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  • A Basic Guide to RTD Measurements

    • 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

       

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  • A Basic Guide to RTD Measurements
  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
  6. IMPORTANT NOTICE
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APPLICATION NOTE

A Basic Guide to RTD Measurements

1 A Basic Guide to RTD Measurements

RTDs, or resistance temperature detectors, are sensors used to measure temperature. These sensors are the among the most accurate temperature sensors available, covering large temperature ranges. However, getting accurate measurements with precision analog-to-digital converters (ADCs) requires attention to detail in design of measurement circuits and calculation of the measurement. This application note starts with an overview of the RTD, discussing their specifications, construction, and details in their use in temperature measurement. Different circuit topologies with precision ADCs are presented for different RTD configurations. Each circuit is shown with a basic design guide, showing calculations necessary to determine the ADC settings, limit measurement errors, and verify that the design fits in the operating range of the ADC.

1 RTD Overview

RTDs are resistive elements that change resistance over temperature. Because the change in resistance is well characterized, they are used to make precision temperature measurements, with capability of making measurements with accuracies of well under 0.1°C. RTDs are typically constructed from a length of wire wrapped around a ceramic or glass core. RTDs may also be constructed from thick film resistors plated onto a substrate. The wire or resistance is typically platinum but may also be made from nickel or copper. The PT100 is a common RTD constructed from platinum with a resistance of 100 Ω at 0°C. RTD elements are also available with 0°C resistances of 200, 500, 1000, and 2000 Ω.

1.1 Callendar-Van Dusen Equation

The relationship between platinum RTD resistance and temperature is described by the Callendar-Van Dusen (CVD) equation. Equation 1 shows the resistance for temperatures below 0°C and Equation 2 shows the resistance for temperatures above 0°C for a PT100 RTD.

Equation 1. For T < 0: RRTD(T) = R0 • {1 + (A • T) + (B • T2) + [(C • T3) • (T – 100)]}
Equation 2. For T > 0: RRTD(T) = R0 • [1 + (A • T) + (B • T2)]

The coefficients in the Callendar-Van Dusen equations are defined by the IEC-60751 standard. R0 is the resistance of the RTD at 0°C. For a PT100 RTD, R0 is 100 Ω. For IEC 60751 standard PT100 RTDs, the coefficients are:

  • A = 3.9083 • 10-3
  • B = –5.775 • 10-7
  • C = –4.183 • 10-12

The change in resistance of a PT100 RTD from –200°C to 850°C is displayed in Figure 1-1.

GUID-65D5D024-694B-4E42-8942-E6A72A3B515E-low.gifFigure 1-1 PT100 RTD Resistance From –200°C to 850°C

While the change in RTD resistance is fairly linear over small temperature ranges, Figure 1-2 displays the resulting non-linearity if an end-point fit is made to the curve shown in Figure 1-1.

GUID-5148431A-598F-4544-8295-0E2EB8498663-low.gifFigure 1-2 PT100 RTD Non-Linearity From –200°C to 850°C

The results show a non-linearity greater than 16 Ω, making a linear approximation difficult over even small ranges. For temperatures greater than 0°C, temperatures can be determined by solving the quadratic from Equation 2. For temperatures lower than 0°C, the third order polynomial of Equation 1 may be difficult to calculate. Using simple microcontrollers, determining the temperature may be computationally difficult and using a look-up table to determine the temperature is common practice.

Newer calibration standards allow for more calculation accuracy using higher order polynomials over segmented temperature ranges, but the Callendar-Van Dusen equation remains a commonly used conversion standard.

 
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