Chapter 4 Theory of OperationSCXI-1503 User Manual 4-6 ni.comchanges in resistance, you must use special configurations that minimizemeasured errors caused by lead-wire resistance.RTD Measurement ErrorsBecause the RTD is a resistive device, you must pass a current through thedevice and monitor the resulting voltage. However, any resistance in thelead wires that connect the measurement system to the RTD adds error tothe readings. For example, consider a 2-wire RTD element connected to ameasurement system that also supplies a constant current, I EX, to excite theRTD. As shown in Figure 4-2, the voltage drop across the lead resistances(labeled R L) adds an error voltage to the measured voltage.Figure 4-2. 2-Wire RTD MeasurementFor example, a lead resistance of 0.3 Ω in each wire adds a 0.6 Ω error tothe resistance measurement. For a platinum RTD at 0 °C with α = 0.00385,the lead resistance equates to an error of approximatelyChapter 2, Connecting Signals, describes different ways of connectingresistive devices to the SCXI system.The Relationship Between Resistance andTemperature in RTDsCompared to other temperature-measurement devices, the output of anRTD is relatively linear with respect to temperature. The temperaturecoefficient, called alpha (α), differs between RTD curves. Althoughvarious manufacturers specify alpha differently, alpha is most commonlyR TRLRLIEX+–V 00.6 Ω0.385 Ω/°C----------------------------- 1.6 °C=
