Minimizing errors in multiplexed 3-wire RTD data acquisition systems
Overview:
The Resistance Temperature Detector (RTD) monitors temperature in many industrial applications. In a distributed control system (DCS) or programmable logic controller (PLC), a data acquisition module can be used to monitor many RTD temperatures installed remotely. In high-performance applications, if each RTD has its own excitation circuit and ADC, it has the best accuracy, but the data acquisition module will be bulky, costly, and power efficient. Multiplexed modules have the features of smaller size, lower cost, and lower power consumption, but may lose some accuracy performance. This article discusses how to minimize multiplex system errors.
The circuit architecture provides two-wire, three-wire, and four-wire RTD configurations, with a two-wire configuration with the lowest cost and a four-wire device with the best accuracy. Three-wire RTDs are commonly used in industrial applications and can be excited by two identical current sources to eliminate pin resistance. When used with a precision reference resistor, the current source error does not affect the measurement accuracy. High-performance ADCs such as the AD7792 and AD7793 integrate an excitation current source for high-precision RTD measurements.
Figure 1 shows an on-chip current source that excites two three-wire RTDs. The RTD channel can be selected by a multiplexer such as the ADG5433 high voltage, latch-proof, triple SPDT switch. 
Figure 1. Two 3-wire RTDs multiplexed into one AD7792/AD7793 ADC
Only one RTD can be measured at a time. S1A, S1B, and S1C Close Measurement RTD #1; S2A, S2B, and S3B Close Measurement RTD #2. A single ADG5433 can switch between two 3-wire RTDs; additional multiplexers can be added to handle more than two sensors. RLXX represents the resistance introduced between the RTD and the measurement system due to the excessive length of the wire and the on-resistance of the switch.
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