Figure 7-1 shows the basic connection diagram for the XTR105. The loop power supply, V_PS, provides power for all circuitry. Output loop current is measured as a voltage across the series load resistor, R_L.

Two matched 0.8mA current sources drive the RTD and zero-setting resistor, R_Z. The instrumentation amplifier input of the XTR105 measures the voltage difference between the RTD and R_Z. The value of R_Z is chosen to be equal to the resistance of the RTD at the low-scale (minimum) measurement temperature. R_Z 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.

R_CM provides an additional voltage drop to bias the inputs of the XTR105 within the common-mode input range. Bypass R_CM with a 0.01μF capacitor to minimize common-mode noise. Resistor R_G sets the gain of the instrumentation amplifier according to the desired temperature range. R_LIN1 provides 2nd-order linearization correction to the RTD, typically achieving a 40:1 improvement in linearity. An additional resistor is required for 3-wire RTD connections (see Figure 6-3).

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

I_O = 4mA + V_IN × (40 / R_G)

(V_IN in volts, R_G in ohms)

where V_IN is the differential input voltage.

A negative input voltage, V_IN, causes the output current to be less than 4mA. Increasingly negative V_IN causes the output current to limit at approximately 2.2mA. See also typical characteristic Under-Scale Current vs Temperature.

Increasingly positive input voltage (greater than the full-scale input) produces increasing output current according to the transfer function, up to the output current limit of approximately 27mA. See also typical characteristic Over-Scale Current vs Temperature.

As evident from the transfer function, if no R_G is used the gain is zero and the output is simply the XTR105’s zero current. The value of R_G varies slightly for 2-wire RTD and 3-wire RTD connections with linearization. R_G can be calculated from the equations given in Figure 7-1 (2-wire RTD connection) and Table 7-1 (3-wire RTD connection).

The I_RET pin is the return path for all current from the current sources and V_REG. The I_RET pin allows any current used in external circuitry to be sensed by the XTR105 and to be included in the output current without causing an error.

Figure 7-1 Basic 2-Wire RTD Temperature Measurement Circuit With Linearization

Table 7-1 R_Z, R_G, R_LIN1, and R_LIN2 Standard 1% Resistor Values for 3-Wire Pt100 RTD Connection With Linearization