SBAS894A April 2018 – October 2018 ADS112C04
Implementing a 2- or 4-wire RTD measurement is very similar to the 3-wire RTD measurement illustrated in Figure 74, except that only one IDAC is required.
Figure 75 shows a typical circuit implementation of a 2-wire RTD measurement. The main difference compared to a 3-wire RTD measurement is with respect to the lead resistance compensation. The voltage drop across the lead resistors, RLEAD1 and RLEAD2, in this configuration is directly part of the measurement (as shown in Equation 27) because there is no means to compensate the lead resistance by use of the second current source. Any compensation must be done by calibration.
Figure 76 shows a typical circuit implementation of a 4-wire RTD measurement. Similar to the 2-wire RTD measurement, only one IDAC is required for exciting and measuring a 4-wire RTD in a ratiometric manner. The main benefit of using a 4-wire RTD is that the ADC inputs are connected to the RTD in the form of a Kelvin connection. Apart from the input leakage currents of the ADC, there is no current flow through the lead resistors RLEAD2 and RLEAD3 and therefore no voltage drop is created across them. The voltage at the ADC inputs consequently equals the voltage across the RTD and the lead resistance is of no concern.
As shown in Equation 28, the transfer function of a 2- and 4-wire RTD measurement differs compared to the one of a 3-wire RTD measurement by a factor of 2 because only one IDAC is used and only one IDAC flows through the reference resistor, RREF.
In addition, the input common-mode voltage and reference voltage is reduced compared to the 3-wire RTD configuration. Therefore, some further modifications may be required in case the 3-wire RTD design is used to measure 2- and 4-wire RTDs as well. If the decreased absolute input voltages does not meet the minimum absolute voltage requirements of the PGA anymore, either increase the value of RREF by switching in a larger resistor or, alternatively, increase the excitation current and decrease the gain at the same time.