Pt100 RTD Current Loop Transmitter Using XTR105

XTR105 is a 4-20mA current loop transmitter integrated circuit, with two precision current sources. The Instrumentation amplifier gain inside this chip can be configured for a wide range of temperature or pressure measurements. This integrated circuit chip provides current excitation for platinum RTD temperature sensors and bridges, instrumentation amplifiers, and current output circuitry on a single integrated circuit.

Two matched 0.8mA current sources drive the RTD and zero-setting resistor, RZ. The instrumentation amplifier input of the XTR105 measures the voltage difference between the RTD and RZ. The value of RZ is chosen to be equal to the resistance of the RTD at the low-scale (minimum) measurement temperature. RZ can be adjusted to achieve 4mA output at the minimum measurement temperature to correct for input offset voltage and reference current mismatch of the XTR105.

Transistor Q1 conducts the majority of the signal-dependent 4-20mA loop current. Using an external transistor isolates the majority of the power dissipation from the precision input and reference circuitry of the XTR105, maintaining excellent accuracy. Since the external transistor is inside a feedback loop, its characteristics are not critical. Requirements are: VCEO = 45V min, ? = 40 min, and PD = 800mW. Power dissipation requirements may be lower if the loop power-supply voltage is less than 36V.

RCM provides an additional voltage drop to bias the inputs of the XTR105 within their common-mode input range. RCM should be bypassed with a 0.01µF capacitor to minimize common-mode noise. Resistor RG sets the gain of the instrumentation amplifier according to the desired temperature range. RLIN1 provides 2nd-order linearization correction to the RTD, typically achieving a 40:1 improvement in linearity.

A resistor and a 7.5-36V power supply could be enough for the powered current loop receiver, but a better current loop receiver with RCV420 chip is recommended. [Circuit’s schematic diagram source: Texas Instruments Application Notes]