Chapter 4
Analog Input
©
National Instruments Corporation
4-25
Connecting Floating Signal Sources
What Are Floating Signal Sources?
A floating signal so
u
rce is not connected to the b
u
ilding gro
u
nd system, b
u
t
has an isolated gro
u
nd-reference point. Some examples of floating signal
so
u
rces are o
u
tp
u
ts of transformers, thermoco
u
ples, battery-powered
devices, optical isolators, and isolation amplifiers. An instr
u
ment or device
that has an isolated o
u
tp
u
t is a floating signal so
u
rce.
When to Use Differential Connections with Floating
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 for the signal.
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 Floating Signal Sources
section for more information abo
u
t differential connections.
When to Use Referenced Single-Ended Connections
with Floating Signal Sources
Only
u
se referenced single-ended inp
u
t connections if the inp
u
t signal
meets the following conditions:
•
The inp
u
t signal can share a common reference point, AI GND, with
other signals that
u
se referenced single-ended.
•
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).
