or die contact. However, if the signal is differential (and each of the thermocouple pairs are at the same temperature), the thermocouple voltages will cancel (and have no net effect on the measurement). Therefore, the net voltage error added by these connections is zero. Figure 9. Unwanted Thermocouple Junction Equation 2 shows how to calculate the temperature (T HOT ) of measuring junction J1. V THOT TCOLD S Equation 2 shows that to determine the temperature of a thermocouple hot junction, the temperature of the cold junction must be known. Classically, the cold junction of a thermocouple is kept in an ice bath as shown in Figure 10 to keep the cold junction at the known reference temperature of 0°C. The low 0°C temperature is another reason this junction is called the cold junction. In reality, it is impractical and inconvenient in most applications to provide a true ice point reference. The National Institute of Standards and Technology (NIST) thermocouple reference tables also assume that the cold junction is at 0°C. (2) TIDU271–May 2014 Submit Documentation Feedback Temperature Sensor Interface Module for Programmable Logic Controllers (PLC) Copyright © 2014, Texas Instruments Incorporated 11