In Figure 7-1, the RTD can be located remotely simply by extending the two connections to the RTD. With this remote 2-wire connection to the RTD, line resistance introduces error. This error can be partially corrected by adjusting the values of R_Z, R_G, and R_LIN1.

A better method for remotely located RTDs is the 3-wire RTD connection (see Figure 6-3). This circuit offers improved accuracy. R_Z’s current is routed through a third wire to the RTD. Assuming line resistance is equal in RTD lines 1 and 2, this produces a small common-mode voltage that is rejected by the XTR105. A second resistor, R_LIN2, is required for linearization.

Note that although the 2-wire and 3-wire RTD connection circuits are very similar, the gain-setting resistor, R_G, has slightly different equations:

where

• R_Z = RTD resistance at T_MIN
• R₁ = RTD resistance at (T_MIN + T_MAX) / 2
• R₂ = RTD resistance at T_MAX

To maintain good accuracy, use at least 1% (or better) resistors for R_G. Table 7-1 provides standard 1% R_G resistor values for a 3-wire Pt100 RTD connection with linearization.