Chapter 3 Signal Connections© National Instruments Corporation 3-7 DAQCard-1200 User ManualThe difference in ground potential between two instruments connected tothe same building power system is typically between 1 mV and 100 mV butcan be much higher if power distribution circuits are not properlyconnected. The connection instructions that follow for grounded signalsources eliminate this ground potential difference from the measuredsignal.Note If you power both the DAQCard-1200 and your PC with a floating power source,such as a battery, your system may be floating with respect to earth ground. In thiscase, treat all of your signal sources as floating sources.Input ConfigurationsYou can configure the DAQCard-1200 for one of three input modes—RSE,NRSE, or DIFF. The following sections discuss the use of single-ended anddifferential measurements, and considerations for measuring both floatingand ground-referenced signal sources. Table 3-2 summarizes therecommended input configurations for both types of signal sources.Differential Connection Considerations (DIFF Configuration)Differential connections are those in which each DAQCard-1200 analoginput signal has its own reference signal or signal return path. Theseconnections are available when you configure the DAQCard-1200 inthe DIFF mode. Each input signal is tied to the positive input of theinstrumentation amplifier, and its reference signal, or return, is tied tothe negative input of the instrumentation amplifier.Table 3-2. Recommended Input Configurations for Ground-Referencedand Floating Signal SourcesType of Signal Recommended Input ConfigurationGround-Referenced(nonisolated outputs, plug-in instruments)DIFFNRSEFloating(batteries, thermocouples, isolated outputs)DIFF with bias resistorsRSE
