Sensor wire breaks are an uncommon but inevitable challenge in industrial environments, requiring fault identification to enable the host to take corrective action. Resistance temperature detectors (RTDs) fall into this category of sensors where wire-break detection is necessary. This application report discusses ways to use features commonly found in delta-sigma (ΔΣ) analog-to-digital converters (ADCs) to help identify wire breaks for all common RTD configurations.
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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:
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.
RTD wire breaks can be detected using just the excitation current sources (IDACs) integrated into the ADC along with voltage reference (VREF) monitoring.
All devices in Table 1-1 integrate two matched current sources, which are commonly used to establish a voltage across an RTD that is then measured by an ADC. For VREF monitoring, many ADCs include a discrete diagnostic circuit that can be read in real-time. Or, another option for VREF monitoring involves reading back the differential reference voltage with an unused measurement channel. Both of these scenarios are discussed in the following subsections.