National Instruments Data Acquisition Device NI USB-621x, Руководство пользователя

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Chapter 4 Analog Input
© National Instruments Corporation 4-29 NI USB-621x User Manual
Figure 4-18. Differential Connections for Floating Signal Sources
with Balanced Bias Resistors
Both inp u ts of the NI-PGIA req u ire a DC path to gro u nd in order for the
NI-PGIA to work. If the so u rce is AC co u pled (capacitively co u pled), the
NI-PGIA needs a resistor between the positive inp u t and AI GND. If the
so u rce has low-impedance, choose a resistor that is large eno u gh not to
significantly load the so u rce b u t small eno u gh not to prod u ce significant
inp u t offset voltage as a res u lt of inp u t bias c u rrent (typically 100 k
Ω
to
1 M
Ω
). In this case, connect the negative inp u t directly to AI GND. If the
so u rce has high o u tp u t impedance, balance the signal path as previo u sly
described u sing the same val u e resistor on both the positive and negative
inp u ts; be aware that there is some gain error from loading down the so u rce,
as shown in Fig u re 4-19.
U S B-6210/6211/6212 Device Config u red in DIFF Mode
PGIA
–
+
–
+
–
+
Flo a ting
S ign a l
S o u rce
Bi as
C u rrent
Ret u rn
P a th s
AI GND
AI S EN S E
Inp u t M u ltiplexer s
Me asu red
Volt a ge
In s tr u ment a tion
Amplifier
AI+
AI–
I/O Connector
Bi as
Re s i s tor s
( s ee text)
V
s
V
m