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
2.8.1.1.1 Design Notes

The RTD is a temperature sensor that changes resistance over temperature. There are several different types of RTD construction, but the resistance for any given temperature is well characterized. RTDs are often used to make precision temperature measurements. Figure 2-8 shows a 2-wire RTD circuit topology for making a temperature measurement used for cold-junction compensation.

The measurement circuit requires:

  • Single dedicated IDAC output pin
  • AINP and AINN inputs
  • External reference input
  • Precision reference resistor

An IDAC current source drives both the RTD and the reference resistor, RREF. Because the same current drives both elements, the ADC measurement is a ratiometric measurement. Calculation for the RTD resistance does not require a conversion to a voltage, but does require a precision reference resistor with high accuracy and low drift.

With IDAC1, the ADC measures the voltage across the RTD using the voltage across RREF as the reference. This provides an output code that is proportional to the ratio of the RTD voltage and the reference voltage as shown in Equation 20. Ratiometric measurements only produce positive output data, assuming zero offset error. For a fully-differential measurement, this is only the positive half of the full-scale range of the ADC, reducing the measurement resolution by one bit. The following equations assume a 16-bit bipolar ADC, with ±VREF as the full-scale range of the ADC.

Equation 20. Output code = 215 • VRTD / VREF = 215 • IIDAC1 • RRTD / (IIDAC1 • RREF)

The currents cancel so that the equation reduces to Equation 21.

Equation 21. Output code = 215 • RRTD / RREF

In the end, the RTD resistance can be represented from the code as a function of the reference resistance.

Equation 22. RRTD = Output code • RREF / 215

The measurement depends on the resistive value of the RTD and the reference resistor RREF, but not on the IDAC1 current value. Therefore, the absolute accuracy and temperature drift of the excitation current does not matter. In a ratiometric measurement, as long as there is no current leakage from IDAC1 outside of this circuit, the measurement depends only on RRTD and RREF. ADC conversions do not need to be translated to voltage. Assuming the ADC has a low gain error, RREF is often the largest source of error. The reference resistor must be a high accuracy precision resistor with low drift. Any error in the reference resistance becomes a gain error in the measurement.

There are many different types of RTDs and several different construction forms. For more detailed information about RTD measurement, see A Basic Guide to RTD Measurements.