tidu271

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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
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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)
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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
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Temperature Sensor Interface Module for Programmable Logic Controllers
(PLC)
Copyright © 2014, Texas Instruments Incorporated
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