©
National Instruments Corporation
13
NI myDAQ User Guide and Specifications
Figure 7.
Differential Connections for Floating Signal Sources without Resistors
For larger source impedances, however, this connection leaves the
differential signal path significantly off balance. Noise that couples
electrostatically onto the positive line does not couple onto the negative line
because it is connected to ground. This noise appears as a differential-mode
signal instead of a common-mode signal, and thus appears in your data.
In this case, instead of directly connecting the negative line to AGND,
connect the negative line to AGND through a resistor that is about
100 times the equivalent source impedance. The resistor puts the signal
path nearly in balance, so that about the same amount of noise couples onto
both 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
and each of the two resistors is 100 k
, the resistors
load down the source with 200 k
and produce a –1% gain error.
+
–
AI+
AGND
Sign
a
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rce
R
s
o
u
rce
<100
Ω
AI–
+
–
+
–
+
–
AI+
AGND
Sign
a
l So
u
rce
R
s
o
u
rce
>100
Ω
AI–
