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

2.6.3 Design Notes

This design should be exactly the same as in the two-wire RTD design in Section 2.1. The IDAC current is being routed through an alternate input and measurements are taken from AIN1 and AIN2. However, the considerations in reference resistor size, IDAC current, reference voltage, and PGA input voltage are exactly the same.

The measurement circuit requires:

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

To verify that the design is within the ADC range of operation, Calculate the voltages for AIN1 and AIN2 and the maximum differential input voltage. Verify that VAIN1 and VAIN2 are within the input range of the PGA given the gain setting and supply voltage. Use the maximum RTD resistance based on the desired temperature measurement.

Equation 65. VAIN1 = IIDAC1 • (RRTD + RREF)
Equation 66. VAIN2 = IIDAC1 • RREF

Additionally, verify that the voltage seen at the IDAC pin (where VAIN0 = VAIN1) is within the current source compliance voltage. When the IDAC output voltage rises too close to AVDD, the IDAC loses compliance and the excitation current is reduced.

The reference resistor, RREF must be a precision resistor with high accuracy and low drift. Any error in the RREF reflects the same error in the RTD measurement. The REFP0 and REFN0 pins are shown connecting to the RREF resistor as a Kelvin connection to get the best measurement of the reference voltage. This eliminates any series resistance as an error from the reference resistance measurement.