National Instruments PXIe-4310, Руководство пользователя

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PXIe-4310 User Manual
Figure 2-2 and 2-3 show the ground referenced and floating sources with low source impedance
(<100
Ω
).
Figure 2-2. Ground Referenced Source Signal Connection
Figure 2-3. Floating Source Signal Connection
For signals with source impedance >100 Ω , it is important to maintain a balanced source
impedance, which means that both terminals of the source have equal impedance to the signal
source ground. Providing a balanced source impedance is critical to achieving optimal noise
rejection, since any noise pickup will induce a noise voltage common to both conductors, which
can then be rejected by the analog input Programmable Gain Instrumentation Amplifier (PGIA).
Noise pickup in a system with an unbalanced source impedance will induce noise voltages of
different amplitudes on each conductor, which appears as a differential voltage that will be
measured as a signal voltage. Additionally, if the isolation voltage (V
ISO
) is a time varying
voltage or noisy DC source, a balanced source impedance is critical in ensuring that V
ISO
is
common to both inputs, which maximizes the ability of the PGIA to reject V
ISO
. A mismatch in
the source impedance can cause a portion of V
ISO
to appear as a differential voltage between AI+
and AI-.
I s ol a ted Gro u nd Ch ass i s Gro u nd
+
–
AI+
AI–
COM
AI Ch a nnel
+
–
V
S IG
+
–
V
I S O
+
–
AI+
AI–
COM
AI Ch a nnel
+
–
V
S IG
I s ol a ted Gro u nd