Error minimization of multiplexed 3-wire RTD data acquisition systems
“Resistance temperature detectors (RTDs) monitor 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 the temperature of many remotely installed RTDs. In high-performance applications, each RTD with its own excitation circuitry and ADC provides the best accuracy, but the data acquisition module will be bulky, expensive, and power-hungry.
Two-wire, three-wire, and four-wire RTD configurations are available, with the two-wire configuration having the lowest device cost and the four-wire device having the best accuracy. Typically used in industrial applications, 3-wire RTDs can be excited with two identical current sources to eliminate pin resistance. When used with precision reference resistors, current source errors do not affect measurement accuracy. High-performance ADCs such as AD7792 and AD7793 integrate excitation current sources for high-precision RTD measurements.
Figure 1 shows an on-chip current source exciting two 3-wire RTDs. RTD channels can be selected by a multiplexer such as the ADG5433 high voltage, latch-up proof, triple SPDT switch.
Figure 1. Two 3-Wire RTDs Multiplexed to One AD7792/AD7793 ADC
Only one RTD can be measured at a time. S1A, S1B, and S1C close to measure RTD #1; S2A, S2B, and S3B close to measure RTD #2. A single ADG5433 can switch two 3-wire RTDs; additional multiplexers can be added to handle more than two sensors. RLXXIndicates the resistance introduced between the RTD and the measurement system due to excessive wire length and the on-resistance of the switch.
Calculate RTD Resistance
Since S1A, S1B, and S1C measure RTD #1 in a closed loop, the RTD resistance can be calculated as:
Therefore, the measured value depends only on RREFThe value (and precision) of . But remember, we assume IOUT1 = IOUT2 and RL1A = RL1B = RL1C. In fact, these current and resistance mismatches are a major source of measurement error.
Effects of Current Source and Line Resistance Mismatch
Next, assume that the two current sources are mismatched, say IOUT2 = (1 + x) IOUT1. Now, consider the following situation:
Note that mismatches cause offset errors as well as gain errors. Offset error is related to the mismatch between the two pin resistances, while gain error is related to the mismatch between the two current sources. If these mismatches are not taken into account, the RTD resistance value calculated from the ADC’s data reading will be inaccurate.
Taking a 200 Ω RTD as an example, Table 1 shows the values obtained without considering the mismatch; where RREF = 1000 Ω, IOUT1 = 1 mA, IOUT2 > IOUT1 (displayed as a percentage), RL1A = 10 Ω, RL1C > RL1A (displayed as resistance value).
Table 1. RTD Measurements Without Mismatch
The data shows that small mismatches can severely impact accuracy, so 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 resistor mismatch can be easily calibrated. However, the mismatch changes with temperature, which makes compensation difficult. Therefore, the device selected should have low drift over temperature.
If IOUT1 ≠ IOUT2and the current source is connected as shown:
Suppose we exchange IOUT1 and IOUT2so that IOUT1 Connect VINC and IOUT2 and connect VIN+:
Now, if we sum the conversion results, and the current sources are connected in the original direction, while swapping the current sources on the second conversion, we get:
Note that the measurement is now independent of the current source mismatch. The only downside is the loss of speed, as each RTD calculation goes through two conversions.
The AD7792 and AD7793 are designed for this application. As shown in Figure 2, an integrated switch simplifies the swapping of current sources to output pins by writing to the I/O registers.
Figure 2. AD7792/AD7793 Functional Block Diagram
Swapping the excitation current sources within the AD7792/AD7793 devices improves the accuracy of the multiplexed RTD measurement circuit.Calculations show the importance of the mismatch between the current source and the line resistance
Kester, Walt, James Bryant, and Walt Jung. “Temperature Sensors.” Sensor Signal Conditioning, Section 7. Analog Devices, Inc., 1999.