Measuring Resistance with the NI 4351 and TBX-68THardware: Modular Instruments>>High-Precision DAQ (Dataloggers)>>PXI-4351
Problem: To measure resistance using an NI 4351 and a TBX-68T terminal block, what is the function of switches S1 and S2 on the TBX-68T? Is one switch for IEX0 and the other switch for IEX1? Also, how do I turn on the current excitation? Solution: The NI 4351 has two current sources (IEX0=25 uA, IEX1=1 mA) that can be used to measure current. Switches S1 and S2 route IEX0 to the channels or internally to the onboard cold-junction compensation (CJC) sensor. If you are not routing IEX0 to any channels for resistance measurements, switches S1 and S2 should both be in the OFF position. This configuration completes the circuit for the current so that an accurate reading can be made on the sensor. When measuring resistance, switches S1 and S2 should be in the ON position. Refer to Figure 2-10, Multiple Transducer Connections to Analog Channels in One Measurement Setup, Channels 0-5 in the NI 435x User Manual at ni.com/manuals for more information. Also be sure to terminate the current circuit in IEX0-. If you are obtaining bad readings when measuring resistance, then the switches are probably in the wrong position. IEX1 is constantly on and has no switches. Note Although using the 1 mA current source may yield greater accuracy, you must verify that this current source does not exceed the voltage limits of the NI 4351 and does not induce self-heating, which is common with thermistors. The other switches on the TBX-68T terminal block are for digital lines. Refer to the CB-27T, TBX-68T, and CB-68T Isothermal Terminal Blocks Installation Guide at ni.com/manuals for more information. Related Links: CB-27T, TBX-68T, AND CB-68T Isothermal Terminal Blocks Installation Guide NI 435x User Manual KnowledgeBase 2RCIL32H: How Can I verify that my Cold-Junction Temperature Sensor is Accurate? Attachments:
Report Date: 03/12/2003 Last Updated: 06/11/2004 Document ID: 2VB7682H |
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