In this article, we introduce three different RTD configurations: 2-wire, 3-wire and 4-wire.

4-wire RTD connection diagram

The 4-wire RTD configuration provides the best performance. Compared with the other two configurations, the only problems faced by the system designer are the cost of the sensor itself and the size of the 4-pin connector. In this configuration, errors caused by the lead wire are inherently eliminated by the return line. The 4-wire configuration uses Kelvin induction, which transmits the excitation current to and from the RTD through two wires, while the other two wires sense the current flowing through the RTD element itself. Errors due to lead resistance are inherently eliminated. The 4-wire configuration requires only one excitation current IOUT, as shown in Figure 1. Three analog pins of ADC are used to realize a single 4-wire RTD configuration: one pin is used to excite the current IOUT.

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Figure 1 Single and multiple 4-wire RTD analog input configuration measurements. (source: ADI)

When designing to use multiple 4-wire RTDs, a single excitation current source can be used, and the excitation current is directed to different RTDS in the system. By placing the reference resistor at the low end of RTD, a single reference resistor can support all RTD measurements; That is, the reference resistance is shared by all RTDS. Note that if the reference input of the ADC has a wide common mode range, the reference resistor can be placed on the high or low side. Therefore, for a single 4-wire RTD, a high-end or low-end reference resistor can be used. However, when multiple 4-wire RTDS are used in the system, it is advantageous to place the reference resistor at the low end, because all RTDS can share a reference resistor. Note that some ADCs contain reference buffers. These buffers may require some space, so a margin resistor is required if buffers are enabled. Enabling the buffer means that a more powerful filter can be connected to the reference pin without causing errors such as gain errors in the ADC.

2-wire RTD connection diagram

The 2-wire RTD configuration is the simplest configuration, as shown in Figure 2. For a 2-wire configuration, only one excitation current source is required. Therefore, three analog pins of ADC are used to realize a single 2-wire RTD configuration: one pin is used to excite current IOUT, and two pins are used as fully differential input channels (ainp and ainm) to detect the voltage at both ends of RTD. When designing to use multiple 2-wire RTDs, a single excitation current source can be used, and the excitation current is directed to different RTDS in the system. By placing the reference resistor at the low end of the RTD according to the 4-wire configuration, a single reference resistor can support all RTD measurements; That is, the reference resistance is shared by all RTDS.

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Figure 2 Single and multiple 2-wire RTD analog input configuration measurements. (source: ADI)

The 2-wire configuration has the lowest accuracy among the three different wiring configurations, because the actual resistance of the measurement point includes the resistance of the sensor and leads RL1 and RL2, so the voltage measurement value on the ADC is increased. If the sensor is remote and the system uses long wires, the error will be serious. For example, a 25 foot length of 24 AWG copper wire has an equivalent resistance of 0.026 Ω / foot (0.08 Ω / meter) × two × 25 feet to 1.3 Ω. Therefore, due to the wire resistance, the wire resistance of 1.3 Ω will produce an error of (1.3 / 0.385) = 3.38 ° C (approximately). The wire resistance will also vary with temperature, which will add additional error.

3-wire RTD connection diagram

Using a 3-wire RTD configuration can significantly improve the significant error caused by the lead resistance of a 2-wire RTD configuration. In this paper, we use the second excitation current (as shown in Figure 3) to eliminate the lead resistance error caused by RL1 and RL2. Therefore, the four analog pins of ADC are used to realize a single 3-wire RTD configuration: two pins are used to excite the current (iout0 and iout1), and two pins are used as fully differential input channels (ainp and ainm) to sense the voltage RTD at both ends.

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Figure 3 Single and multiple 3-wire RTD analog input configuration measurements. (source: ADI)

There are two ways to configure a 3-wire RTD circuit. Method 1 places the reference resistor on the top, so the first excitation current iout0 flows to R ref, RL1 and then RTD, and the second current flows through the RL2 lead resistance and generates a voltage to offset the voltage drop on the RL1 lead resistance. Therefore, the well matched excitation current completely eliminates the error caused by the lead resistance. If there is some mismatch in the excitation current, the use of this configuration can minimize the impact of the mismatch. The same current flows to RTD and R ref; Therefore, any mismatch between the two iouts will only affect the lead resistance calculation. This configuration is useful when measuring a single RTD.

When measuring multiple 3-wire RTDs, it is recommended to use a reference resistor at the bottom (method 2), so only a single reference resistor can be used to minimize the overall cost. However, in this configuration, one current flows through the RTD and both currents flow through the reference resistor. Therefore, any mismatch in IOUT will affect the value of reference voltage and the elimination of lead resistance. When there is excitation current mismatch, this configuration will have greater error than method 1. There is a mismatch between the two methods of IO configuration to improve the accuracy of UT calibration. The first is to calibrate by chopping (exchanging) excitation current, measure each phase, and then average the two measurements. Another solution is to measure the actual excitation current itself and then use the calculated mismatch to compensate for the mismatch in the microcontroller. For more details on these calibrations, see cn-0383.

Jellenie Rodriguez is an application engineer in the precision Converter Technology Department of ADI. Her focus is on precision for DC measurement Σ-Δ ADC。 In 2011, she graduated from sevice college with a bachelor’s degree in electronic engineering.

Mary McCarthy is an application engineer at ADI. She joined ADI in 1991 and worked in the linear and precision technology application department in cork, Ireland, focusing on precision sigma delta converters. Mary graduated from cork University in 1991 with a bachelor’s degree in electronic and electrical engineering.

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