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Minimizing Errors in Multiplexed 3-Wire RTD Data-Acquisition Systems
By Henry He
Resistance temperature detectors (RTDs) monitor temperature in many industrial applications. In a distributed control system (DCS) or programmable logic controller (PLC), one data-acquisition module may monitor the temperature of many remotely located RTDs. In high-performance applications, the best accuracy will be obtained when each RTD has its own excitation circuit and ADC, but the data-acquisition module will be large, expensive, and power hungry. Multiplexing leads to a smaller, lower cost, lower power module, but some accuracy can be lost. This article discusses how to minimize errors in a multiplexed system.
Figure 1 shows two 3-wire RTDs excited by the on-chip current sources. The RTD channel is selected by a multiplexer, such as the ADG5433 high-voltage, latch-up proof, triple SPDT switch.
Figure 1. Two 3-wire RTDs multiplexed into one AD7792/AD7793 ADC.
Only one RTD can be measured at one time. S1A, S1B, and S1C are closed to measure RTD #1; S2A, S2B, and S2C are closed to measure RTD #2. A single ADG5433 can switch two 3-wire RTDs; additional multiplexers can be added to handle more than two sensors. RLXX represents the resistance introduced by long wires between the RTD and the measurement system, plus the on resistance of the switches.
Calculating the RTD Resistance
Thus, the measurement depends only on the value (and accuracy) of RREF. Remember, however, that we assumed
Impact of Mismatched Current Sources and Wire Resistors
Note that the mismatch creates both an offset error and a gain error. The offset error is related to the mismatch between the two lead resistances, while the gain error is related to the mismatch between the two current sources. If these mismatches are not taken into consideration, the calculated value of the RTD resistance, based on the data read from the ADC, will be incorrect.
Using a 200-Ω RTD as an example, Table 1 shows the acquired values when the mismatches are not considered, given
Table 1. Measured RTD Values When Mismatches Are Not Considered
The transfer function is linear, so initial errors due to current source and resistance mismatches can be calibrated out easily. Unfortunately, the mismatch varies with temperature, making it difficult to compensate. Hence, it’s important to use devices that have low drift over temperature.
With IOUT1 ≠ IOUT2, and the current sources connected as shown:
Assume we swap IOUT1 and IOUT2, so that IOUT1 now connects to VIN– and IOUT2 now connects to VIN+:
Now, if we sum the results from a conversion with the current sources connected in the original orientation and a second conversion with the current sources swapped, the result is
Note that the measurement is now independent of current source mismatch. The only downside is the loss of speed, because two conversions are needed for each RTD calculation.
The AD7792 and AD7793 are designed for this application. As shown in Figure 2, integrated switches make it easy to swap the current sources to the output pins by writing to an I/O register.
Figure 2. Functional block of AD7792/AD7793.
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