SBAA274A September   2018  – March 2023 ADS1118 , ADS1119 , ADS1120 , ADS112C04 , ADS112U04 , ADS1146 , ADS1147 , ADS1148 , ADS114S06 , ADS114S06B , ADS114S08 , ADS114S08B , ADS1219 , ADS1220 , ADS122C04 , ADS122U04 , ADS1246 , ADS1247 , ADS1248 , ADS124S06 , ADS124S08 , ADS125H02 , ADS1260 , ADS1261 , ADS1262 , ADS1263

 

  1.   A Basic Guide to Thermocouple Measurements
  2.   Trademarks
  3. 1Thermocouple Overview
    1. 1.1 Seebeck Voltage
    2. 1.2 Thermocouple Types
      1. 1.2.1 Common Thermocouple Metals
      2. 1.2.2 Thermocouple Measurement Sensitivity
        1. 1.2.2.1 Calculating Thermoelectric Voltage from Temperature
        2. 1.2.2.2 Calculating Temperature From Thermoelectric Voltage
      3. 1.2.3 Thermocouple Construction
      4. 1.2.4 Tolerance Standards
    3. 1.3 Thermocouple Measurement and Cold-Junction Compensation (CJC)
    4. 1.4 Design Notes
      1. 1.4.1 Identify the Range of Thermocouple Operation
      2. 1.4.2 Biasing the Thermocouple
      3. 1.4.3 Thermocouple Voltage Measurement
      4. 1.4.4 Cold-Junction Compensation
      5. 1.4.5 Conversion to Temperature
      6. 1.4.6 Burn-out Detection
  4. 2Thermocouple Measurement Circuits
    1. 2.1 Thermocouple Measurement With Pullup and Pulldown Bias Resistors
      1. 2.1.1 Schematic
      2. 2.1.2 Pros and Cons
      3. 2.1.3 Design Notes
      4. 2.1.4 Measurement Conversion
      5. 2.1.5 Generic Register Settings
    2. 2.2 Thermocouple Measurement With Biasing Resistors Attached to the Negative Lead
      1. 2.2.1 Schematic
      2. 2.2.2 Pros and Cons
      3. 2.2.3 Design Notes
      4. 2.2.4 Measurement Conversion
      5. 2.2.5 Generic Register Settings
    3. 2.3 Thermocouple Measurement With VBIAS for Sensor Biasing and Pullup Resistor
      1. 2.3.1 Schematic
      2. 2.3.2 Pros and Cons
      3. 2.3.3 Design Notes
      4. 2.3.4 Measurement Conversion
      5. 2.3.5 Generic Register Settings
    4. 2.4 Thermocouple Measurement With VBIAS For Sensor Biasing and BOCS
      1. 2.4.1 Schematic
      2. 2.4.2 Pros and Cons
      3. 2.4.3 Design Notes
      4. 2.4.4 Measurement Conversion
      5. 2.4.5 Generic Register Settings
    5. 2.5 Thermocouple Measurement With REFOUT Biasing and Pullup Resistor
      1. 2.5.1 Schematic
      2. 2.5.2 Pros and Cons
      3. 2.5.3 Design Notes
      4. 2.5.4 Measurement Conversion
      5. 2.5.5 Generic Register Settings
    6. 2.6 Thermocouple Measurement With REFOUT Biasing and BOCS
      1. 2.6.1 Schematic
      2. 2.6.2 Pros and Cons
      3. 2.6.3 Design Notes
      4. 2.6.4 Measurement Conversion
      5. 2.6.5 Generic Register Settings
    7. 2.7 Thermocouple Measurement With Bipolar Supplies And Ground Biasing
      1. 2.7.1 Schematic
      2. 2.7.2 Pros and Cons
      3. 2.7.3 Design Notes
      4. 2.7.4 Measurement Conversion
      5. 2.7.5 Generic Register Settings
    8. 2.8 Cold-Junction Compensation Circuits
      1. 2.8.1 RTD Cold-Junction Compensation
        1. 2.8.1.1 Schematic
          1. 2.8.1.1.1 Design Notes
          2. 2.8.1.1.2 Measurement Conversion
          3. 2.8.1.1.3 Generic Register Settings
      2. 2.8.2 Thermistor Cold-Junction Compensation
        1. 2.8.2.1 Schematic
        2. 2.8.2.2 Design Notes
        3. 2.8.2.3 Measurement Conversion
        4. 2.8.2.4 Generic Register Settings
      3. 2.8.3 Temperature Sensor Cold-Junction Compensation
        1. 2.8.3.1 Schematic
        2. 2.8.3.2 Design Notes
        3. 2.8.3.3 Measurement Conversion
        4. 2.8.3.4 Generic Register Settings
  5. 3Summary
  6. 4Revision History

1.2.2.2 Calculating Temperature From Thermoelectric Voltage

Making the reverse conversion, Inverse polynomial functions calculate the temperature based on the thermocouple voltage. The equations for inverse polynomial functions are of the form shown in Equation 2.

Equation 2. t90 = d0 + d1E + d2E2 + … + diEi

where

  • E is in microVolts and t90 is in degrees Celsius

As an example, the inverse function for a K-type thermocouple is shown in Table 1-3. Polynomials are constructed over three smaller ranges of the full temperature range. For each range, the temperature is described with a high order polynomial.

Table 1-3 ITS-90 Temperature Coefficients for a K-Type Thermocouple
Temperature Range:−200°C to 0°C0°C to 500°C500°C to 1372°C
Voltage Range−5891 μV to 0 μV0 μV to 20644 μV20644 μV to 54886 μV
d0
d1
d2
d3
d4
d5
d6
d7
d8
d9		0.000 000 0
2.517 346 2 x 10–2
–1.166 287 8 x 10–6
–1.083 363 8 x 10–9
–8.977 354 0 x 10–13
–3.734 237 7 x 10–16
–8.663 264 3 x 10–20
–1.045 059 8 x 10–23
–5.192 057 7 x 10–29		0.000 000 0
508 355 x 10–2
7.860 106 x 10–8
–2.503 131 x 10–10
8.315 270 x 10–14
–1.228 034 x 10–17
9.804 036 x 10–22
–4.413 030 x 10–26
1.057 734 x 10–30
–1.052 755 x 10–35		–1.318 058 x 102
4.830 222 x 10–2
–1.646 031 x 10–6
5.464 731 x 10–11
–9.650 715 x 10–16
8.802 193 x 10–21
–3.110 810 x 10–26
Error Range0.04°C to –0.02°C0.04°C to –0.05°C0.06°C to –0.05°C

Table 1-2 and Table 1-3 show the complexity of direct and inverse polynomial equations. The mathematical operations used to calculate these high order equations without loss of precision can take a significant amount of computational processing with high resolution, floating-point numbers. This type of computation is generally not suited for embedded processing or microcontrollers. In many cases, it is far more efficient to determine the temperature through interpolation using a lookup table.