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tidu271

Theory of Operation

Theory of Operation www.ti.com Sample 1: Measurement with normal current routing as shown in Figure 43. Figure 43. Excitation Current Chopping → Sample 1 Sample 2: Measurement with swapped current routing as shown in Figure 44. Figure 44. Excitation Current Chopping → Sample 2 Mismatch corrected reading 0.5 Sample 1 Sample 2 (29) 5.2.11 Noise Considerations and Input Filter RTD voltage output signals are typically in the millivolt range, which makes them susceptible to noise. A first-order differential and common-mode RC filter (R F1 , R F2 , C DIF1 , C CM1 , C CM2 ) is placed on the ADC inputs, as well as on the reference inputs (R F3 , R F4 , C DIF2 , C CM3 , C CM4 ) to eliminate high-frequency noise in RTD measurements. For best performance, it is recommended to match the corner frequencies of the input and reference filters. More detailed information on matching the input and reference filters can be found in the application report, RTD Ratiometric Measurements and Filtering Using the ADS1148 and ADS1248 Family of Devices, SBAA201. The differential filters chosen for this application were designed to have a –3 dB corner frequency at least 10 times larger than the bandwidth of ADC. The selected ADS1220 sampling rate of 20 SPS results in a –3 dB bandwidth of 13.1 Hz. The cut off frequency chosen for this design is higher to account for a faster sampling rate. For proper operation, the differential cutoff frequencies of the reference and input low-pass filters must be well matched. Matching the frequencies can be difficult because, as the resistance of the RTD changes over the span of the measurement, the filter cutoff frequency changes as well. To mitigate this effect, the two resistors used in the input filter (RI1 and RI2) are chosen to be more than an order of magnitude larger than the RTD. Limiting the resistors to at most 20 kΩ keeps DC offset errors low due to input bias current. 36 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 I1 R I1 C I_CM1 R RTD R I2 C I_DIFF GAIN ΔΣ ADC R ZERO I2 REFP REFN C I_CM2 C R_CM1 C R_CM2 C R_DIFF R R1 R R2 The –3 dB cutoff frequency of differential input filter at 186 Ω RTD resistance (at mid-scale temperature) can be calculated as given in Equation 30. 1 F 3 dB _ I _ DIFF 2 CIDIFF RI1 RRTD RI2 F 3 dB _ I _ DIFF 402.1Hz The –3 dB cutoff frequency of differential reference filter can be calculated as given in Equation 31. 1 F 3 dB _ R _ DIFF 2 CR DIFF RR1 RREF RR2 R REF Figure 45. Common Mode and Differential Mode Filters on RTD Input and Reference R I1 = R I2 = 4.12 K C I_DIFF = 0.047 µF To ensure that mismatch of the common-mode filtering capacitors is not translated to a differential voltage, the common-mode capacitors (CI_CM1 and CI_CM2) are 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. C I_CM1 = C I_CM2 = 4700 pF Although it is not always possible to exactly match the corner frequencies of all the filters, a good compromise is to attempt to balance the corner frequencies of the input path differential filter and the reference path differential filter, because these filters have a dominant effect in the performance. RR1 = RR2 = 4.7 K CR_DIFF = 0.033 µF (30) F 3 dB _ R _ DIFF 405.83 Hz (31) TIDU271–May 2014 Submit Documentation Feedback Temperature Sensor Interface Module for Programmable Logic Controllers (PLC) Copyright © 2014, Texas Instruments Incorporated 37