Floating Signal Voltage Reading Saturates to Full-Scale

Updated Mar 15, 2018

Reported In

Hardware

  • PCI-6220
  • PXI-6220
  • BNC-2121
  • BNC-2111
  • BNC-2120
  • SC-2050
  • SC-2345
  • Multifunction I/O Device

Software

  • DAQExpress

Driver

  • NI-DAQmx

Other

   

Issue Details

Why does my floating signal source measurement saturate to full-scale (positive or negative) voltage?

Solution

A variety of phenomena, such as the instrumentation amplifier input bias currents, can cause the voltage level of a floating source to go out of the valid measurable range of the DAQ board. To anchor this voltage level to some reference bias resistors (10k-100kOhm) should be used to provide a DC path from the instrumentation amplifier inputs to its ground.
 
  • For DC-coupled signals only 1 resistor is necessary and should be placed across the (-) terminal of the signal input and the measurement system ground (AIGND).
     
  • For AC-coupled signals, in addition to the (-) bias resistor mentioned above, another resistor should be placed across the (+) terminal of the signal input and AIGND.

Additional Information

Consult the User Manual that shipped with your DAQ board, or the provided link for online versions of the DAQ manuals, for additional information.

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