® FOUR-WIRE RTD CURRENT-LOOP TRANSMITTER: Four-Wire Connections to an RTD Allow the RTD to be Remotely Located from Active Circuitry, Yet Maintain Accuracy by David Jones, and Mark Stitt XTR105 is an application-specific IC containing onboard transducer excitation, signal conditioning instrumentation amplifier, linearization circuitry, and two-wire current-loop output on a single chip. Excitation consists of two on-chip current sources. One current source excites the RTD and a second current source drives a reference resistor to set system “zero.” Platinum Resistance Temperature Detectors (RTD)s are favored for demanding temperature measurement applications, such as process control, because of their excellent accuracy and sensor interchangeability. Combining an RTD with a Burr-Brown XTR105 forms a complete precision temperature to 4-to-20mA current-loop transmitter solution. Current-mode transmission gives superior noise rejection and is immune to errors resulting from series line resistance. XTR105 is a two-wire transmitter that uses the same wirepair for both signal and power supply, which reduces expensive wiring cost and simplifies installation. When the RTD is located near the XTR105, a basic “twowire” solution can be implemented, as shown in Figure 1. Errors due to wiring resistance are minimal and the XTR105 will provide good accuracy. However, when the RTD is RLIN IREF (2 ea.) 0.8mA 4-to-20mA +IN 14 13 1 12 3 10 9 RG XTR105 8 100Ω RTD 4 –IN 2 B E 0.01µF 7 6 RZ 100Ω RCM 2kΩ 0.01µF FIGURE 1. Basic RTD to 4-to-20mA Two-Wire Tansmitter Using XTR105. Internal 0.8mA current sources excite the RTD and zero-setting resistor. RCM provides additional voltage drop to set the XTR105 inputs within their valid common-mode input range. Optional resistor, RLIN can be used for linearization of the RTD. See the XTR105 data sheet for additional details. The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems. © 1999 Burr-Brown Corporation AB-151 Printed in U.S.A. October, 1999 located remotely, wiring resistance adds to the RTD impedance and can cause significant error. The “three-wire” connection (shown in Figure 2) mitigates the problem, but depends on matching wiring resistance in RW1 and RW2 to achieve best accuracy. Applications where matching wiring resistance is not practical can use the “four-wire” connection shown in Figure 3. The voltage across the RTD is accurately measured through RW2 and RW3. Since these connect to high-impedance amplifier inputs, the small bias currents flowing in these connections produce little error. Buffer amplifier, A1, drives the zero- set resistor, RZ, which is excited by the second current source at XTR105 (pin 14). Power to A1 is provided by a 5.1V voltage regulator, internal to the XTR105. All currents return through pin 6, assuring current-loop accuracy. In the “four-wire” solution, the RTD is connected remotely through four wires, designated RW1 to RW4. RTD excitation current from XTR105 (pin 1) flows to the RTD through RW1 and RW4. Since excitation is derived from a high impedance current source, voltage drops in the wiring do not substantially affect the accuracy of the current delivered to the RTD. System span is determined by RG. Adjust RZ and RG to set system zero and span according to the procedures described in the XTR105 data sheet. If linearization is desired, use the calculations described in the data sheet and subtract the wire resistance, RW1, from the external linearization resistor. RLIN RLIN RW1(1) IREF (2 ea.) 0.8mA 4-to-20mA +IN 14 13 1 12 3 3-Wire Connection 10 9 RG XTR105 8 100Ω RTD 4 –IN RW2 (1) 2 B E 0.01µF 7 6 RZ 100Ω RCM 2kΩ RW3(1) 0.01µF NOTE: (1) Wiring resistance. FIGURE 2. Three-Wire RTD to 4-to-20mA, Two-Wire Transmitter using XTR105. When the RTD is located remotely, this “three-wire” solution can improve accuracy. For linearization, a second resistor is required with this solution. See the XTR105 data sheet for additional details. 2 RLIN RW1(1) IREF (2 ea.) 0.8mA 4-to-20mA RW2(1) +IN 14 13 1 12 3 10 9 RG XTR105 8 4 –IN 100Ω RTD VREG 4-Wire Connection 2 11 B E 0.01µF 7 6 RZ 100Ω RW3(1) RW4(1) A1 OPA234 RCM 2kΩ 0.01µF NOTE: (1) Wiring resistance. FIGURE 3. Four-Wire RTD to 4-to-20mA, Two-Wire Transmitter using XTR105. When remote location distances are great and wiring resistance mismatch can be substantial, better precision is possible with a “four-wire” solution. 3