Using Differential Connections For Floating Signal Sources - National Instruments Data Acquisition Device NI USB-621x User Manual

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Using Differential Connections for Floating Signal
Sources
It is important to connect the negative lead of a floating source to AI GND
(either directly or through a bias resistor). Otherwise, the source may float
out of the maximum working voltage range of the NI-PGIA and the DAQ
device returns erroneous data.
The easiest way to reference the source 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– without using resistors. This connection works
well for DC-coupled sources with low source impedance (less than 100 Ω).
Floating
Signal
Source
Impedance
<100 Ω
Figure 4-16. Differential Connections for Floating Signal Sources
However, for larger source impedances, this connection leaves the DIFF
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 DIFF-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).
+
V
s
–
without Bias Resistors
4-27
Chapter 4 Analog Input
AI+
AI–
AI SENSE
AI GND
NI USB-621x User Manual