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tidu271

Theory of Operation 5.1

Theory of Operation 5.1 Thermocouple Input Channel www.ti.com 5 V 5 V 0.1 F 0.1 F 5 V REFP0 REFN0 R B1 R F1 C CM1 AIN0 10 A to 1.5 mA AVDD Internal Reference Reference Mux DVDD Device Thermocouple R B2 R F2 C DIF C CM2 AIN1 AIN2 Mux PGA 24-bit ADC Digital Filter and SPI Interface CS SCLK DIN DOUT/DRDY DRDY AIN3 Low Drift Oscillator Precision Temp Sensor AVSS CLK DGND Figure 2. Thermocouple Measurement Circuit Using ADS1220 6 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 5.1.1 Thermocouple Fundamentals – “Back to Basics” NOTE: Any conductor subjected to thermal gradient will generate a voltage between its ends. A simple metal bar produces a voltage when there is a temperature difference between the two ends. The electrons at the hot end are more thermally agitated than the electrons at the cooler end. The more thermally agitated electrons on the hot end begin to diffuse towards the cooler end. The redistribution of electrons creates a negative charge at the cooler end and an equal positive charge at the hotter end. This process produces an electrostatic voltage between the two ends. The open circuit voltage produced in this way is known as “Seebeck Voltage.” The phenomenon is called the “Seebeck Effect”. The Seebeck Effect was first discovered by German scientist Thomas Johann Seebeck in 1821. Direct measurement of Seebeck Voltage of a single metal bar is impossible. Another similar metal bar will also produce the same Seebeck Voltage, cancelling each other and resulting in 0 V at the measuring point. However, a single wire does not form as a thermocouple. A thermocouple is formed when two dissimilar metals are bonded together electrically to form two junctions as shown in Figure 4. Figure 3. Voltage Developed Due to Temperature Gradient V S T T HOT COLD Figure 4. Thermocouple The magnitude and direction of open circuit voltage developed between the two ends is proportional to the temperature difference, as shown in Figure 4 and Equation 1. where S is the Seebeck coefficient or thermoelectric sensitivity When designing with thermocouples, it is important to understand that thermocouples are bipolar. Bipolar means thermocouples can produce a positive or a negative voltage, depending on whether or not the measured temperature is higher or lower than the system temperature, respectively. (1) TIDU271–May 2014 Submit Documentation Feedback Temperature Sensor Interface Module for Programmable Logic Controllers (PLC) Copyright © 2014, Texas Instruments Incorporated 7