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 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

−n

Π

2

radians where n

is the order (number of poles) of the filter. The

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

0.67fc

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

Settling Time (% of final value)

% Overshoot

±1%

±0.1%

±0.01%

Butterworth

2

4

6

8

4

11

14

16

Bessel

2

4

6

8

0.4

0.8

0.6

0.1

Chebyschev (ripple

±0.25dB)

2

4

6

8

11

18

21

23

-

-

-

-

Chebyschev (ripple

±1dB)

2

4

6

8

21

28

32

34

-

-

-

-

Design of 2nd order active low pass filter (Sallen and Key configuration unity gain-op-amp)

LS204

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