Data Acquisition

Analog and digital signals 21Four-wire RTD measurementA better method of excitation and measurement, especially when the wire lead lengths aregreater than a few meters in length, is the four-wire RTD configuration shown in Figure2.8.Figure 2.8Four-wire RTD measurementsRTDs are commonly packaged with four (4) leads, two current leads to provide theexcitation current for the device, and two voltage leads for measurement of the voltagedeveloped. This configuration eliminates the voltage drops caused by excitation currentthrough the lead resistances (R L1 and R L4 ). Since negligible current flows in the voltagelead resistances, (R L2 and R L3 ) only the voltage drop across the resistance R T of the RTDis measured.Three-wire RTD measurementA reduction in cost is possible with the elimination of one of the wire leads. In the threewireconfiguration shown in Figure 2.9, only one lead R L1 adds an error to the RTDvoltage measured.Figure 2.9Three-wire RTD measurementsSelf-heatingAnother consequence of current excitation of the RTD is the possible effect that internalheating of the device may have on the accuracy of the actual temperature measurements