Remote 3-wire RTD temperature measurement via 4-20mA current loop

The loop uses the XTR112 4-20mA current transmitter to measure the temperature of a remote 3-wire RTD. This device provides two matched current sources for RTD excitation and linear impedance compensation. The built-in linearization circuit provides a 2nd order correction term to correct the RTD, achieving a 40:1 linearity improvement. IR1 acts as the excitation for the RTD. IR2 acts as the compensation current flowing through RZ and RLINE1. By selecting the RZ resistor value to be the same as the resistance at the lowest temperature of the RTD, the built-in instrumentation amplifier (INA) is dedicated to measuring the temperature depending on the difference in RTD impedance.

RCM is used to provide an additional voltage drop to bias the input of the XTR112 to keep it within the common-mode input range. Its 0.01μF bypass capacitor minimizes common-mode noise. RG is used to set the INA gain. A small portion of the INA output voltage is fed back through resistors RLIN1 and RLIN2 for second-order linearization. This output voltage is converted to a current internally and then added to the return current IRET to produce an output current of IO = 4mA + VIN·40/RG.

In the current loop section, transistor Q1 conducts most of the signal-dependent 4-20mA loop current. This isolates most of the power dissipation from the internal high-precision circuitry, maintaining excellent accuracy. For detailed resistor value calculations for different temperature ranges, please refer to the XTR112 data sheet.

The diagram of remote positioning RTD temperature measurement is as follows: