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)

3.3 Wire-Break Detection in a 4-Wire RTD System

Four-wire RTDs are the most accurate configuration because they do not suffer from the challenges inherent to the 3-wire RTD implementations: multiple measurements in the case of the one-IDAC system, or mismatched current sources in the case of the two-IDAC systems. As a result, 4-wire RTDs are also the most expensive sensors and consume the most PCB space because they require four terminals. Four-wire RTDs are generally used where temperature measurement accuracy is the most critical system parameter.

Similar to the 3-wire RTD fault detection methods, the VREF monitors can be used to detect the majority of the wire breaks in a 4-wire RTD system. Under normal operating conditions, any combination of wire breaks that include a break in lead 1 or lead 4 always eliminates the path to ground for the IDAC, resulting in a fault detectable by the VREF monitor. Figure 3-10 shows this result in a low-side RREF configuration, though the same detection scheme can be used for a high-side RREF.

GUID-20210107-CA0I-PSTQ-G5KJ-2KQ60BD1BVX2-low.gifFigure 3-10 Lead 1 (Left) and Lead 4 (Right) Breaks in a 4-Wire RTD System

The only cases where the VREF monitor does not detect a wire break in a 4-wire RTD system is if only lead 2, lead 3, or both lead 2 and lead 3 break. In these cases, a wire break likely causes the ADC output to deviate from the expected value, but the cause cannot be assumed to be a wire break as opposed to some other system anomaly.

Therefore, perform a diagnostic measurement routine using the IDACs in the ADC similar to those described in the previous sections. Furthermore, as previously described, the routine is different depending if a low-side or high-side RREF configuration is used.