Rockwell Automation 1746-NI8 SLC 500 Analog Input Modules User Manual - Manual (Page 39)

4–8 Preliminary Operating Considerations
Publication 1746Ć6.8 - April 1997
The cut-off frequency for each input channel is defined by its filter
frequency selection. Choose a filter frequency so that your fastest
changing signal is below that of the filter’s cut-off frequency. The
cut-off frequency should not be confused with update time. The
cut-off frequency relates how the digital filter attenuates frequency
components of the input signal. The update time defines the rate at
which an input channel is scanned and its channel data word is
updated.
Aliasing Frequency
Aliasing is a natural characteristic of discrete time sampling of
analog signals. This can result in erroneous data in the data channel
word. Aliasing is usually not a problem because the duration of the
high frequency signal is much shorter than the program scan time.
Aliasing begins at a lower frequency when more channels are
enabled. Anti-aliasing filters are available, but it is recommended
that you first check your transducer for malfunctions as the filters are
fairly expensive.
Aliasing occurs when unwanted signals, at a frequency greater than
the effective sampling rate, are present. The effective sampling rate
is determined by the number of enabled channels (n). The first
aliasing frequency occurs at: f
a
= 1 / (n * 0.00075). Subsequent
aliasing frequencies occur at integer multiples of f
a
. The following
table shows the first alias frequency, based on the number of
channels enabled.
Aliasing Frequency for Number of Channels Enabled
1
Channel
2
Channels
3
Channels
4
Channels
5
Channels
6
Channels
7
Channels
8
Channels
1333 Hz 666 Hz 444 Hz 333 Hz 267 Hz 222 Hz 190 Hz 167 Hz