SBAA275A June   2018  – March 2023 ADS1120 , ADS112C04 , ADS112U04 , ADS1147 , ADS1148 , ADS114S06 , ADS114S06B , ADS114S08 , ADS114S08B , ADS1220 , ADS122C04 , ADS122U04 , ADS1247 , ADS1248 , ADS124S06 , ADS124S08 , ADS125H02 , ADS1260 , ADS1261 , ADS1262 , ADS1263

 

  1.   A Basic Guide to RTD Measurements
  2. 1RTD Overview
    1. 1.1 Callendar-Van Dusen Equation
    2. 1.2 RTD Tolerance Standards
    3. 1.3 RTD Wiring Configurations
    4. 1.4 Ratiometric Measurements
      1. 1.4.1 Lead Resistance Cancellation
      2. 1.4.2 IDAC Current Chopping
    5. 1.5 Design Considerations
      1. 1.5.1 Identify the RTD Range of Operation
      2. 1.5.2 Set the Excitation Current Sources and Consider RTD Self Heating
      3. 1.5.3 Set Reference Voltage and PGA Gain
      4. 1.5.4 Verify the Design Fits the Device Range of Operation
      5. 1.5.5 Design Iteration
  3. 2RTD Measurement Circuits
    1. 2.1  Two-Wire RTD Measurement With Low-Side Reference
      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  Two-Wire RTD Measurement With High-Side Reference
      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  Three-Wire RTD Measurement, Low-Side Reference
      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
      6. 2.3.6 Chopping IDAC Currents for Matching
    4. 2.4  Three-Wire RTD Measurement, Low-Side Reference, One IDAC Current Source
      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 Configuration Register Settings
    5. 2.5  Three-Wire RTD Measurement, High-Side Reference
      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 Configuration Register Settings
    6. 2.6  Four-Wire RTD Measurement, Low-Side Reference
      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 Configuration Register Settings
    7. 2.7  Two Series Two-Wire RTD Measurements, Low-Side Reference
      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 Configuration Register Settings
    8. 2.8  Two Series Four-Wire RTD Measurements
      1. 2.8.1 Schematic
      2. 2.8.2 Pros and Cons
      3. 2.8.3 Design Notes
      4. 2.8.4 Measurement Conversion
      5. 2.8.5 Configuration Measurement Settings
    9. 2.9  Multiple Two-Wire RTD Measurements
      1. 2.9.1 Schematic
      2. 2.9.2 Pros and Cons
      3. 2.9.3 Design Notes
      4. 2.9.4 Measurement Conversion
      5. 2.9.5 Configuration Register Settings
    10. 2.10 Multiple Three-Wire RTD Measurements
      1. 2.10.1 Schematic
      2. 2.10.2 Pros and Cons
      3. 2.10.3 Design Notes
      4. 2.10.4 Measurement Conversion
      5. 2.10.5 Configuration Register Settings
    11. 2.11 Multiple Four-Wire RTD Measurements in Parallel
      1. 2.11.1 Schematic
      2. 2.11.2 Pros and Cons
      3. 2.11.3 Design Notes
      4. 2.11.4 Measurement Conversion
      5. 2.11.5 Configuration Register Settings
    12. 2.12 Universal RTD Measurement Interface With Low-Side Reference
      1. 2.12.1 Schematic
      2. 2.12.2 Pros and Cons
      3. 2.12.3 Design Notes
        1. 2.12.3.1 Universal Measurement Interface - Two-Wire RTD
        2. 2.12.3.2 Universal Measurement Interface - Three-Wire RTD
        3. 2.12.3.3 Universal Measurement Interface - Four-Wire RTD
      4. 2.12.4 Measurement Conversion
        1. 2.12.4.1 Two-Wire Measurement
        2. 2.12.4.2 Three-Wire Measurement
        3. 2.12.4.3 Four-Wire Measurement
      5. 2.12.5 Configuration Register Settings
    13. 2.13 Universal RTD Measurement Interface With High-Side Reference
      1. 2.13.1 Schematic
      2. 2.13.2 Pros and Cons
      3. 2.13.3 Design Notes
        1. 2.13.3.1 Universal Measurement Interface, High-Side Reference - Two-Wire RTD
        2. 2.13.3.2 Universal Measurement Interface, High-Side Reference - Three-Wire RTD
        3. 2.13.3.3 Universal Measurement Interface, High-Side Reference - Four-Wire RTD
      4. 2.13.4 Measurement Conversion
        1. 2.13.4.1 Two-Wire Measurement
        2. 2.13.4.2 Three-Wire Measurement
        3. 2.13.4.3 Four-Wire Measurement
      5. 2.13.5 Configuration Register Settings
  4. 3Summary
  5. 4Revision History

