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)

3.1 Wire-Break Detection Using 2-Wire RTDs

Two-wire RTDs provide low-accuracy measurements because no option exists to compensate for lead resistance. However, 2-wire RTDs are low cost and easy to implement, making them attractive for cost-sensitive applications. Moreover, wire-break detection in 2-wire RTDs is straightforward, although determining which wire (or wires) is no longer attached is not possible.

Figure 3-1 shows what happens when lead 2 breaks in the 2-wire RTD configuration: Figure 3-1a shows the low-side RREF implementation and Figure 3-1b shows the high-side RREF implementation. Although Figure 3-1 shows a break in lead 2, the same detection scheme applies to a break in lead 1 or if both wires break.

GUID-20210107-CA0I-9QJ0-CKZG-DZRBWFLBPJHQ-low.gifFigure 3-1 Lead 2 Broken in a 2-Wire RTD Implementation: Low-Side (Left) and High-Side (Right) RREF

For both reference configurations, a broken wire (or wires) results in no current flow through RREF because the path to ground is disconnected. This event drops the voltage across RREF to 0 V, thereby setting the VREF monitor flag high. Therefore, to identify a break in either wire in a 2-wire RTD configuration using either a low-side or high-side RREF, use the VREF monitor in the ADC. Table 3-1 summarizes the wire-break detection method and expected results for all wire-break conditions using a 2-wire RTD.

Table 3-1 Wire-Break Detection Summary for 2-Wire RTD Systems (Low-Side or High-Side RREF)
Lead 1Lead 2Wire-Break Detection Method and Result
ConnectedBroken

  • VREF monitor → VREF voltage ≈ 0 V

BrokenConnected
BrokenBroken