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)

2.1 Detecting a Wire Break Using a Continuous VREF Monitor

Figure 2-1 shows a 4-wire RTD using a low-side reference resistor (RREF) as an example of how to use IDACs and VREF monitoring to detect an RTD wire break. One IDAC is used for this RTD configuration, flowing through lead 1, RRTD, lead 4, and finally through RREF to ground. The ADC measures the RTD voltage at AINP and AINN and uses the voltage established across RREF as the reference voltage.

GUID-20210107-CA0I-DMQT-BV76-HTHDRHGWW6LF-low.gifFigure 2-1 IDAC Current Flow in a 4-Wire RTD With a Low-Side RREF

When a wire breaks, the current may no longer flow through the RTD, or more specifically through RREF. With no current flowing through RREF, the ADC reads a differential reference voltage of approximately 0 V between REFP and REFN. For example, Figure 2-2 shows that the IDAC current return path becomes an open circuit if lead 4 breaks.

GUID-20210107-CA0I-JKLP-4HTZ-MQ42RXCFSQQL-low.gifFigure 2-2 Detecting a Wire Break Using a Continuous VREF Monitor

In Figure 2-2, the near-zero differential voltage applied between REFP and REFN trips the VREF monitor in the ADC, which has a typical threshold of a few hundred millivolts. This monitor sets a flag in the ADC STATUS byte that can be read continuously by the host to determine corrective action. In other words, this monitoring is completed in real-time without interrupting the precision RTD measurement.