National Instruments DAQCard-1200 User Manual

Also see for DAQCard-1200: User manual

Contents

National Instruments DAQCard-1200 User Manual Manual pdf 28 page image

Chapter 3 Signal ConnectionsDAQCard-1200 User Manual 3-6 © National Instruments CorporationThe instrumentation amplifier applies gain, common-mode voltagerejection, and high-input impedance to the analog input signals connectedto the DAQCard-1200. Signals are routed to the positive and negativeinputs of the instrumentation amplifier through input multiplexers on theDAQCard-1200. The instrumentation amplifier converts two input signalsto a signal that is the difference between the two input signals multiplied bythe gain setting of the amplifier. The amplifier output voltage is referencedto the DAQCard-1200 ground. The DAQCard-1200 ADC measures thisoutput voltage when it performs A/D conversions.All signals must be referenced to ground, either at the source device or atthe DAQCard-1200. If you have a floating source, you must use aground-referenced input connection at the DAQCard-1200. If you have agrounded source, you must use a nonreferenced input connection at theDAQCard-1200.Types of Signal SourcesWhen configuring the input mode of the DAQCard-1200 and makingsignal connections, you must first determine whether the signal source isfloating or ground referenced. This section describes these two types ofsignals.Floating Signal SourcesA floating signal source is not connected in any way to the building groundsystem but has an isolated ground-reference point. Some examples offloating signal sources are outputs of transformers, thermocouples,battery-powered devices, optical isolator outputs, and isolation amplifiers.You must tie the ground reference of a floating signal to theDAQCard-1200 analog input ground to establish a local or onboardreference for the signal. Otherwise, the measured input signal varies orappears to float. An instrument or device that supplies an isolated outputfalls into the floating signal source category.Ground-Referenced Signal SourcesA ground-referenced signal source is connected in some way to thebuilding system ground and is therefore already connected to a commonground point with respect to the DAQCard-1200, assuming that the PC isplugged into the same power system. Nonisolated outputs of instrumentsand devices that plug into the building power system fall into this category.