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Theory of Operation

Theory of Operation The behavior of resistance for PT100 RTD from –200°C to 850°C is shown in Figure 28. 400 360 320 RTD( Temp) R LINFIT ( Temp) Resistance (Ohms) 280 240 200 160 120 80 40 0 200 95 10 115 220 325 430 535 640 745 850 Temperature (C) Figure 28. RTD Resistance and Linear Fit Line versus Temperature The change in RTD resistance is piecewise linear over small temperature ranges. An end-point curve fitting shows RTDs have a second-order, nonlinearity of approximately 0.375% as shown in Figure 29 . To correct this correct this nonlinearity error requires implementing a digital linearization technique. 20 Resistance Error and Nonlinearity %FSR 5 18 16 4.5 4 Resistance Error (Ohms) 14 3.5 12 3 Nonlin Ohms ( Temp) 10 2.5 Nonlin Error ( Temp) 8 2 6 1.5 Resistance Nonlinearity (%) 4 2 1 0.5 0 200 95 10 115 220 325 430 535 640 745 0 850 Temp Temperature (C) Figure 29. Resistance Error and Nonlinearity versus Temperature 26 Temperature Sensor Interface Module for Programmable Logic Controllers TIDU271–May 2014 (PLC) Submit Documentation Feedback Copyright © 2014, Texas Instruments Incorporated Theory of Operation The three techniques for RTD linearization are as follows: 1. Simple Linear Approximation Method 2. Piecewise Linear Approximation Method 3. Callendar-Van Dusen Equations Method A brief comparison is also given in Table 6. Table 6. Linear Approximation Methods METHOD ADVANTAGE DISADVANTAGE OTHER REMARKS Simple Linear Approximation Piecewise Linear Approximation Callendar-Van Dusen Equations Easy to implement Fast execution Accurate over small temperature ranges Smaller code size Accuracy depends on the entries in the lookup table Fast execution Most accurate Least accurate over wide temperature range High accuracy More entries in lookup table Bigger code size Extremely Processor Intensive Time consuming Requires Square root and Power functions on the processor Simple method when memory size is limited and temperature range is smaller Most practical method implement by most of the embedded systems Not practical in most applications This reference design implements the piecewise linear approximation method using the manufacturer’s lookup table with more number of entries to achieve better accuracy. 5.2.2 RTD Resistance Measurement The basic principle required for RTD measurement is based on Ohm’s Law. Ohm's law equation is as follows: V R I where • R = Resistance of the RTD element • I = Known excitation current • V = Voltage across RTD element (16) As an RTD is a passive sensor, an RTD does not produce output on its own. An RTD requires a known constant current source for the RTD's'excitation to produce a useful voltage output proportional to the resistance of the RTD. The resulting voltage output is then amplified and fed to an ADC for measurement. By this method, an unknown resistance of an RTD can be easily calculated and then transformed to temperature using a software algorithm. Depending on the nominal resistance of the RTD, different excitation currents can be used. The excitation current flow through an RTD should be minimized to avoid self-heating. In general, approximately 1 mA or less current is used. I1 V IN+ R RTD V DIFF GAIN ΔΣ ADC V IN- Figure 30. Simplified RTD Measurement Circuit TIDU271–May 2014 Submit Documentation Feedback Temperature Sensor Interface Module for Programmable Logic Controllers (PLC) Copyright © 2014, Texas Instruments Incorporated 27