National Instruments USB-6008 User Manual page 15

• The leads connecting the signal to the device are greater than 3 m (10 ft).
• The input signal requires a separate ground-reference point or return signal.
• The signal leads travel through noisy environments.
• Two analog channels, AI+ and AI-, are available for the signal.
DIFF signal connections reduce noise pickup and increase common-mode noise rejection.
DIFF signal connections also allow input signals to float within the working voltage of the
device.
Refer to the Taking Differential Measurements
connections.
When to Use Referenced Single-Ended (RSE) Connections with Floating
Signal Sources
Only use RSE input connections if the input signal meets all of the following conditions:
• The input signal can share a common reference point, GND, with other signals that use
RSE.
• Your application permits the use of the ±10 V input range.
• The leads connecting the signal to the device are less than 3 m (10 ft).
DIFF input connections are recommended for greater signal integrity for any input signal that
does not meet the preceding conditions.
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 PGA 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 Taking Referenced Single-Ended Measurements
about RSE connections.
Ground-Referenced Signal Sources
A ground-referenced signal source is a signal source connected to the building system ground.
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. Non-isolated 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. Follow the connection instructions
14 | ni.com | NI USB-6008/6009 User Guide
section for more information about differential
section for more information