2.3.6 Chopping IDAC Currents for Matching

As previously mentioned, IDAC current matching is important. The impact of IDAC current mismatch is small for lead compensation because the additional error is small. However, IDAC current mismatch results in a gain error in the RTD measurement. As an example if IDAC2 is larger than IDAC1 by 1%, the reference would be 0.5% larger than expected, resulting in a 0.5% gain error:

Equation 33. VREF = (IIDAC1 + 1.01 • IIDAC1) • RREF = 2.01 • IIDAC1 • RREF

This gain error due to mismatched IDAC current sources can be removed by chopping the IDAC currents. Chopping is achieved by making a measurement and averaging this first measurement with a second measurement after the IDAC currents have been swapped. Starting with the original configuration, the input voltage and reference voltage are given in the following. Equation 34 shows the first measurement, while Equation 35 shows the reference voltage.

Equation 34. VMEAS1 = [IIDAC1 • (RRTD + RLEAD1)] − (IIDAC2 • RLEAD2)
Equation 35. VREF = (IIDAC1 + IIDAC2) • RREF

If the IDACs are swapped so that IDAC2 is sourced from AIN0, and IDAC1 is sourced from AIN3, the reference voltage stays the same. However, the second measurement now becomes:

Equation 36. VMEAS2 = [IIDAC2 • (RRTD + RLEAD1)] − (IIDAC1 • RLEAD2)

Averaging the first input measurement and the second input measurement, the result is:

Equation 37. (VMEAS1 + VMEAS2) / 2 = [(IIDAC1 + IIDAC1) • (RRTD + RLEAD1) / 2] − [(IIDAC1 + IIDAC1) • RLEAD2 / 2]

The resulting ADC measurement is:

Equation 38. Averaged output code = 223 • Gain • {[(IIDAC1 + IIDAC1) • (RRTD + RLEAD1) / 2] − [(IIDAC1 + IIDAC1) • RLEAD2 / 2]} / (IIDAC1 + IIDAC2) • RREF

Using averaging, the (IIDAC1 + IIDAC2) terms cancel; and if the lead wire resistances are equal, they are cancelled as well:

Equation 39. Averaged output code = 223 • Gain • RRTD / (2 • RREF) = 222 • Gain • RRTD / RREF
Equation 40. RRTD = RREF • Averaged output code / (222 • Gain)

With averaging, the ADC output code is no longer dependent on IDAC current matching, resulting in a more accurate measurement.

For chopping IDAC currents, set the register values:

  • For the first measurement, select multiplexer settings for AINP and AINN to measure leads 1 and 2 of the three-wire RTD
  • Enable the PGA, set gain to desired value
  • Select data rate and digital filter settings
  • Select reference input to measure RREF for ratiometric measurement
  • Enable the internal reference (the IDAC requires an enabled internal reference)
  • Set IDAC magnitude and select IDAC1 output pin to drive lead 1 of the RTD and select IDAC2 output pin to drive lead 2 of the RTD
  • For the second measurement, swap the IDAC output pins, select IDAC2 output pin to drive lead 1 of the RTD and select IDAC1 output pin to drive lead 2 of the RTD
  • Average the first and second measurements