16 | ni.com | NI USB-6001/6002/6003 User Guide
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 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
for grounded signal sources to eliminate this ground potential difference from the measured
signal.
For more information about field wiring and noise considerations for analog signals, go to
ni.com/info and enter Info Code rdfwn3.
When to Use Differential Connections with Ground-Referenced Signal
Sources
Use DIFF input connections for any channel that meets any of the following conditions:
• The input signal is low level and requires greater accuracy.
• 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 input 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 section for more information about differential
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 differential
connections instead.
As shown in the bottom right cell of Table 6, there can be a potential difference between AI GND
and the ground of the sensor. In RSE mode, this ground loop causes measurement errors.
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