Chapter 4 Analog Input© National Instruments Corporation 4-27 NI USB-621x User ManualUsing Differential Connections for Floating SignalSourcesIt is important to connect the negative lead of a floating source to AI GND(either directly or through a bias resistor). Otherwise, the source may floatout of the maximum working voltage range of the NI-PGIA and the DAQdevice returns erroneous data.The easiest way to reference the source to AI GND is to connect thepositive side of the signal to AI+ and connect the negative side of the signalto AI GND as well as to AI– without using resistors. This connection workswell for DC-coupled sources with low source impedance (less than 100 Ω).Figure 4-16. Differential Connections for Floating Signal Sourceswithout Bias ResistorsHowever, for larger source impedances, this connection leaves the DIFFsignal path significantly off balance. Noise that couples electrostaticallyonto the positive line does not couple onto the negative line because it isconnected to ground. This noise appears as a DIFF-mode signal instead ofa common-mode signal, and thus appears in your data. In this case, insteadof directly connecting the negative line to AI GND, connect the negativeline to AI GND through a resistor that is about 100 times the equivalentsource impedance. The resistor puts the signal path nearly in balance,so that about the same amount of noise couples onto both connections,yielding better rejection of electrostatically coupled noise. Thisconfiguration does not load down the source (other than the veryhigh input impedance of the NI-PGIA).–+Impedance<100 ΩAI GNDAI+AI–AI SENSEVsFloatingSignalSource
