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tidu271

Theory of Operation

Theory of Operation www.ti.com However, the thermocouple sensor output that is connected to the inputs in this design does not make use of the entire full-scale range. Instead, the output is limited to V IN = ±54.886 mV (max). Accordingly, this reduced input signal relaxes the V CM restriction to 1.078 V ≤ V CM ≤ 3.922 V. 200 AINP 25 pF A1 R AIN_P R C ADC VCM = 2.5V 54.886mV AIN_N 200 A2 Figure 20. Thermocouple Sensor Output = Input to PGA AINN 25 pF Figure 21. PGA with Gain Programmed to 32 V/V Figure 22. PGA Output = ADC Input The thermocouple signal conditioning circuit includes a simple bias generation through R PU and R PD , which basically centers the thermocouple between supply and ground. This is often an ideal common-mode input for PGA. R PU and R PD should be sized as high as possible to minimize the current flowing through the thermocouple without introducing too much additional noise. The thermal noise or Johnson noise for a resistor can be given as: VN R PU 2 4KTR(BW) where • R = R PU + R PD • K = Boltzmann constant= 1.38 × 10 –23 J/K • T = Temperature in Kelvin = 273.15 + 85°C (max) = 358.15 K (max) • BW = Bandwidth = External input filter bandwidth = 152 Hz (11) For this design, the variables are sized to restrict their total noise contribution to less than the peak-topeak input referred noise of the ADC. R PD Peak-to-Peak Input Referred Noise 4 KT BW A 1 MΩ resistor each is sufficient to accomplish the task without adding too much noise. 2 (12) 20 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 5.1.7 Accuracy Calculation K-Type Thermocouple Voltage at 1372°C temperature produces 54.886 mV. ADS1220’s Full Scale Range = FSR = ±0.064 V = ±64 mV PGA Gain = 32 V/V 2 2 2 2 2 2 Total Unadjusted Error (TUE) (CJC Error) (ADC Temp Error) 2 2 2 2 2 ADC Error Contribution INL OFFSET OFFSET DRIFT GAIN GAIN DRIFT Individual errors using typical ADC specifications are: INL = ±15 PPM of FSR = ±15 PPM × 55 mV = ±0.385 µV Offset error = ±10 µV (from datasheet graph) Error due to offset drift = ±0.08 µV/°C × (60°C – 0) = ±4.8 µV Gain error = ±300 PPM of FSR = ±300 PPM × 55 mV = ±16.5 µV Error due to gain drift = ±1 PPM/°C × (60°C – 0) = ±3.3 µV ADC Error Contribution = ±17.52 µV ADC Error Contribution 17.52 V This yields temperature error 0.431 C AverageThermocouple Sensitivity 40.6 V C ADC Temperature Error ≈ ±0.5°C (Typical) For TMP275, the temperature accuracy is ±0.5°C 2 2 Total Unadjusted Error (TUE) (0.5C) (0.5C) 0.707C (excluding thermocouple sensor error) For LM94022, the temperature accuracy is ±1.5°C 2 2 Total Unadjusted Error (TUE) (1.5C) (0.5C) 1.58C (excluding thermocouple sensor error) (13) Accuracy/Resolution of ADC + V1 J2 + + 0.1 µF 1 µF AIN P AIN N PGA Voltage References ADC SPI Interface CS SCLK DIN DOUT/DRDY J3 0.1 µF Oscillator Temperature Sensor Accuracy of Thermocouple Temp. Sensor Accuracy of Reference Junction Temperature Sensor Figure 23. Parts of Circuit Affecting Accuracy TIDU271–May 2014 Submit Documentation Feedback Temperature Sensor Interface Module for Programmable Logic Controllers (PLC) Copyright © 2014, Texas Instruments Incorporated 21