How to Determine If an Input Signal Is Ground-Referenced or Floating

Updated Aug 2, 2018

Issue Details

I do not know if my input signal is a ground-referenced signal source or a floating signal source. How can I determine this?

Solution

  • A ground-referenced signal source is connected in some way to the building ground and is, therefore, already connected to a common ground point with respect to the data acquisition (DAQ) device (assuming that the computer is plugged into the same power system.) Non-isolated outputs of instruments and devices that plug into the building power system fall into this category.
  • A floating signal source is not connected in any way to the building ground system but, it has an isolated ground-reference point. Some examples of floating signal sources are outputs of transformers, thermocouples, battery-powered devices, optical isolator outputs, and isolation amplifiers. An instrument or device that has an isolated output is a floating signal source. You must connect the ground reference of a floating signal to your DAQ device analog input ground to establish a local or onboard reference for the signal. Otherwise, the measured input signal varies as the source floats outside the common-mode input range.

Additional Information

The difference in ground potential between two instruments connected to the same building power system is typically between 1 and 100 mV, but can be much higher if power distribution circuits are improperly connected. If a grounded signal source is incorrectly measured, this difference can appear as an error in the measurement. The connection instructions for grounded signal sources are designed to eliminate this ground potential difference from the measured signal. You can find these in the appropriate manual. For the main DAQ series you can find it in the Related Link section.
 

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