National Instruments NI 784xR User Manual

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Contents

National Instruments NI 784xR User Manual Manual pdf 29 page image

Chapter 2 Hardware Overview of the NI 78xxRR Series Intelligent DAQ User Manual 2-8 ni.comTypes of Signal SourcesWhen configuring the input channels and making signal connections,you must first determine whether the signal sources are floating or groundreferenced. The following sections describe these two signal types.Floating Signal SourcesA floating signal source is not connected to the building ground system butinstead has an isolated ground-reference point. Some examples of floatingsignal sources are outputs of transformers, thermocouples, battery-powereddevices, optical isolator outputs, and isolation amplifiers. An instrumentor device that has an isolated output is a floating signal source.You must connect the ground reference of a floating signal to theNI 783xR/784xR/785xR AIGND through a bias resistor to establish a localor onboard reference for the signal. Otherwise, the measured input signalvaries as the source floats out of the common-mode input range.Ground-Referenced Signal SourcesA ground-referenced signal source is connected to the building systemground, so it is already connected to a common ground point with respectto the NI 783xR/784xR/785xR, assuming that the computer is plugged intothe same power system. Instruments or devices with nonisolated outputsthat plug into the building power system are ground referenced signalsources.The difference in ground potential between two instruments connected tothe same building power system is typically between 1 and 100 mV. Thisdifference can be much higher if power distribution circuits are improperlyconnected. If a grounded signal source is improperly measured, thisdifference might appear as a measurement error. The connectioninstructions for grounded signal sources are designed to eliminate thisground potential difference from the measured signal.Input ModesThe following sections discuss single-ended and differential measurementsand considerations for measuring both floating and ground-referencedsignal sources.