LS204

Application information for active low-pass filters

9/16

Figure 13.

Filter configuration

Three parameters are needed to characterize the frequency and phase response of a 2nd

order active filter:

●

the gain (Gv),

●

the damping factor (

ξ ) or the Q factor (Q = 2 ξ )1),

●

the cut-off frequency (Fc).

The higher order response is obtained with a series of 2nd order sections. A simple RC

section is introduced when an odd filter is required.

The choice of

ξ (or Q factor) determines the filter response (see Table 6).

Table 6.

Filter response to

ξ or Q factor

Filter response

ξ

Q

Cut-off frequency (Fc)

Bessel

Frequency at which phase shift is -90°C

Butterworth

Frequency at which Gv = -3 dB

Chebyschev

Frequency at which the amplitude response passes through

specified max. ripple band and enters the stop bank.

C2

R2

R1

Vin

C1

Vout

3

2

-------

1

3

-------

2

2

-------

1

2

-------

2

2

-------

1

2

-------