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Chapter 2 Connecting Signals
© National Instruments Corporation 2-5 SCXI-1141/1142/1143 User Manual
Figure 2-2 illustrates how to connect a non-referenced (floating) signal
source to an SCXI channel.
Figure 2-2. Floating Signal Connection
For AC-coupled signals, connect an external resistor from the AC-coupled
input channel to ground. This provides a DC path for the amplifier input
bias current. Typical resistor values range from 100 kΩ to 10 MΩ. This
solution, although necessary, lowers the input impedance of the channel
and introduces an additional DC offset voltage proportional to the product
of the input bias current and the resistor value used. Using a 1 MΩ resistor
results in ±200 µV of offset, which is insignificant in most applications.
However, if you use larger-valued bias resistors, significant input offset can
result. Lower-valued bias resistors increase loading of the source, which
can result in gain error.
Figures 2-3 through 2-5 illustrate how to connect AC-coupled signals.
Figure 2-3. Ground-Referenced AC-Coupled Signal Connection
A GND
100 kΩ
A
SCXI-1141/1142/1143
IN–
IN+
1µF
1MΩ
SCXI-1141/1142/1143
IN–
IN+