LS204N Ver la hoja de datos (PDF) - STMicroelectronics
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LS204N Datasheet PDF : 10 Pages
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LS204
APPLICATION INFORMATION :
Active low-pass filter
BUTTERWORTH
The Butterworth is a ”maximally flat” amplitude
response filter (figure 10) Butterworth filters are
used for filtering signals in data acquisition systems
to prevent aliasing errors in samples-data applica-
tions and for general purpose low-pass filtering.
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 :
Flattest possible amplitude response
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
−nΠ
2
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 :
. Selectivity not as great as Chebyschev or But-
terworth
. Very little overshoot response to step inputs
. Fast rise time
CHEBYSCHEV
Chebyschev filters have greater selectivity than
either 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 :
Greater selectivity
Very non-linear phase response
High overshoot response to step inputs
The table below shows the typical overshoot and settling 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%
Butterworth
2
4
1.1Fcsec.
1.7Fcsec.
1.9Fcsec.
4
11
1.7/fc
2.8/fc
3.8/fc
6
14
2.4/fc
3.9/fc
5.0/fc
8
16
3.1/fc
5.1/fc
7.1/fc
Bessel
2
0.4
0.8/fc
1.4/fc
1.7/fc
4
0.8
1.0/fc
1.8/fc
2.4/fc
6
0.6
1.3/fc
2.1/fc
2.7/fc
8
0.1
1.6/fc
2.3/fc
3.2/fc
Chebyschev (ripple ±0.25dB)
2
4
6
8
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
-
Chebyschev (ripple ±1dB)
2
4
6
8
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)
6/10
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