LS204

Application information for active low-pass filters

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4

Application information for active low-pass filters

4.1

Butterworth

The Butterworth is a "maximally flat" amplitude response filter (Figure 11).

Butterworth filters are used for filtering signals in data acquisition systems to prevent

aliasing errors in samples-data applications and for general purpose low-pass filtering.

The cut-off frequency, Fc, is the frequency at which the amplitude response is down 3 dB.

The attenuation rate beyond the cut-off 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

4.2

Bessel

The Bessel is a type of “linear phase” filter.

Because of their linear phase characteristics, these filters approximate a constant time delay

over a limited frequency range. Bessel filters pass transient 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

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.

Table 4 can be used to obtain the -3 dB frequency of the filter.

Other characteristics:

●

Selectivity not as great as Chebyschev or Butterworth

●

Very little overshoot response to step inputs

●

Fast rise time

4.3

Chebyschev

Chebyschev filters have greater selectivity than either Bessel or Butterworth at the expense

of ripple in the passband (Figure 12).

Chebyschev filters are normally designed with peak-to-peak ripple values from 0.2 dB to

2dB.

Table 4.

-3 dB frequency of the filter

2 Poles

4 Poles

6 Poles

8 Poles

-3 dB frequency

0.77 Fc

0.67

Fc

0.57

Fc

0.50

Fc

n

π

–

2

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