Chapter 4
Analog Input
X Series User Manual
4-20
ni.com
Connecting Ground-Referenced Signal Sources
What Are Ground-Referenced Signal Sources?
A gro
u
nd-referenced signal so
u
rce is a signal so
u
rce connected to the
b
u
ilding system gro
u
nd. It is already connected to a common gro
u
nd point
with respect to the device, ass
u
ming that the comp
u
ter is pl
u
gged into the
same power system as the so
u
rce. Non-isolated o
u
tp
u
ts of instr
u
ments and
devices that pl
u
g into the b
u
ilding power system fall into this category.
The difference in gro
u
nd potential between two instr
u
ments connected to
the same b
u
ilding power system is typically between 1 and 100 mV, b
u
t the
difference can be m
u
ch higher if power distrib
u
tion circ
u
its are improperly
connected. If a gro
u
nded signal so
u
rce is incorrectly meas
u
red, this
difference can appear as meas
u
rement error. Follow the connection
instr
u
ctions for gro
u
nded signal so
u
rces to eliminate this gro
u
nd potential
difference from the meas
u
red signal.
When to Use Differential Connections with
Ground-Referenced Signal Sources
Use DIFF inp
u
t connections for any channel that meets any of the
following conditions:
•
The inp
u
t signal is low level (less than 1 V).
•
The leads connecting the signal to the device are greater than 3 m
(10 ft).
•
The inp
u
t signal req
u
ires a separate gro
u
nd-reference point or ret
u
rn
signal.
•
The signal leads travel thro
u
gh noisy environments.
•
Two analog inp
u
t channels, AI+ and AI–, are available.
DIFF signal connections red
u
ce noise pick
u
p and increase common-mode
noise rejection. DIFF signal connections also allow inp
u
t signals to float
within the common-mode limits of the NI-PGIA.
Refer to the
Using Differential Connections for Ground-Referenced Signal
Sources
section for more information abo
u
t differential connections.
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