LS404 데이터 시트보기 (PDF) - STMicroelectronics
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LS404 Datasheet PDF : 11 Pages
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LS404
APPLICATION INFORMATION: Active low-pass filter
BUTTERWORTH
The Butterworth is a "maximally flat" amplitude re-
sponse filter (figure 10) Butterworth filters are
used for filtering signals in data acquisition sys-
tems to prevent aliasing errors in samples-data
applications and for general purpose low-pass fil-
tering.
The cut-off frequency Fc, is the frequency at which
the amplitude response is down 3dB. The attenu-
ation rate beyond the cutoff frequency is n6 dB per
octave of frequency where n is the order (number
of poles) of the filter.
Other characteristics :
u Flattest possible amplitude response
u Excellent gain accuracy at low frequency
end of passband
BESSEL
The Bessel is a type of “linear phase” filter. Be-
cause of their linear phase characteristics, these
filters approximate a constant time delay over a
limited frequency range. Bessel filters pass tran-
sient waveforms with a minimum of distortion.
They are also used to provide time delays for low
pass filtering of modulated waveforms and as a
“running average” type filter.
The maximum phase shift is
–----n2---π---
radians where
n is the order (number of poles) of the filter. The
cut-off frequency fc, is defined as the frequency at
which the phase shift is one half of this value.
For accurate delay, the cut-off frequency should
be twice the maximum signal frequency.
The following table can be used to obtain the -3dB
frequency of the filter.
2 Pole 4 Pole 6 Pole 8 Pole
-3dB Frequency 0.77fc 0.67fc 0.57fc 0.50fc
Other characteristics :
u Selectivity not as great as Chebyschev or
Butterworth
u Very little overshoot response to step inputs
u Fast rise time
CHEBYSCHEV
Chebyschev filters have greater selectivity than ei-
ther Bessel ro Butterworth at the expense of ripple
in the passband (figure 11).
Chebyschev filters are normally designed with
peak-to-peak ripple values from 0.2dB to 2dB.
Increased ripple in the passband allows increased
attenuation above the cut-off frequency.
The cut-off frequency is defined as the frequency
at which the amplitude response passes through
the specificed maximum ripple band and enters
the stop band.
Other characteristics :
u Greater selectivity
u Very non-linear phase response
u High overshoot response to step inputs
The table below shows the typical overshoot and setting time response of the low pass filters to a step
input.
Number of Poles
Peak
Overshoot
% Overshoot
Settling Time (% of final value)
±1%
±0.1%
±0.01%
2
Butterworth
4
6
8
2
Bessel
4
6
8
2
Chebyschev (ripple ±0.25dB)
4
6
8
2
Chebyschev (ripple ±1dB)
4
6
8
4
1.1Fc sec. 1.7Fc sec. 1.9Fc sec.
11
1.7/fc
2.8/fc
3.8/fc
14
2.4/fc
3.9S/fc
5.0S/fc
14
3.1/fc
5.1/fc
7.1/fc
0.4
0.8/fc
1.4/fc
1.7/fc
0.8
1.0/fc
1.8/fc
2.4/fc
0.6
1.3/fc
2.1/fc
2.7/fc
0.1
1.6/fc
2.3/fc
3.2/fc
11
1.1/fc
1.6/fc
-
18
3.0/fc
5.4/fc
-
21
5.9/fc
10.4/fc
-
23
8.4/fc
16.4/fc
-
21
1.6/fc
2.7/fc
28
4.8/fc
8.4/fc
-
32
8.2/fc
16.3/fc
-
34
11.6/fc
24.8/fc
-
Design of 2nd order active low pass filter (Sallen and Key configuration unity gain op-amp)
6/11
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