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NI myDAQ User Guide
connections, yielding better rejection of electrostatically coupled noise. This configuration does
not load down the source.
Figure 8.
Differential Connections for Floating Signal Sources with a Single Resistor
You can fully balance the signal path by connecting another resistor of the same value between
the positive input and AGND, as shown in Figure 9. This fully balanced configuration offers
slightly better noise rejection, but has the disadvantage of loading the source down with the
series combination (sum) of the two resistors. If, for example, the source impedance is 2 k
Ω
and
each of the two resistors is 100 k
Ω
, the resistors load down the source with 200 k
Ω
and produce
a -1% gain error.
Figure 9.
Differential Connections for Floating Signal Sources with Two Resistors
Both positive and negative analog input lines require a DC path to ground in order for the
instrumentation amplifier to work. If the source is AC coupled (capacitively coupled), a resistor
is needed between the positive input and AGND. If the source has low impedance, choose a
resistor that is large enough not to significantly load the source but small enough not to produce
significant input offset voltage as a result of input bias current (typically 100 k
Ω
to 1 M
Ω
). In
this case, connect the negative input directly to AGND. If the source has high output impedance,
balance the signal path as previously described using the same value resistor on both the positive
and negative inputs.
+
–
+
–
AI+
AGND
S
ign
a
l
S
o
u
rce
R
s
o
u
rce
>100
Ω
AI–
+
–
+
–
AI+
AGND
S
ign
a
l
S
o
u
rce
R
s
o
u
rce
>100
Ω
AI–
