LS404 STMicroelectronics, LS404 Datasheet - Page 6

• Manufacturer Part Number: LS404
• Description: Low-noise bipolar op-amps
• Manufacturer: STMicroelectronics

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 :
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
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.
The table below shows the typical overshoot and setting time response of the low pass filters to a step
input.
Design of 2nd order active low pass filter (Sallen and Key configuration unity gain op-amp)
6/11
Butterworth
Bessel
Chebyschev (ripple ±0.25dB)
Chebyschev (ripple ±1dB)
Flattest possible amplitude response
Excellent gain accuracy at low frequency
end of passband
–
---------- -
Number of Poles
n
2
radians where
2
4
6
8
2
4
6
8
2
4
6
8
2
4
6
8
% Overshoot
Overshoot
Peak
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.
Other characteristics :
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 :
0.4
0.8
0.6
0.1
14
14
18
21
23
21
28
32
34
-3dB Frequency
11
11
4
Selectivity not as great as Chebyschev or
Butterworth
Very little overshoot response to step inputs
Fast rise time
Greater selectivity
Very non-linear phase response
High overshoot response to step inputs
1.1Fc sec.
11.6/fc
1.7/fc
2.4/fc
3.1/fc
0.8/fc
1.0/fc
1.3/fc
1.6/fc
1.1/fc
3.0/fc
5.9/fc
8.4/fc
1.6/fc
4.8/fc
8.2/fc
±1%
Settling Time (% of final value)
2 Pole
0.77fc
1.7Fc sec.
3.9S/fc
10.4/fc
16.4/fc
16.3/fc
24.8/fc
±0.1%
4 Pole
2.8/fc
5.1/fc
1.4/fc
1.8/fc
2.1/fc
2.3/fc
1.6/fc
5.4/fc
2.7/fc
8.4/fc
0.67fc
6 Pole
0.57fc
1.9Fc sec.
±0.01%
5.0S/fc
3.8/fc
7.1/fc
1.7/fc
2.4/fc
2.7/fc
3.2/fc
-
-
-
-
-
-
-
8 Pole
0.50fc