Rockwell Automation 1746-NI16V SLC 500 Analog Input Modules User Manual - Manual (Page 45)

Publication 1746-UM001A-US-P
Preliminary Operating Considerations 4-9
Channel Filter Frequency
Selection The module uses a digital low-pass filter that provides noise rejection
for the input signals. The digital filter is programmable, allowing you
to select from eight filter frequencies for each group of four channels.
Selecting a low value (i.e., 6 Hz) for the channel filter frequency
provides the best noise rejection for that group of channels. Selecting
a high value for the channel filter frequency provides less noise
rejection, but faster data response time. See page 4-10 for more
information on noise rejection.
The table on page 4-7 shows the available filter frequencies and
module data update times for each filter frequency
Channel Frequency
Channel Cut-Off Frequency
The channel filter frequency selection determines a channel’s cut-off
frequency, also called the -3 dB frequency. The cut-off frequency is
defined as the point on the input channel frequency response curve
where frequency components of the input signal are passed with 3 dB
of attenuation. All frequency components at or below the cut-off
frequency are passed by the digital filter with less than 3 dB of
attenuation. All frequency components above the cut-off frequency
are increasingly attenuated.
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.