National Instruments Multifunction RIO NI R Series, Руководство пользователя

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Chapter 2 Hardware Overview of the NI 78xxR
R Series Multifunction RIO User Manual 2-8 ni.com
Types of Signal Sources
When config
u
ring the inp
u
t channels and making signal connections,
yo
u
m
u
st first determine whether the signal so
u
rces are floating or gro
u
nd
referenced. The following sections describe these two signal types.
Floating Signal Sources
A floating signal so
u
rce is not connected to the b
u
ilding gro
u
nd system b
u
t
instead 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 isolator o
u
tp
u
ts, 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.
Yo
u
m
u
st connect the gro
u
nd reference of a floating signal to the
NI 783 x R/784 x R/785 x R AIGND thro
u
gh a bias resistor to establish a local
or onboard reference for the signal. Otherwise, the meas
u
red inp
u
t signal
varies as the so
u
rce floats o
u
t of the common-mode inp
u
t range.
Ground-Referenced Signal Sources
A gro
u
nd-referenced signal so
u
rce is connected to the b
u
ilding system
gro
u
nd, so it is already connected to a common gro
u
nd point with respect
to the NI 783 x R/784 x R/785 x R, ass
u
ming that the comp
u
ter is pl
u
gged into
the same power system. Instr
u
ments or devices with nonisolated o
u
tp
u
ts
that pl
u
g into the b
u
ilding power system are gro
u
nd referenced signal
so
u
rces.
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. This
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 improperly meas
u
red, this
difference might appear as a meas
u
rement error. The connection
instr
u
ctions for gro
u
nded signal so
u
rces are designed to eliminate this
gro
u
nd potential difference from the meas
u
red signal.
Input Modes
The following sections disc
u
ss single-ended and differential meas
u
rements
and considerations for meas
u
ring both floating and gro
u
nd-referenced
signal so
u
rces.