In the three-wire configuration, the matched IDAC1 and IDAC2 excitation currents flow though the wire resistances connecting the RTD sensor to the inputs of the ADC. Given that typically the lead wire resistances are equal, and the excitation current sources are closely matched, the errors due to the line series resistance cancel; this configurations allows the RTD sensor to be remotely placed away from the ADC.

In noisy industrial environments, where the sensor wiring is prone to noise interference, the addition of low-pass RC filters can be beneficial. However, when adding the filter resistors in the excitation current path, the input common-mode range is exceeded and the drift and mismatch of the resistors results in errors. The solution is to connect the IDAC excitation current in the configuration as shown in Figure 3-5. The IDAC current is sourced from another set of unused input channels or in the case of the ADS1248, from the IEXC1 and IEXC2 pins. In this configuration, the series resistors of the low-pass filter are outside the excitation current path.

Figure 3-5 Example of a Three-Wire Ratiometric Configuration with RC Filters