LMF100 datasheet(20/28 Pages) NSC

20 / 28 page

2.0 Modes of Operation (Continued)

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

Number of

Adjustable

Notes

Resistors

CLK/f0

3No

(2)

May need input buffer.

OBP1 = −Q

OLP = + 1

Poor dynamics

OBP2 = +1

for high Q.

Useful for high

frequency applications.

Yes (above

CLK/50 or

CLK/100)

Universal State-

Yes

Variable Filter. Best

general-purpose mode.

As above, but also

Yes

includes resistor-

tuneable notch.

Gives Allpass res-

ponse with H

OAP = −1

and H

OLP = −2.

Gives flatter allpass

Yes

response than above

if R

1 = R2 = 0.02R4.

Yes

Single pole.

(2)

OLP1 = + 1

Yes

Single pole.

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

CLK). The various clocking options are summarized in the

following table.

Clocking Options

Power Supply

Clock Levels

LSh

−5V and +5V

TTL (0V to +5V)

+5V

−5V and +5V

CMOS (−5V to +5V)

+5V

0V and 10V

TTL (0V to 5V)

+10V

0V and 10V

CMOS (0V to +10V)

+5V

+10V

−2.5V and

+2.5V

CMOS

+2.5V

(−2.5V to +2.5V)

0V and 5V

TTL (0V to +5V)

Power Supply

Clock Levels

LSh

0V and 5V

CMOS (0V to +5V)

+2.5V

+5V

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

center frequency, f

0, can be made equal to either fCLK/100 or

CLK/50. f0 can be very accurately set (within ±0.6%)byus-

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

CLK/f0 ratio can be altered by external resistors as

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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