Ground-Referenced Signal Sources
A ground-referenced signal source is connected to the building system ground, so it is already connected to a common ground point with respect to the device, assuming that the computer is plugged into the same power system as the source. Nonisolated outputs of instruments and devices that plug into the building power system fall into this category.
The difference in ground potential between two instruments connected to the same building power system is typically between 1 mV and 100 mV, but the difference can be much higher if power distribution circuits are improperly connected. If a grounded signal source is incorrectly measured, this difference can appear as measurement error. The connection instructions for grounded signal sources eliminate this ground potential difference from the measured signal.