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

1.3 Thermocouple Measurement and Cold-Junction Compensation (CJC)

As discussed earlier, the thermocouple generates a voltage related to the temperature difference between the thermocouple junction and the leads to attached to the cold junction at the isothermal block (see Figure 1-1). However, the voltage created from the thermocouple is non-linear depending on the temperature of the cold junction. Cold-junction compensation is required to accurately determine the thermocouple junction temperature based on the cold junction temperature.

With cold-junction compensation, the leads of the thermocouple must be at the same known temperature. In thermocouple measurement systems, there is a cold-junction block which connects the thermocouple lead to the ADC measurement. This block holds both thermocouple leads at the same temperature and is often a connector made from a large metal mass, with thermal capacitance. In some applications, it may be sufficient to maximize the copper fill around the junctions of the PCB, layering the connection between metal fill between top and bottom layers. Because air currents may affect the temperature, an enclosure around the block may be necessary.

An accurate measurement of the cold junction block acts as the reference temperature of the cold-junction. This reference measurement is often made through a diode, thermistor, or RTD. If the reference temperature at TCJ is known, then the thermocouple temperature at TTC is computed based on the thermocouple voltage. The process of accounting for TCJ is called cold junction compensation because it is generally assumed that TCJ is the cold temperature.

In the classical method of setting the cold-junction temperature the leads of the thermocouple are placed in an ice bath, ensuring that the reference temperature is 0°C. However, in most systems the cold-junction temperature is measured separately with a device such as an RTD or thermistor.

Once the reference temperature is measured, the thermocouple voltage for that temperature (relative to 0°C) can be determined and added to the measured voltage on the thermocouple leads. This compensation gives the voltage that would have been developed if TCJ had been at 0°C. Note that this voltage is required when referencing the NIST charts, since the chart values are specified relative to 0°C.

Thermocouple voltages are non-linear with temperature. Therefore you cannot simply add the temperature of the cold-junction to the temperature computed from the thermocouple voltage. To accurately determine the thermocouple temperature, the proper method is to:

  1. Convert the cold-junction temperature (TCJ) to a voltage (VCJ)
  2. Add the cold-junction voltage to the measured thermocouple voltage (VCJ + VTC)
  3. Convert the summed cold-junction voltage and thermocouple voltage to the thermocouple temperature (TTC)

Conversion tables and polynomial equations used to determine thermocouple temperature from the thermoelectric voltage is found at the NIST website at http://srdata.nist.gov/its90/menu/menu.html.