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tidu271

Theory of Operation

Theory of Operation www.ti.com Thermocouple manufacturers usually provide a lookup table that offer excellent accuracy for linearization of a specific type of thermocouple. The granularity on these lookup tables is also very precise—approximately 1°C for each lookup value. To save microcontroller memory and development time, an interpolation technique applied to these values can be used. An example of this method when converting from voltage to temperature with eight lookup table entries is shown in Figure 13. To perform a linear interpolation using a lookup table, first compare the measured thermocouple voltage to values given in the lookup table, until the lookup table value exceeds the measured value being converted. Then, use Equation 4 to convert the measured thermocouple voltage to temperature, where V LT is the voltage lookup table array and T LT is the temperature lookup table array. This operation involves four additions, one multiplication, and one division step, respectively. This operation can be done easily on most 16- and 32-bit microcontrollers. Converting from temperature to voltage is the same, except that the lookup tables and the temperature variables are reversed, as shown in Equation 4. Figure 13. V-to-T Conversion Block Diagram for Thermocouple VIN VLT n 1 LT LT LT VLTn VLT n 1 T T n 1 T n T n 1 TIN TLT n 1 LT LT LT TLTn TLT n 1 V V n 1 V n V n 1 (4) 14 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 Use the following steps for thermocouple linearization using the lookup table: 1. Read the voltage from temperature sensor IC (LM94022 and/or TMP275) placed inside the isothermal block. For example, reading TMP275 will give a count equal to 1A0h, while reading LM94022 will give a voltage output of 1.356 V. 2. Use temperature sensor IC’s mV/°C transfer function or lookup table to convert this voltage to temperature. This temperature corresponds to cold junction temperature. For TMP275, a count 1A0h corresponds to 26°C temperature. For LM94022, a voltage output 1.356 V also corresponds to 26°C temperature (which means cold junction temperature is 26°C). 3. Translate this cold-junction temperature reading into the corresponding voltage for a K-type thermocouple at that temperature using lookup. The resulting voltage is V REF = 1.041 mV at 26°C. 4. Read the voltage on thermocouple input channel. For example, the thermocouple voltage measured is V = 3.055 mV. 5. Add the voltage obtained in to the voltage read in step 2 to the voltage read in step 3. V TC = V + V REF = 3.055 mV + 1.041 mV = 4.096 mV. 6. Translate this voltage reading into the corresponding temperature using the lookup table. V TC = 4.096 mV corresponds to a thermocouple temperature of 100°C. Figure 14. K-Type Thermocouple lookup Table The mV/°C transfer function and/or the look tables for LM94022 (with hardwired gain = 01) and TMP275 can be found in their respective datasheets. TIDU271–May 2014 Submit Documentation Feedback Temperature Sensor Interface Module for Programmable Logic Controllers (PLC) Copyright © 2014, Texas Instruments Incorporated 15