SBAA483 February   2021 ADS1120 , ADS112C04 , ADS112U04 , ADS114S06 , ADS114S06B , ADS114S08 , ADS114S08B , ADS1220 , ADS122C04 , ADS122U04 , ADS124S06 , ADS124S08 , ADS125H02 , ADS1260 , ADS1261 , ADS1262 , ADS1263

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
  4. 2Features Used to Detect Wire Breaks in RTD Systems
    1. 2.1 Detecting a Wire Break Using a Continuous VREF Monitor
    2. 2.2 Detecting a Wire Break Using a Periodic VREF Monitor
    3. 2.3 Detecting a Wire Break Using Separate Analog Inputs
  5. 3Wire-Break Detection Methods for Different RTD Configurations
    1. 3.1 Wire-Break Detection Using 2-Wire RTDs
    2. 3.2 Wire-Break Detection Using 3-Wire RTDs
      1. 3.2.1 Wire-Break Detection in a One-IDAC, 3-Wire RTD System
        1. 3.2.1.1 Detecting a Break in Lead 2 in a One-IDAC, 3-Wire RTD System
          1. 3.2.1.1.1 Detecting a Break in Lead 2 in a One-IDAC, 3-Wire RTD System Using a High-Side RREF
        2. 3.2.1.2 Wire-Break Detection Summary for a One-IDAC, 3-Wire RTD System
      2. 3.2.2 Wire-Break Detection in a Two-IDAC, 3-Wire RTD System
        1. 3.2.2.1 Detecting Lead 1 or 2 breaks in a two IDAC, 3-wire RTD system using a low-side RREF
        2. 3.2.2.2 Detecting Lead 1 or 2 Breaks in a Two-IDAC, 3-Wire RTD System Using a High-Side RREF
        3. 3.2.2.3 Wire-Break Detection Summary for a Two-IDAC, 3-Wire RTD System
    3. 3.3 Wire-Break Detection in a 4-Wire RTD System
      1. 3.3.1 Detecting Lead 2 and Lead 3 Breaks in a 4-Wire RTD System Using a Low-Side RREF
      2. 3.3.2 Detecting Lead 2 and Lead 3 Breaks in a 4-Wire RTD System Using a High-Side RREF
      3. 3.3.3 Wire-Break Detection Summary for a 4-Wire RTD System
  6. 4Settling Time Considerations for RTD Wire-Break Detection
  7. 5Summary
  8.   A How Integrated PGA Rail Detection Helps Identify Wire Breaks
  9.   B Pseudo-Code for RTD Wire-Break Detection
    1.     B.1 Pseudo-Code for a 2-Wire RTD System (Low-Side or High-Side RREF)
    2.     B.2 Pseudo-Code for a One-IDAC, 3-Wire RTD System (Low-Side or High-Side RREF)
    3.     B.3 Pseudo-Code for a Two-IDAC, 3-Wire RTD System (Low-Side or High-Side RREF)
    4.     B.4 Pseudo-Code for a 4-Wire RTD System (Low-Side or High-Side RREF)

1 Introduction

Resistance temperature detectors (RTDs) are highly-accurate sensors that can measure wide temperature ranges. RTDs are used in a variety of industrial applications, including analog input modules in programmable logic controllers (PLC), temperature transmitters, and patient monitoring equipment. These dynamic commercial and industrial environments occasionally result in RTD wires becoming damaged or disconnected. For reliable operation, these fault conditions must be detected so the host controller can either correct the fault if possible, or put the system in a default state if necessary.

To aid this process, this application report defines a methodology for broken-wire detection in all common RTD configurations using the integrated features in precision ΔΣ ADCs. This document begins by describing these features in detail, including how they work and why they are useful for wire-break detection. The subsequent sections present each RTD configuration, stepping through what occurs as each wire (or group of wires) breaks and how these breaks are identified. Finally, additional features to aid in wire-break detection and pseudo code are included in Appendix A and Appendix B, respectively. This information generally applies to many precision ΔΣ ADCs, including the devices in Table 1-1:

Table 1-1 Precision ΔΣ ADCs for RTD Measurements
Cost-Optimized Low Power Lowest Noise
ADS1120 ADS114S06B ADS125H02
ADS112C04 ADS114S08B ADS1260
ADS112U04 ADS114S06 ADS1261
ADS1220 ADS114S08 ADS1262
ADS122C04 ADS124S06 ADS1263
ADS122U04 ADS124S08 —

Finally, this application report assumes a general understanding of the various RTD configurations and how they work under normal operating conditions. To learn more about these topics, see the related A Basic Guide to RTD Measurements application report.