Chapter 4
Analog Input
4-32
ni.com
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 differential 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.
Differential signal connections red
u
ce noise pick
u
p and increase
common-mode noise rejection. Differential 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
section for more information abo
u
t differential connections.
When to Use Non-Referenced Single-Ended
Connections with Ground-Referenced Signal
Sources
Only
u
se non-referenced single-ended inp
u
t connections if the inp
u
t signal
meets the following conditions:
•
The inp
u
t signal is high-level (greater than 1 V).
•
The leads connecting the signal to the device are less than 3 m (10 ft).
•
The inp
u
t signal can share a common reference point with other
signals.
Differential inp
u
t connections are recommended for greater signal integrity
for any inp
u
t signal that does not meet the preceding conditions.
