The sensor interface XTR112U and XTR114 are monolithic 4-20mA two-wire current transmitters. They provide a complete current excitation high impedance platinum RTD temperature sensor and bridge, instrumentation amplifier, current output circuit in one integrated circuit. The 250µa sensor interface XTR112U has two current sources and the XTR114 has two 100µa RTD excitations. The general purpose linearizer provides a 2nd order corrected RTD, typically achieving a 40:1 linearity improvement. The instrumentation amplifier gain can be configured for a wide temperature or pressure measurement range. The total unadjusted error of the entire current transmitter is low enough to be used without adjustment in many applications. This includes zero output current drift, a duty cycle supply voltage down to 7.5V across drift and nonlinear sensor interfaces XTR112U and XTR114. Both are available in the SO-14 surface mount package and are specified for use in the -40°C to +85°C industrial temperature range.

Basic connection diagram for sensor interface XTR112U and XTR114. The loop power supply VPS provides power to all circuits. The output loop current is measured as the voltage across the series load resistance (RL). Two matched current sources drive RTD and zero resistance resistor RZ. These current sources are the 250µA sensor interface XTR112U and the 100µA XTR114. Their instrumentation amplifier input measures the voltage difference between 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 the mismatch between the input bias voltage and reference current of the sensor interface XTR112U and XTR114. The RCM provides additional voltage drop to bias the input of the XTR112 and XTR114 within the common-mode input range. The RCM should bypass the 0.01µf capacitor to minimize common mode noise. Resistor RG sets the gain of the instrumentation amplifier based on the desired temperature range. RLIN1 provides second-order linearization correction for RTDs, typically achieving a 40:1 improvement in linearity. For a three-wire RTD connection, an additional resistor is required, see Figure 3.

The transfer function through the complete instrumentation amplifier and voltage-to-current converter is:

IO = 4mA + VIN?(40/RG) (VIN is in volts and RG is in ohms) where VIN is the differential input voltage. It is obvious from the transfer function that if RG is not used, the gain is zero and the output is only zero current for XTR. The RG value varies slightly for linearized two-wire RTD and three-wire RTD connections. RG can be calculated from the equations given in Figure 1 (two-wire RTD connection) and Table I (three-wire RTD connection). The IRET pin is the current source and return path for all current in VREG. The IRET pin allows any current used in external circuitry to be sensed by the XTR112 and XTR114 and included in the output current without causing errors. The VREG pin provides an on-chip voltage source of approximately 5.1V, suitable for powering external input circuits (see Figure 6). The VREG is capable of supplying approximately 2.1mA to the XTR112 and 2.4mA to the XTR114. Exceeding these values may affect the 4mA zero output. Both products sink about 1mA