Chapter 5
Adding Components for Special Functions
SCB-68 Shielded Connector Block User Manual
5-14
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To prevent aliasing, remove all signal components with frequencies greater
than the Nyquist frequency from input signals before those signals are
sampled. Once a data sample is aliased, it is impossible to accurately
reconstruct the original signal.
To design a lowpass filter that attenuates signal components with a
frequency higher than half of the Nyquist frequency, substitute the half
Nyquist value for the
f
c
value in Equation 5-6.
The following devices provide antialiasing filters and do not need to have
the filters implemented at the SCB-68 terminal block:
•
NI PCI/PXI-61
XX
(not including the NI PCI-6110/6111)
•
NI PCI-445
X
•
NI PCI-455
X
Special Consideration for Analog Input Channels
Filtering increases the settling time of the instrumentation amplifier to the
time constant of the filter used. Adding RC filters to scanning channels
greatly reduces the practical scanning rate, since the instrumentation
amplifier settling time can be increased to 10
T
or longer, where
T
= (
R
)(
C
).
You can use RC filters with single-ended or differential inputs.
Special Consideration for Analog Output Signals
Lowpass filters can smooth stairstep-like curves on AO signals. If the
curves are not smoothed, the AO signals can be a hazard for some external
circuitry connected to it. Figure 5-15 shows the output of a lowpass filter
when a stairstep-like signal is the input.
