National Instruments Data Acquisition Device NI USB-621x - Manual (Page 66)

Chapter 4 Analog Input
© National Instruments Corporation 4-33 NI USB-621x User Manual
In the single-ended modes, more electrostatic and magnetic noise couples
into the signal connections than in DIFF configurations. The coupling is the
result of differences in the signal path. Magnetic coupling is proportional
to the area between the two signal conductors. Electrical coupling is a
function of how much the electric field differs between the two conductors.
With this type of connection, the NI-PGIA rejects both the common-mode
noise in the signal and the ground potential difference between the signal
source and the device ground.
Refer to the Using Non-Referenced Single-Ended (NRSE) Connections for
Ground-Referenced Signal Sources section for more information about
NRSE connections.
When to Use Referenced Single-Ended (RSE)
Connections with Ground-Referenced Signal
Sources
Do not use RSE connections with ground-referenced signal sources.
Use NRSE or DIFF connections instead.
As shown in the bottom-rightmost cell of Table 4-2, there can be a potential
difference between AI GND and the ground of the sensor. In RSE mode,
this ground loop causes measurement errors.
Using Differential Connections for
Ground-Referenced Signal Sources
Figure 4-22 shows how to connect a ground-referenced signal source to the
USB-6210/6211/6212 device configured in DIFF mode.