14 MULTIPLE FEEDBACK LOOP CONFIGURATION

The multi-loop approach to building bandpass filters is high-

ly flexible and stable yet uses few external components

Figure 5 shows the MF8’s internal operational amplifier and

two second-order filter stages with three external resistors

in a fourth-order multiple feedback configuration Higher-or-

der filters may be built by adding more second-order sec-

tions and feedback resistors as in

Figure 6 The filter’s re-

sponse is determined by the clock frequency the clock-to-

center-frequency ratio the ratios of the feedback resistor

values and the Qs of the second-order filter sections The

design procedure for multiple feedback filters can be broken

down into a few simple steps

1) Determine the characteristics of the desired filter This

will depend on the requirements of the particular applica-

tion For a given application the required bandpass re-

sponse can be shown graphically as in

Figure 7 which

shows the limits for the filter response

Figure 7 also makes

use of several parameters that must be known in order to

design a filter These parameters are defined below in terms

of

Figure 7

TLH8694 – 15

FIGURE 7 Graphical representation of the amplitude

response specifications for a bandpass filter The

filter’s response should fall within the shaded area

TLH8694 – 13

FIGURE 5 General fourth-order multiple-feedback bandpass filter circuit MF8 pin numbers are shown

TLH8694 – 14

FIGURE 6 By adding more second-order filter stages and feedback resistors

higher order multiple-feedback filters may be built

10