Effects Of Filter Frequency On Noise Rejection; Cut-Off Frequency - Spectrum Controls 1769 Series User Manual

Input
Filter
4.17 Hz
10.0 Hz
16.7 Hz
19.6 Hz
62 Hz
470 Hz
7
Use channel update rates for channels 1 through 4 when CJCs are not being sampled.
User's Manual Pub. 0300244-01 Rev. A
Chapter 4: Module, Data, Status, and Channel Configuration
Table 4-6 (Filter Effects)
Channel
Update Channel
Time Update
(Ch 1 to 4) Time
Normal During CJC
Conversion Sample
Mode (Ch 0 & 5)
246 msec 500 msec
106 msec 220 msec
66 msec 140 msec
57 msec 122 msec
26 msec 52 msec
8 msec 24 msec

Effects of Filter Frequency on Noise Rejection

The filter frequency that you choose for a module channel determines the amount of
noise rejection for the inputs. A lower frequency (4.17 Hz versus 470 Hz) provides better
noise rejection and improves repeatability, but also increases channel update time. A
higher filter frequency provides lower noise rejection, but decreases the channel update
time and negatively affects repeatability.
When selecting a filter frequency, be sure to consider the cut-off frequency to obtain
acceptable noise rejection. Choose a filter frequency so that your fastest-changing signal
is below that of the filter's cut-off frequency.
Table 4-6 above lists the expected normal mode rejection for each of the filter settings.
Note: Transducer power supply noise, transducer circuit noise, or process variable
irregularities may also be sources of normal mode noise.

Cut-Off Frequency

The filter cut-off frequency, -3 dB, is the point on the frequency response curve where
frequency components of the input signal are passed with 3 dB of attenuation. Table 4-6
shows cut-off frequencies for the supported filters.
All input 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 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 the update time.
The cut-off frequency relates to 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.
Cut-Off
7
Frequency Repeatability
1 Hz See Appendix A
2 Hz See Appendix A
4 Hz 2x(4.17 Hz values)
From Appendix A
5 Hz 2x(4.17 Hz values)
From Appendix A
14 Hz 4x(4.17 Hz values
From Appendix A)
109 Hz 10x(4.17 Hz values)
From Appendix A
4-7
NMRR NMRR
50 Hz 60 Hz
Rejection Rejection
74 dB 74 dB
70 dB 70 dB
65 dB NA
NA 74 dB
NA NA
NA NA