TABLE 1. Summary of Modes. Realizable filter types (e.g. low-pass) denoted by asterisks.

Unless otherwise noted, gains of various filter outputs are inverting and adjustable by resistor ratios.

Mode

BP

LP

HP

N

AP

Number of

Adjustable

Notes

Resistors

f

1

**

*

3No

(2)

May need input buffer.

1a

H

H

2

No

Poor dynamics

H

for high Q.

1b

**

*

3

No

Useful for high

frequency applications.

Yes (above

2

**

*

3f

f

Universal State-

3

**

*

4

Yes

Variable Filter. Best

general-purpose mode.

As above, but also

3a

**

*

*

7

Yes

includes resistor-

tuneable notch.

Gives Allpass res-

4

**

*

3

No

ponse with H

and H

Gives flatter allpass

5

**

*

4

Yes

response than above

if R

6a

**

3

Yes

Single pole.

(2)

6b

H

2

Yes

Single pole.

6c

**

3

No

Single pole.

7

2

Yes

Summing integrator with

adjustable time constant.

3.0 Applications Information

The LMF100 is a general purpose dual second-order state

variable filter whose center frequency is proportional to the

frequency of the square wave applied to the clock input

(f

following table.

Clocking Options

Power Supply

Clock Levels

LSh

V

D

+

−5V and +5V

TTL (0V to +5V)

0V

+5V

−5V and +5V

CMOS (−5V to +5V)

0V

+5V

0V and 10V

TTL (0V to 5V)

0V

+10V

0V and 10V

CMOS (0V to +10V)

+5V

+10V

−2.5V and

+2.5V

CMOS

0V

+2.5V

(−2.5V to +2.5V)

0V and 5V

TTL (0V to +5V)

0V

0V

Power Supply

Clock Levels

LSh

V

D

+

0V and 5V

CMOS (0V to +5V)

+2.5V

+5V

By connecting pin 12 to the appropriate dc voltage, the filter

center frequency, f

f

ing a crystal clock oscillator, or can be easily varied over a

wide frequency range by adjusting the clock frequency. If de-

sired, the f

in

Figures 10, 11, 12, 13, 14, 15 and Figure 16. This is useful

when high-order filters (greater than two) are to be realized

by cascading the second-order sections. This allows each

stage to be stagger tuned while using only one clock. The fil-

ter Q and gain are set by external resistor ratios.

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

ther section of the LMF100. These are illustrated in

Figures

1, 2, 3, 4 and Figure 5 along with their transfer functions and

some related equations.

Figure 6 shows the effect of Q on

the shapes of these curves.

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