National Instruments BNC-2090A User Manual

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National Instruments BNC-2090A User Manual Manual pdf 18 page image

Chapter 2 Connecting Signals© National Instruments Corporation 2-3 BNC-2090A User Manualoptical isolators, and isolation amplifiers. The ground reference of afloating source must be tied to the ground of the DAQ device to establish alocal or on-device reference for the signal.To provide a return path for the instrumentation amplifier bias currents,floating sources must have a 10–100 kΩ resistor to AI GND on one inputif DC-coupled, or both inputs if AC-coupled. For more information aboutconnections to floating signal sources and differential inputs, refer toE Series User Manual or the M Series User Manual. Refer to the InstallingBias Resistors section of Chapter 3, Signal Conditioning, for informationabout installing bias resistors.Measuring Ground-Referenced Signals inDifferential Input ModeA grounded signal source is connected in some way to the building systemground; therefore, the signal source is already connected to a commonground point with respect to the DAQ device (assuming the host computeris plugged into the same power system). The non-isolated outputs ofinstruments and devices that plug into the building power system fall intothis category.If the DAQ device is configured for differential inputs, ground-referencedsignal sources connected to the BNC-2090A need no special componentsadded to the BNC-2090A. You can leave the inputs of the BNC-2090A inthe factory-default configuration, with the 0 Ω jumpers in the two seriespositions, C and D, as listed in Table 3-1, Channel Component Positions.Refer to Chapter 3, Signal Conditioning, for information about buildingsignal-conditioning circuitry, such as filters and attenuators, in theopen-component positions.Connecting Single-Ended Analog Input SignalsComplete the following steps to measure a single-ended (SE) analog inputsignal.1. Connect the BNC cable to one of the AI <0..15> BNC connectors onthe front panel.2. Move the corresponding SE/DIFF switch to the SE position. On theBNC-2090A front panel, a line indicates which SE/DIFF switchcorresponds to each AI <0..15> BNC connector.3. Move the RSE/NRSE switch to select how the ground signal is routed.All single-ended signals share the one RSE/NRSE switch. For moreinformation about non-referenced single-ended (NRSE) and