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Volume 47 – September 2013

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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.

Circuit Structure
RTDs are available in 2-wire, 3-wire, and 4-wire configurations, where 2-wire devices are the least expensive and 4-wire devices are the most accurate. Commonly used in industrial applications, 3-wire RTDs can be excited by two identical current sources to cancel out lead resistance. When used with a precision reference resistor, current source errors do not affect the measurement accuracy. High-performance ADCs, such as the AD7792 and AD7793, integrate the excitation current sources, making them ideal for high-accuracy RTD measurements.

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
With S1A, S1B, and S1C closed to measure RTD #1, the resistance of the RTD can be calculated as follows:

Thus, the measurement depends only on the value (and accuracy) of RREF. Remember, however, that we assumed
IOUT1 = IOUT2 and RL1A = RL1B = RL1C. In fact, mismatches in these currents and resistances are the main source of measurement error.

Impact of Mismatched Current Sources and Wire Resistors
Next, assume that the two current sources are mismatched, such that IOUT2 = (1 + x) IOUT1. Now, consider the following:

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 RREF = 1000 Ω, IOUT1 = 1 mA, IOUT2 > IOUT1 by the percentage shown, RL1A = 10 Ω, and RL1C > RL1A by the resistance shown.

Table 1. Measured RTD Values When Mismatches Are Not Considered

                    RL1CRL1A

(IOUT2IOUT1)/IOUT1

0.01 Ω

0.1 Ω

1 Ω

0.1%

199.88

199.79

198.89

0.5%

199.44

199.35

198.45

1.0%

198.90

198.81

197.90


Minimizing the Errors
The data shows that small mismatches will degrade the accuracy severely, and that well matched current sources and switches should be used to improve performance.

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 IOUT1IOUT2, 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.

Conclusion
Swapping the excitation current sources within the AD7792/AD7793 can improve accuracy in a multiplexed RTD measurement circuit. Calculations show the importance of mismatches between current sources and wire resistances.

References
Kester, Walt, James Bryant, and Walt Jung. “Temperature Sensors.” Sensor Signal Conditioning, Section 7. Analog Devices, Inc., 1999.

Author
Henry He [henry.he@analog.com] joined Analog Devices in 2012 as a field application engineer in Beijing, China. Prior to joining ADI, Henry worked for GE Energy and SUPCON as a hardware engineer. He received his BS and MS from Zhejiang University, both in industrial automation.
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