National Instruments E Series User Manual

Also see for E Series: Programmer's manual User manual User manual User manual User manual

Contents

National Instruments E Series User Manual Manual pdf 48 page image

Chapter 4 Signal ConnectionsDAQCard E Series User Manual4-12 National Instruments CorporationTypes of Signal SourcesWhen configuring the input channels and making signal connections,you must first determine whether the signal sources are floating orground-referenced. The following sections describe these two types ofsignals.Floating Signal SourcesA floating signal source is one that is not connected in any way to thebuilding ground system but, rather, has an isolated ground-referencepoint. Some examples of floating signal sources are outputs oftransformers, thermocouples, battery-powered devices, optical isolatoroutput, and isolation amplifiers. An instrument or device that has anisolated output is a floating signal source. You must tie the groundreference of a floating signal to your DAQCard analog input ground toestablish a local or onboard reference for the signal. Otherwise, themeasured input signal varies as the source floats out of thecommon-mode input range.Ground-Referenced Signal SourcesA ground-referenced signal source is one that is connected in someway to the building system ground and is, therefore, already connectedto a common ground point with respect to the DAQCard, assuming thatthe computer is plugged into the same power system. Nonisolatedoutput of instruments and devices that plug into the building powersystem falls into this category.The difference in ground potential between two instruments connectedto the same building power system is typically between 1 and 100 mVbut can be much higher if power distribution circuits are not properlyconnected. If a grounded signal source is improperly measured, thisdifference may appear as an error in the measurement. The connectioninstructions for grounded signal sources are designed to eliminate thisground potential difference from the measured signal.Input ConfigurationsYou can configure your DAQCard for one of three input modes—NRSE, RSE, or DIFF. The following sections discuss the use ofsingle-ended and differential measurements and considerations formeasuring both floating and ground-referenced signal sources.