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6
G
TYPICAL APPLICATION CIRCUIT
APPLICATION INFORMATION
PCB LAYOUT
Special attention must be paid to component layout when
designing serial digital interfaces for HDTV. Figures 8
through 11 show the artwork for a four layer printed circuit
evaluation board for the GS1508. The schematic is shown in
Figure 7. An FR-4 dielectric can be used, however,
controlled impedance transmission lines are required for
PCB traces longer than approximately 1cm. Note the
following
PCB
artwork
features
performance:
used
to
optimize
PCB trace width for HD rate signals is closely matched to
SMT component width to minimize reflections due to
change in trace impedance
The PCB ground plane is removed under the GS1508
output components to minimize parasitic capacitance.
The PCB ground plane is removed under the GS1508 R
SET
pin and resistor to minimize parasitic capacitance
Input and output BNC connectors are surface mounted in-
line to eliminate a transmission line stub caused by a BNC
mounting via High speed traces are curved to minimize
impedance changes due to change of PCB trace width.
TABLE 2: Typical Application Circuit Component Values
COMPONENT
VALUE
COMMENT
R
SET
53.6
For 800mV Output
Z
0
50
For 50
PCB trace impedance
L
COMP
12nH
Typical value. Component value will vary depending on PCB layout.
C
COMP
0.5pF
Typical value. Component value will vary depending on PCB layout.
SDO
SDO
GND
V
CC
SDI
SDI
V
EE
R
SET
53.6
Z
0
Z
0
0μ1
V
CC
L
COMP
L
COMP
4μ7
4μ7
OUT
OUT
75
75
C
COMP
C
COMP
PECL INPUT
1n
75
75
V
CC
100n
+
+
1
2
3
4
8
7
6
5
GS1508
All resistors in ohms,
all capacitors in farads,
unless otherwise shown.