LS204C STMicroelectronics, LS204C Datasheet - Page 6

• Manufacturer Part Number: LS204C
• Description: HIGH PERFORMANCE DUAL OPERATIONAL AMPLIFIERS
• Manufacturer: STMicroelectronics

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 F
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
is the order (number of poles) of the filter. The
6/10
Design of 2nd order active low pass filter (Sallen and Key configuration unity gain-op-amp)
The table below shows the typical overshoot and settling time response of the low pass filters to a step input.
Butterworth
Bessel
Chebyschev (ripple 0.25dB)
Chebyschev (ripple 1dB)
Flattest possible amplitude response
Excellent gain accuracy at low frequency end
of passband
c
, is the frequency at which
n
2
Number of
Poles
radians where n
2
4
6
8
2
4
6
8
2
4
6
8
2
4
6
8
% Overshoot
Overshoot
Peak
0.4
0.8
0.6
0.1
11
14
16
11
18
21
23
21
28
32
34
4
cut-off frequency f
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.
.
Other characteristics :
.
.
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 :
.
.
-3dB Frequency
Selectivity not as great as Chebyschev or But-
terworth
Very little overshoot response to step inputs
Fast rise time
Greater selectivity
Very non-linear phase response
High overshoot response to step inputs
1.1F
11.6/f
1.7/f
2.4/f
3.1/f
0.8/f
1.0/f
1.3/f
1.6/f
1.1/f
3.0/f
5.9/f
8.4/f
1.6/f
4.8/f
8.2/f
1%
c
sec.
Settling Time (% of final value)
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
2 Pole
0.77f
, is defined as the frequency at
c
1.7F
10.4/f
16.4/f
16.3/f
24.8/f
2.8/f
3.9/f
5.1/f
1.4/f
1.8/f
2.1/f
2.3/f
1.6/f
5.4/f
2.7/f
8.4/f
0.1%
4 Pole
0.67fc
c
sec.
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
6 Pole
0.57fc
1.9F
0.01%
3.8/f
5.0/f
7.1/f
1.7/f
2.4/f
2.7/f
3.2/f
c
-
-
-
-
-
-
-
-
8 Pole
sec.
0.50fc
c
c
c
c
c
c
c