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LT1568 Datasheet PDF : 16 Pages
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LT1568
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:
eOUT
eIN
=
–
DCGAIN
s2 + 2πfO
Q
• (2πfO)2
s + (2πfO
)2
where eOUT is either OUTA or OUTB,
DCGAIN
=
R2 ,
R1
fO
=
2π
1
R2 • R3 • C1• C2
and
[ ] Q
=
C1•
2π • C1• C2 • R1• R2 • R3 • fO
R1• (R2 + R3) + R2 • R3 – C2 • R1• R2
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
ANALOG
GROUND
PLANE
BAT54S
V–
1 V+
V+ 16
2
15
INVA INVB
3
14
SA
SB
4
LT1568 13
OUTA OUTB
5
12
OUTA OUTB
6
11
GNDA GNDB
7
NC
10
EN
8 V–
V– 9
V+
0.1µF
0.1µF
Single Supply Power and Ground Connections
ANALOG
GROUND
PLANE
0.1µF
1 V+
V+ 16
2
15
INVA INVB
3
14
SA
SB
4
LT1568 13
OUTA OUTB
5
12
OUTA OUTB
6
11
GNDA GNDB
7
NC
10
EN
8 V–
V– 9
V+
0.1µF
SINGLE POINT
SYSTEM GROUND
10
DIGITAL GROUND PLANE
(IF ANY)
1568 F01a
SINGLE POINT
SYSTEM GROUND
DIGITAL GROUND PLANE
(IF ANY)
1568 F01b
Figure 1. Dual and Single Supply and Ground Plane Connections
1568f
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