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

2.4.3 Design Notes

In this topology, two measurements are taken for lead resistance cancellation. In the first measurement, the ADC measures the voltage across the RTD and the resistance for lead 1 as driven by the single excitation current source. In the second measurement, the ADC measures the resistance for lead 3 as driven by the same excitation current source. This method assumes that the resistance in lead 1 and lead 3 are equal. By subtracting the second measurement from the first, the RTD resistance can be accurately measured, and the lead resistance cancelled.

The measurement circuit requires:

  • Single dedicated IDAC output pin
  • AINP and AINN inputs
  • A measurement for the RTD and a parasitic lead resistance
  • A second measurement to measure an equivalent lead resistance to cancel
  • External reference input
  • Precision reference resistor

Starting with IDAC1 driving AIN0, the voltage at AIN1 and AIN2 can be calculated. For the first measurement:

Equation 41. VAIN1 = IIDAC1 • (RRTD + RLEAD1 + RLEAD3 + RREF)
Equation 42. VAIN2 = IIDAC1 • (RLEAD3 + RREF)
Equation 43. VMEAS1 = VAIN1 – VAIN2 = IIDAC1 • (RRTD + RLEAD1)

Because current does not flow through lead 2, there is no RLEAD2 term in the measurement. For the second measurement, the ADC measures the voltage from AIN2 to AIN3.

Equation 44. VAIN3 = IIDAC1 • RREF
Equation 45. VMEAS2 = VAIN2 – VAIN3 = IIDAC1 • RLEAD3

VMEAS2 yields the measurement of the lead 3 resistance. Subtracting VMEAS2 from VMEAS1, the result is:

Equation 46. VMEAS1 − VMEAS2 = [IIDAC1 • (RRTD + RLEAD1)] − (IIDAC1 • RLEAD3)

Assuming the resistance from lead 1 equals the resistance from lead 3, the result is:

Equation 47. VMEAS1 − VMEAS2 = IIDAC1 • (RRTD + RLEAD1 − RLEAD3) = IIDAC1 • RRTD

For both VMEAS1 and VMEAS2, the reference resistor shunts IIDAC1 for a reference voltage of:

Equation 48. VREF = IIDAC • RREF

As with the previous examples, start the design with the expected usable range of the RTD. The reference resistor and IDAC current values are chosen to place the input voltage within the PGA range, while ensuring that the IDAC is operating within its compliance voltage. As in all ratiometric measurements, the reference resistor, RREF must be a precision resistor with high accuracy and low drift.

To verify that the design is within the PGA range of operation, start by calculating the voltages of AIN1 and AIN2 and the maximum differential input voltage. Assuming the lead resistances are small and can be ignored, Equation 41 and Equation 42 reduce to Equation 49 and Equation 50. 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 49. VAIN1 = IIDAC1 • (RRTD + RREF)
Equation 50. VAIN2 = IIDAC1 • RREF

Additionally, verify the output voltage of the IDAC sources calculated from VAIN0 and VAIN3 are low enough from AVDD to be within the compliance voltage of the IDAC current source. Because the voltage for IDAC1 always be higher than that of IDAC2, it is sufficient to calculate the output voltage at VAIN0 to verify the IDAC compliance voltage. This calculation is already shown in Equation 49, because VAIN0 is the same potential as VAIN1.

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.