Connecting Ground-Referenced Signal Sources; What Are Ground-Referenced Signal Sources; When To Use Differential Connections With Ground-Referenced Signal Sources - National Instruments DAQ X Series User Manual

Chapter 4 Analog Input

Connecting Ground-Referenced Signal Sources

What Are 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.
When to Use Differential Connections with
Ground-Referenced Signal Sources
Use DIFF input connections for any channel that meets any of the
following conditions:
•
•
•
•
•
DIFF signal connections reduce noise pickup and increase common-mode
noise rejection. DIFF signal connections also allow input signals to float
within the common-mode limits of the NI-PGIA.
Refer to the
Sources
X Series User Manual
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
The input signal is low level (less than 1 V).
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.
Using Differential Connections for Ground-Referenced Signal
section for more information about differential connections.
4-20 ni.com