MF5 datasheet(12/18 Pages) TI1

12 / 18 page

20 Modes of Operation (Continued)

TABLE I Summary of Modes Realizable filter types (eg low-pass) denoted by asterisks Unless otherwise noted

gains of various filter outputs are inverting and adjustable by resistor ratios

Mode

Number of

Adjustable

Notes

resistors

fCLK fo

3No

(2)

May need input buf-

HOBP1ebQ

HOLPea1

fer Poor dynamics

HOBP2ea1

for high Q

Yes (above

3fCLK 50 or

fCLK 100)

Universal State-

Yes

Variable Filter Best

general-purpose mode

As above but also

Yes

includes resistor-

tuneable notch

Gives Allpass res-

ponse with HOAPeb1

and HOLPeb2

Gives flatter allpass

response than above

if R1eR2e002R4

Single pole

(2)

HOLPea1

Single pole

HOLP2e

30 Applications Information

The MF5 is a general-purpose second-order state variable

filter whose center frequency is proportional to the frequen-

cy of the square wave applied to the clock input (fCLK) By

connecting pin 9 to the appropriate DC voltage the filter

center frequency fo can be made equal to either fCLK 100

or fCLK 50 fo can be very accurately set (within g06%) by

using a crystal clock oscillator or can be easily varied over

a wide frequency range by adjusting the clock frequency If

desired the fCLK fo ratio can be altered by external resis-

tors as in

Figures 9 10 11 13 14 and 15 The filter Q and

gain are determined by external resistors

All of the five second-order filter types can be built using the

MF5 These are illustrated in

Figures 1 through 5 along with

their transfer functions and some related equations

Figure

6 shows the effect of Q on the shapes of these curves

When filter orders greater than two are desired two or more

MF5s can be cascaded The MF5 also includes an uncom-

mitted CMOS operational amplifier for additional signal pro-

cessing applications

31 DESIGN EXAMPLE

An example will help illustrate the MF5 design procedure

For the example we will design a 2nd order Butterworth

low-pass filter with a cutoff frequency of 200 Hz and a pass-

band gain of b2 The circuit will operate from a g5V power

supply and the clock amplitude will be g5v (CMOS) levels)

From

the

specifications

the

filter

parameters

are

foe200 Hz HOLPeb2 and for Butterworth response

Qe0707

In section 20 are several modes of operation for the MF5

each having different characteristics Some allow adjust-

ment of fCLK fo others produce different combinations of

filter types some are inverting while others are non-invert-

ing etc These characteristics are summarized in Table I To

keep the example simple we will use mode 1 which has

notch bandpass and lowpass outputs and inverts the sig-

nal polarity Three external resistors determine the filter’s Q

and gain From the equations accompanying

Figure 7

QeR3 R2 and the passband gain HOLP ebR2 R1 Since

the input signal is driving a summing junction through R1

the input impedance will be equal to R1 Start by choosing a

value for R1 10k is convenient and gives a reasonable input

impedance For HOLP eb2 we have

R2 ebR1HOLP e 10k c 2 e 20k

For Q e 0707 we have

R3 e R2Q e 20k c 0707 e 1414k Use 15k

For operation on g5V supplies Va is connected to a5V

Vb to b5V and AGND to ground The power supplies

should be ‘‘clean’’ (regulated supplies are preferred) and

01 mF bypass capacitors are recommended

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