Chapter 4
Analog Input
©
National Instruments Corporation
4-27
Using Differential Connections for Floating Signal
Sources
It is important to connect the negative lead of a floating so
u
rce to AI GND
(either directly or thro
u
gh a bias resistor). Otherwise, the so
u
rce may float
o
u
t of the maxim
u
m working voltage range of the NI-PGIA and the DAQ
device ret
u
rns erroneo
u
s data.
The easiest way to reference the so
u
rce to AI GND is to connect the
positive side of the signal to AI+ and connect the negative side of the signal
to AI GND as well as to AI– witho
u
t
u
sing resistors. This connection works
well for DC-co
u
pled so
u
rces with low so
u
rce impedance (less than 100
Ω
).
Figure 4-16.
Differential Connections for Floating Signal Sources
without Bias Resistors
However, for larger so
u
rce impedances, this connection leaves the DIFF
signal path significantly off 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. This noise appears as a DIFF-mode signal instead of
a common-mode signal, and th
u
s appears in yo
u
r data. In this case, instead
of directly connecting the negative line to AI GND, connect the negative
line to AI GND thro
u
gh a resistor that is abo
u
t 100 times the eq
u
ivalent
so
u
rce impedance. The resistor p
u
ts the signal path nearly in balance,
so that abo
u
t the same amo
u
nt of noise co
u
ples onto both connections,
yielding better rejection of electrostatically co
u
pled noise. This
config
u
ration does not load down the so
u
rce (other than the very
high inp
u
t impedance of the NI-PGIA).
–
+
Imped
a
nce
<100
Ω
AI GND
AI+
AI–
AI
S
EN
S
E
V
s
Flo
a
ting
S
ign
a
l
S
o
u
rce
