© National Instruments | 2-9
NI SCB-68A User Manual
However, for larger source impedances, 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 AI GND, connect the negative line to
AI GND 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 (other than the very high input impedance of the NI-PGIA).
Figure 2-5. Differential Connections for Floating Signal Sources with Single Bias Resistor
You can fully balance the signal path by connecting another resistor of the same value between
the positive input and AI GND, as shown in Figure 2-6. 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 kΩ and
each of the two resistors is 100 kΩ, the resistors load down the source with 200 kΩ and produce
a –1% gain error.
–
+
R is about
100 times
source
impedance
of sensor
AI GND
R
V
s
Floating
Signal
Source
AI+
AI–
AI SENSE
DAQ Device
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