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# tidu271

## Theory of Operation

Theory of Operation www.ti.com The resistance of the PT100 changes from 18.52 Ω at –200°C to 390.481 Ω at 850°C. The line resistance R LEAD depends on the distance of the sensor from the measurement setup. Assume R LEAD is equal to 5 Ω. The positive resistance swing is from 100 Ω to 390.481 Ω, which is about 290.481 Ω. The negative resistance swing is from 100 Ω to 18.52 Ω, which is about 81.48 Ω. The IDAC current should be 1 mA or less to minimize the self-heating error. The IDAC current chosen is 500 µA. Therefore, maximum and minimum input voltages to the PGA are 390.481 Ω × 500 µA = 195.2405 mV and 18.52 Ω × 500 µA = 9.26 mV, respectively. The external reference resistor, R REF , serves two purposes. First, the external reference resistor sets the external reference voltage for ADC. Second, the external reference resistor determines the input common mode voltage of the PGA. It is a best practice to set the common mode voltage around mid-supply (AVDD – AVSS) / 2 = (5 V – 0 V) / 2 = 2.5 V. Therefore, the reference voltage chosen is 2.49 V, which also depends on an easily available resistance value and excitation current. The sum of both currents flows through a precision low-drift reference resistor, R REF. The voltage, V REF , generated across the reference resistor is given in Equation 23. VREF IDAC1 IDAC2 RREF Because IDAC1 = IDAC2 = 500 µA, VREF 2 IDAC1 RREF Solving for R REF : VREF 2.49 V RREF 2.49 K 2 IDAC1 2 500 A Then, find out the PGA gain: VREF 2.49 V GAINPGA 12.75 V/V VIN MAX 195.2405 mV The closest gain supported by the PGA is 8 V/V. (23) (24) (25) (26) Common Mode Input Voltage Figure 41. PGA Gain The signal of an RTD is of a pseudo-differential nature. The negative input must be biased at a voltage other than 0 V and the positive input can then swing up to 195.2405 mV above the negative input. 34 Temperature Sensor Interface Module for Programmable Logic Controllers TIDU271–May 2014 (PLC) Submit Documentation Feedback Copyright © 2014, Texas Instruments Incorporated