Rockwell Automation 1746-NT8 SLC 500 Thermocouple/mV Analog Input Module User Manual - Manual (Page 34)

Publication 1746-UM022B-EN-P - January 2005
3-4 Considerations Before Using Your Module
The following table shows the available filter frequencies, cut-off
frequency, step response, and a DC effective resolution for each filter
frequency.
Cut-off frequency, Step Response Time, and Effective Resolution (Based on Filter
Frequency)
The step response is calculated by a 4 x (1/filter frequency) settling
time.
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, as shown in the graphs on page 3-5.
The cut-off frequency for each input channel is defined by its filter
frequency selection. The table above shows the input channel cut-off
frequency for each filter frequency. 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 updated.
Filter Frequency Cut-Off Frequency Step Response ADC Effective
Resolution
10 Hz 2.62 Hz 400 ms 20.5
50 Hz 13.1 Hz 80 ms 19.0
60 Hz 15.72 Hz 66.7 ms 19.0
250 Hz 65.5 Hz 16 ms 15.5