LT1568

10

1568f

APPLICATIO S I FOR ATIO

The LT1568 has been designed to make the implementa-

tion of high frequency filtering functions very easy. Inter-

nal low noise amplifiers and capacitors are configured in

a topology that requires only three external resistors to

implement a 2nd order filter stage. The two 2nd order

stages can be used independently or cascaded for simple

4th order filter functions. With two stages integrated on

the same die, the matching of the independent sections is

better than what can be achieved with separate amplifier

components.

OPERATING WITH SINGLE OR DUAL SUPPLIES

Figure 1 shows the recommended connection of an analog

ground plane with the LT1568 biased from either symmetri-

cal dual (

±V) power supplies or a single supply. Connec-

tion of the two GND pins is important to properly DC bias

the internal amplifiers. The use of a ground plane helps to

minimize noise and stray components to preserve signal

integrity and maintain frequency response accuracy.

When biasing from a dual supply, it is recommended that

a Schottky diode clamp (BAT54S) be added as shown.

These diodes ensure that improper supply voltages,

through either reverse polarity or power-up sequencing,

do not damage the LT1568.

SIMPLE FILTER IMPLEMENTATIONS

The basic 2nd order filter block of the LT1568, with three

external resistors connected as shown in the Block Dia-

gram, has the following lowpass transfer function:

e

e

DC

f

s

f

Q

sf

OUT

IN

GAIN

O

O

O

=

π

()

+ π

+π

()

–

•2

2

2

2

2

2

DC

R

R

f

RR

C

C

GAIN

O

==

π

2

1

1

22

3

1

2

,

••

•

and

Q

CC

R

R

R

f

CR

R

R

R

R

C

R

R

O

=

π

+

[]

21

2

1

2

3

11

2

3

2

3

2

1

2

•

•

•

•••

••

•

–

•

•

The typical values of the internal capacitors are:

C1= 105.7pF

C2 = 141.3pF

These filter functions assume ideal amplifiers.

Dual Supply Power and Ground Connections

Single Supply Power and Ground Connections

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

INVA

SA

OUTA

OUTA

GNDA

NC

INVB

SB

OUTB

OUTB

GNDB

EN

LT1568

BAT54S

0.1

µF

0.1

µF

SINGLE POINT

SYSTEM GROUND

1568 F01a

ANALOG

GROUND

PLANE

DIGITAL GROUND PLANE

(IF ANY)

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

INVA

SA

OUTA

OUTA

GNDA

NC

INVB

SB

OUTB

OUTB

GNDB

EN

LT1568

0.1

µF

0.1

µF

SINGLE POINT

SYSTEM GROUND

1568 F01b

ANALOG

GROUND

PLANE

DIGITAL GROUND PLANE

(IF ANY)

Figure 1. Dual and Single Supply and Ground Plane Connections