This three-part article series discusses the history and design challenges for designing a resistance temperature detector (RTD)-based temperature measurement system. In part one, we covered temperature measurement challenges, RTD types, different configurations, and the RTD configuration circuit. In this article, we cover the three different RTD configurations: 2-wire, 3-wire, and 4-wire.
4-Wire RTD Connection Diagram
A 4-wire RTD configuration offers the best performance. The only issue that system designers face is the cost of the sensor itself and the size of the 4-pin connector compared to the other two configurations. In this configuration, the errors due to the lead wires are inherently removed by the return wires. A 4-wire configuration uses Kelvin sensing with two wires to carry the excitation current to and from the RTD, while the remaining two wires sense the current across the RTD element itself. Errors due to lead resistance are inherently removed. A 4-wire configuration only requires one excitation current IOUT, as shown in Figure 1. Three analog pins from the ADC are used to implement a single 4-wire RTD configuration: one pin for excitation current, IOUT, and two pins as a fully differential input channel (AINP and AINM) used for sensing the voltage across the RTD.
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Figure 1. Single and multiple 4-wire RTD analog input configuration measurements. (Source: Analog Devices)
When the design uses multiple 4-wire RTDs, a single excitation current source can be used with the excitation current being directed to the different RTDs in the system. By placing the reference resistor on the low side of the RTD, a single reference resistor can support all the RTD measurements; that is, the reference resistor is shared by all RTDs. Note that the reference resistor can be placed on the high side or low side if the ADC’s reference input has wide common-mode range. So, for a single 4-wire RTD, either the reference resistor on the high side or low side can be used. However, when using multiple 4-wire RTDs in a system, placing the reference resistor on the low side is advantageous as one reference resistor can be shared by all RTDs. Note that some ADCs include reference buffers. These buffers may require some headroom, so a headroom resistor is then required if the buffer is enabled. Enabling the buffer means that more robust filtering can be connected to the reference pins without causing errors such as gain errors within the ADC.
2-Wire RTD Connection Diagram
The 2-wire RTD configuration is the simplest configuration and is shown in Figure 2. For the 2-wire configuration, only one excitation current source is required. Thus, three analog pins from the ADC are used to implement a single 2-wire RTD configuration: one pin for excitation current, IOUT, and two pins as a fully differential input channel (AINP and AINM) used for sensing the voltage across the RTD. When the design uses multiple 2-wire RTDs, a single excitation current source can be used with the excitation current being directed to the different RTDs in the system. By placing the reference resistor in the low side of the RTD as per the 4-wire configuration, a single reference resistor can support all the RTD measurements; that is, the reference resistor is shared by all RTDs.
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Figure 2. Single and multiple 2-wire RTD analog input configuration measurement. (Source: Analog Devices)
The 2-wire configuration is the least accurate of the three different wiring configurations since the actual resistance at the point of measurement includes both the resistances of the sensor and the lead wires RL1 and RL2, thus increasing the voltage measurement across the ADC. If the sensor is remote and the system uses a very long wire, then the errors will be significant. For example, a 25-foot length of a 24 AWG copper wire will have an equivalent resistance of 0.026 Ω/foot (0.08 Ω/meter) × 2 × 25 foot is to 1.3 Ω. Therefore, 1.3 Ω wire resistance produces an error of (1.3/0.385) = 3.38°C (approximately) due to wire resistance. The wire resistance also changes with temperature, which adds additional error.
3-Wire RTD Connection Diagram
The significant error due to lead-wire resistances of the 2-wire RTD configuration can be significantly improved by using a 3-wire RTD configuration. In this article, we use a second excitation current (shown in Figure 3) to cancel the lead-wire resistance errors produced by RL1 and RL2. Thus, four analog pins from the ADC are used to implement a single 3-wire RTD configuration: two pins for excitation currents (IOUT0 and IOUT1) and two pins as a fully differential input channel (AINP and AINM) used for sensing the voltage across the RTD.
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Figure 3. Single and multiple 3-wire RTD analog input configuration measurement. (Source: Analog Devices)
There are two ways to configure a 3-wire RTD circuit. Method 1 places the reference resistor on the top side so the first excitation current IOUT0 flows to RREF, RL1 then to RTD, and the second current flows through the RL2 lead resistance and develops a voltage that cancels the voltage dropped across the RL1 lead resistance. So, well matched excitation currents null the error due to the lead resistance completely. If the excitation currents have some mismatch, the impact of the mismatch is minimized using this configuration. The same current flows to the RTD and RREF; thus, any mismatch between the two IOUTs affects the lead resistance calculation only. This configuration is useful when measuring a single RTD.
When measuring multiple 3-wire RTDs, a reference resistor on the bottom side is recommended (Method 2) so only a single reference resistor can be used, which minimizes the overall cost. However, in this configuration, one current flows through the RTD while both currents flow through the reference resistor. So, any mismatch in IOUT can affect the value of the reference voltage along with the lead resistance cancellation. When excitation current mismatch is present, this configuration will have greater error than Method 1. There are two possible ways to calibrate the mismatch and mismatch drift between IOUT, hence improving the accuracy of the second configuration. First is calibrating by chopping (swapping) the excitation currents, performing a measurement on each phase, and then averaging the two measurements. Another solution is to measure the actual excitation currents themselves and then use the calculated mismatch to compensate for the mismatch in the microcontroller. More details regarding these calibrations are discussed in CN-0383.
In part 3, we’ll cover RTD system optimization, selection of external components, and how to evaluation the final RTD system.
|Jellenie Rodriguez is an applications engineer at Analog Devices within the Precision Converter Technology Group. Her focus is on precision sigma-delta ADCs for DC measurements. She joined ADI in 2012 and graduated from San Sebastian College-Recoletos de Cavite with a bachelor’s degree in electronics engineering in 2011. She can be reached at firstname.lastname@example.org.|
|Mary McCarthy is an applications engineer at Analog Devices. She joined ADI in 1991 and works in the Linear and Precision Technology Applications Group in Cork, Ireland, focusing on precision sigma-delta converters. Mary graduated with a bachelor’s degree in electronic and electrical engineering from University College Cork in 1991. She can be reached at email@example.com.|
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