National Instruments Multifunction RIO NI R Series, User Manual

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Chapter 2 Hardware Overview of the NI 78xxR
© National Instruments Corporation 2-7 R Series Multifunction RIO User Manual
Connection of AI signals to the NI 783 x R/784 x R/785 x R depends on the
inp
u
t mode of the AI channels yo
u
are
u
sing and the type of inp
u
t signal
so
u
rce. With different inp
u
t modes, yo
u
can
u
se the instr
u
mentation
amplifier in different ways. Fig
u
re 2-4 shows a diagram of the
NI 783 x R/784 x R/785 x R instr
u
mentation amplifier.
Figure 2-4. NI 783 x R/784 x R/785 x R Instrumentation Amplifier
The instr
u
mentation amplifier applies common-mode voltage rejection
and presents high inp
u
t impedance to the AI signals connected to the
NI 783 x R/784 x R/785 x R. Inp
u
t m
u
ltiplexers on the device ro
u
te signals to
the positive and negative inp
u
ts of the instr
u
mentation amplifier. The
instr
u
mentation amplifier converts two inp
u
t signals to a signal that is the
difference between the two inp
u
t signals. The amplifier o
u
tp
u
t voltage is
referenced to the device gro
u
nd. The NI 783 x R/784 x R/785 x R ADC
meas
u
res this o
u
tp
u
t voltage when it performs A/D conversions.
Yo
u
m
u
st reference all signals to gro
u
nd either at the so
u
rce device or at the
NI 783 x R/784 x R/785 x R. If yo
u
have a floating so
u
rce, reference the signal
to gro
u
nd by
u
sing RSE inp
u
t mode or the DIFF inp
u
t mode with bias
resistors. Refer to the Differential Connections for Nonreferenced or
Floating Signal Sources section of this chapter for more information abo
u
t
these inp
u
t modes. If yo
u
have a gro
u
nded so
u
rce, do not reference the
signal to AIGND. Yo
u
can avoid this reference by
u
sing DIFF or NRSE
inp
u
t modes.
+
+
–
–
V
m
= [V
in+
– V
in–
]
V
in+
V
in–
V
m
Instrumentation
Amplifier
Measured
Voltage