Spectrum Controls 1746sc-NI8u - Manual (Page 38)

26 SLC 500 ™ Universal Analog Input Module
Table 3.1 Cut-off frequency, step response time, and
effective resolution (based on filter
frequency)
Filter Cut-Off Step ADC Effective
Frequency Frequency Response Resolution
10 Hz 2.62 Hz 300 ms 20.5
50 Hz 13.1 Hz 60 ms 19.0
60 Hz 15.72 Hz 50 ms 19.0
250 Hz 65.5 Hz 12 ms 15.5
The step response is calculated by a 3 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 below.
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.