LS404

7/11

Fixed R = R1 = R2, we have (see figure 13)

Figure 13 : Filter Configuration

Three parameters are needed to characterize the

frequency and phase response of a 2nd order ac-

tive filter: the gain (Gv), the damping factio (

ξ) or

the Q factor (Q = 2

(fc).

The higher order response are obtained with a se-

ries 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 1).

Table 1

EXAMPLE

Figure 14 : 5th Order Low-pass Filter (Butterworth) with Unity Gain configuration

1

R

----

ζ

-------

1

R

----

1

-----------

C2

R2

R1

Vin

C1

Vout

Filter Response

ξ

Q

Cuttoff Frequency fc

Bessel

Frequency at which Phase Shift is -90°C

Butterworth

Frequency at which Gv = -3dB

Chebyschev

Frequency at which the amplitude response

passes through specified max. ripple band and

enters the stop bank.

3

2

-------

1

3

-------

2

2

-------

1

2

-------

2

2

-------

1

2

-------

C2

R2

R1

C1

Ri

Ci

C4

R4

R3

C3