National Instruments NI 4350 User Manual

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National Instruments NI 4350 User Manual Manual pdf 45 page image

Chapter 3 NI 4350 OperationNI 4350 User Manual3-24 ©National Instruments Corporationespecially useful when your NI 4350 instrument is operating at anambient temperature other than that of calibration (23° C typical). Usethe 4-wire mode in LabVIEW while reading the offset for resistancemeasurements.Note: When using VirtualBench-Logger along with NI 4350 accessories—PSH32-TC6, CB-27T, TC-2190, or TBX-68T—auto-zeroing isimplemented automatically.Programmable Ground-ReferencingAlways switch off ground-referencing on the channel connected to aresistor excited by the current source. The current source returnterminal IEX– is tied to –2.5 V through a resistor. This causes anyresistor excited by the current source to be ground-referenced.Otherwise, the leakage current flowing into the resistor can causeerroneous measurement for all channels that use the current source.Programmable Open-Thermocouple DetectionAlways switch off open-thermocouple detection on the channelconnected to a resistor. Otherwise, the leakage current flowing into theresistor can cause erroneous measurement for all channels that use thecurrent source.Connecting to External CircuitsSee Figure 3-10 for an example of how different transducers connect toanalog channels in the same measurement setup. To measure the valueof a resistor accurately, make sure the resistor is not electricallyconnected to any other circuits. Erroneous or misleading readings canresult if the resistor you are measuring is electrically connected toexternal circuits that supply voltages or currents or is connected toexternal circuits that change the effective resistance of that resistor.Two-Wire, Three-Wire, and Four-WireMeasurementsThe discussion in Connecting Your RTD on whether to use two-wire,three-wire, or four-wire, earlier in this chapter, applies to any resistancemeasurement. Choose the appropriate measurement technique for yourapplication as shown in Table 3-5.