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2 1 2 1 2 1 2 1 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 Theory of Operation To ensure that mismatch of the common-mode filtering capacitors is not translated to a differential voltage, the common-mode capacitors (CR_CM1 and CR_CM2) were chosen to be 10 times smaller than the differential capacitor. This common-mode capacitors' size results in a common-mode cutoff frequency that is roughly 10 times larger than the differential filter, making the matching of the common-mode cutoff frequencies less critical. CR_CM1 = CR_CM2 = 3300 pF C27 0.1µF C13 AVDD_ISO RTD_IEXC2 RTD_IEXC2 RTD_IEXC1 RTD_IEXC1 RTD_AINP RTD_AINN RTD_AINP RTD_AINN R11 249 R12 249 0.1µF SIG_GND D10 DESD1P0RFW-7 K C A 3 K A SIG_GND D6 DESD1P0RFW-7 3 C C14 4700pF R9 4.12k C17 0.047µF R13 4.12k D11 DFLS1200-7 SIG_GND C51 1000pF D5 P4SMA13CA D3 DFLS1200-7 J1 1 2 3 4 OSTVN04A150 C24 4700pF C54 1000pF D7 P4SMA13CA C47 1000pF D4 P4SMA13CA SIG_GND SIG_GND SIG_GND SIG_GND C49 0.1µF C59 AVDD_ISO SIG_GND 0.1µF SIG_GND RTD_REFP0 RTD_REFP0 R41 249 K C52 3300pF R38 4.22k RTD_REFN0 RTD_REFN0 A A D16 DESD1P0RFW-7 3 C R40 249 D17 DESD1P0RFW-7 K C 3 C53 0.033µF R39 4.22k R37 3.24k C58 1000pF D9 P4SMA13CA C56 3300pF SIG_GND SIG_GND SIG_GND SIG_GND Figure 46. Common Mode and Differential Mode Filters Implemented in Design for RTD The device uses a linear-phase finite impulse response (FIR) digital filter that performs both filtering and decimation of the digital data stream coming from the modulator. The digital filter is automatically adjusted for the different data rates and always settles within a single cycle. Only at data rates of 5 SPS and 20 SPS can the filter be configured to reject 50-Hz or 60-Hz line frequencies or to simultaneously reject 50 Hz and 60 Hz. Two bits (50/60[1:0]) in the configuration register are used to configure the filter accordingly. Figure 47. Details View of 50 Hz and 60 Hz Simultaneous Rejection at 20 SPS Data Rate 38 Temperature Sensor Interface Module for Programmable Logic Controllers TIDU271–May 2014 (PLC) Submit Documentation Feedback Copyright © 2014, Texas Instruments Incorporated Theory of Operation 5.2.12 Piecewise Linear Approximation Method RTD 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 48. To perform a linear interpolation using a lookup table, first compare the measured RTD resistance to values given in the lookup table, until the lookup table value exceeds the measured value that is being converted. Then, use Equation 32 to convert the measured RTD resistance to temperature, where R LT is the resistance 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. Figure 48. V-to-T Conversion Block Diagram for RTD T TLT n 1 TLT n TLT n 1 LT LT RIN R n 1 RLT n R n 1 There is a trade-off between memory size and the piecewise linear approximation method’s accuracy. The more entries on the lookup table, the more accurate the results are. However, the larger number of entries on the lookup table uses more memory space. (32) Figure 49. Temperature Error versus Temperature Using Different Sizes of lookup Table TIDU271–May 2014 Submit Documentation Feedback Temperature Sensor Interface Module for Programmable Logic Controllers (PLC) Copyright © 2014, Texas Instruments Incorporated 39