Chapter 2
Hardware Overview of the NI 78xxR
R Series Multifunction RIO User Manual
2-12
ni.com
Differential Connections for Nonreferenced or
Floating Signal Sources
Fig
u
re 2-7 shows how to connect a floating signal so
u
rce to a channel on
the NI 783
x
R/784
x
R/785
x
R config
u
red in DIFF inp
u
t mode.
Figure 2-7.
Differential Input Connections for Nonreferenced Signals
Fig
u
re 2-7 shows two bias resistors connected in parallel with the signal
leads of a floating signal so
u
rce. If yo
u
do not
u
se the resistors and the
so
u
rce is tr
u
ly floating, the so
u
rce might not remain within the
common-mode signal range of the instr
u
mentation amplifier, ca
u
sing
erroneo
u
s readings. Yo
u
m
u
st reference the so
u
rce to AIGND by
connecting the positive side of the signal to the positive inp
u
t of the
instr
u
mentation amplifier and connecting the negative side of the signal to
AIGND and to the negative inp
u
t of the instr
u
mentation amplifier witho
u
t
resistors. This connection works well for DC-co
u
pled so
u
rces with low
so
u
rce impedance, less than 100
Ω
.
For larger so
u
rce impedances, this connection leaves the differential signal
path significantly o
u
t of balance. Noise that co
u
ples electrostatically onto
the positive line does not co
u
ple onto the negative line beca
u
se it is
connected to gro
u
nd. Hence, this noise appears as a differential-mode
signal instead of a common-mode signal, and the instr
u
mentation amplifier
does not reject it. In this case, instead of directly connecting the negative
DIFF Input Mode Selected
–
+
–
+
AISENSE
AIGND
V
m
AI+
AI–
–
+
V
s
I/O Connector
Floating
Signal
Source
Bias
Current
Return
Paths
Measured
Voltage
Instrumentation
Amplifier
Bias
Resistors
(see text)
Summary of Contents for Multifunction RIO NI R Series
Page 1: ...PXI 7813...
