Resistance temperature detectors (RTD) can be used to monitor temperatures in many industrial applications. In a distributed control system (DCS) or programmable logic controller (PLC), a data acquisition module can be used to monitor the temperature of many RTD installations at a distance. In high performance applications, the best precision is achieved if each RTD is equipped with an excitation circuit and ADC, but the data acquisition module will be bulky, costly, and power consuming.

The multiplexing module has the characteristics of small size, low cost and low power consumption, but it may lose some accuracy performance. This article discusses how to minimize errors in a multiplexing system.

1. Circuit structure

RTD resistors are available in two-wire, three-wire and four-wire configurations, with the two-wire configuration having the lowest cost and the four-wire configuration having the best accuracy. Three-wire RTDS, commonly used in industrial applications, can be excited by two identical current sources to eliminate pin resistance. When used with precision reference resistors, the current source error does not affect the measurement accuracy. High performance ADCs such as AD7792 and AD7793 integrate excitation current sources for high precision RTD measurements.

Figure 1 shows two three-wire RTDS excited by an on-chip current source. The RTD channel can be selected by a multiplexer, such as ADG5433 high voltage, latch latch, three-way SPDT switch.

Only one RTD can be measured at a time. S1A, S1B and S1C closed measurement RTD #1; Closure measurement RTD #2 for S2A, S2B and S3B. A single ADG5433 can switch two three-wire RTD; Additional multiplexers can be added to handle more than two sensors. RLXX represents the resistance introduced between THE RTD and the measuring system due to the length of the wire and the on-resistance of the switch.

2, calculate the RTD resistance

Since S1A, S1B and S1C closed loop measures RTD #1, the RTD resistance can be calculated as follows:

Therefore, the measured value depends only on the value (and accuracy) of the RREF. But remember, we assume IOUT1 = IOUT2, and RL1A = RL1B = RL1C. In fact, these current and resistance mismatches are a major source of measurement errors.

3. The effect of mismatch between current source and line resistance

Next, assume that the two current sources are mismatched, such as IOUT2 = (1 + x) IOUT1. Now, consider the following:

Note that mismatches lead to misalignment errors as well as gain errors. The misalignment error is related to the mismatch between the two pin resistors, while the gain error is related to the mismatch between the two current sources. If these mismatches are not taken into account, the RTD resistance values calculated from the ADC data readings will be inaccurate.

Taking 200 ω RTD as an example, Table 1 shows the value obtained without considering the mismatch. Where, RREF = 1000 ω, IOUT1 = 1 mA, IOUT2 > IOUT1(shown as percentage), RL1A = 10 ω, RL1C > RL1A(shown as resistance).

4. Minimize errors

The data show that even small mismatches can seriously affect accuracy, so a well-matched current source and switch should be used to improve performance. The transfer function is linear, so initial errors caused by current source and resistance mismatches can be easily calibrated. However, the mismatch changes with temperature, which makes compensation difficult.

Therefore, the selected device should have a low drift characteristic with the change of temperature. If IOUT1 ≠ IOUT2, and the current source is connected as shown in the figure:

Suppose we swap IOUT1 and IOUT2 so that IOUT1 is connected to VIN – and IOUT2, and VIN+ is connected:

Now, if we sum the results of the conversion, and the current sources are connected in the initial direction, and the current sources are exchanged during the second conversion, we get:

Note that the measured values are now independent of the current source mismatch. The only drawback is the loss of speed, since each RTD calculation requires two conversions.

AD7792 and AD7793 are designed for this application. As shown in Figure 2, the integrated switch simplifies switching the current source to the output pin by writing to the I/O register.

Switching the excitation current source in the AD7792/AD7793 device can improve the accuracy of the multiplexed RTD measurement circuit. The calculation shows the importance of the mismatch problem between the current source and the line resistance.

Summary: ADI official website

Download resources: download.csdn.net/download/m0…