Chapter 4
Analog Input
©
National Instruments
4-17
X Series User Manual
Figure 4-6.
Differential Connections for Floating Signal Sources with
Balanced Bias Resistors
Both inp
u
ts of the NI-PGIA req
u
ire a DC path to gro
u
nd in order for the
NI-PGIA to work. If the so
u
rce is AC co
u
pled (capacitively co
u
pled), the
NI-PGIA needs a resistor between the positive inp
u
t and AI GND. If the
so
u
rce has low-impedance, choose a resistor that is large eno
u
gh not to
significantly load the so
u
rce b
u
t small eno
u
gh not to prod
u
ce significant
inp
u
t offset voltage as a res
u
lt of inp
u
t bias c
u
rrent (typically 100 k
Ω
to
1 M
Ω
). In this case, connect the negative inp
u
t directly to AI GND. If the
so
u
rce has high o
u
tp
u
t impedance, balance the signal path as previo
u
sly
described
u
sing the same val
u
e resistor on both the positive and negative
inp
u
ts; be aware that there is some gain error from loading down the so
u
rce,
as shown in Fig
u
re 4-7.
MIO X
S
erie
s
Device Config
u
red in Differenti
a
l Mode
PGIA
–
+
–
+
–
+
Flo
a
ting
S
ign
a
l
S
o
u
rce
Bi
as
C
u
rrent
Ret
u
rn
P
a
th
s
AI GND
AI
S
EN
S
E
Inp
u
t M
u
ltiplexer
s
Me
asu
red
Volt
a
ge
In
s
tr
u
ment
a
tion
Amplifier
AI+
AI–
I/O Connector
Bi
as
Re
s
i
s
tor
s
(
s
ee text)
V
s
V
m
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