LMH1981 Datasheet, PDF (12/13 Pages) National Semiconductor (TI) – Multi-Format Video Sync Separator

Application Information (Continued)
FIGURE 13. Video Format Output for Progressive Format, 480p
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PCB LAYOUT CONSIDERATIONS
LMH1981 IC Placement
The LMH1981 should be placed such that critical signal
paths are short and direct to minimize PCB parasitics from
degrading the high-speed video input and logic output sig-
nals.
Ground Plane
A two-layer, FR-4 PCB is sufficient for this device. One of the
PCB layers should be dedicated to a single, solid ground
plane that runs underneath the device and connects the
device GND pins together. The ground plane should be used
to connect other components and serve as the common
ground reference. It also helps to reduce trace inductances
and minimize ground loops. Try to route supply and signal
traces on another layer to maintain as much ground plane
continuity as possible.
Power Supply Pins
The power supply pins should be connected together using
short traces with minimal inductance. When routing the sup-
ply traces, be careful not to disrupt the solid ground plane.
For high frequency bypassing, place 0.1 µF SMD ceramic
bypass capacitors with very short connections to power
supply and GND pins. Two or three ceramic bypass capaci-
tors can be used depending on how the supply pins are
connected together. Place a 10 µF SMD tantalum bypass
capacitor nearby all three power supply pins for low fre-
quency supply bypassing.
R_EXT Resistor
The R_EXT resistor should be a 10 kΩ 0.1% SMD resistor with
a temperature coefficient under 100 ppm/˚C. Place R_EXT as
close as possible to the device and connect to pin 1 and the
ground plane using the shortest possible connections. All
input and output signals must be kept away from this pin to
prevent unwanted signals from coupling into this pin.
Video Input
The input signal path should be routed using short, direct
traces between video source and input pin. Use a 75Ω input
termination and a SMD capacitor for AC coupling the video
input to pin 4.
Output Routing
The output signal paths should be routed using short, direct
traces to minimize parasitic effects that may degrade these
high-speed logic signals. This is especially important for the
horizontal sync output, in which it is critical to minimize
timing jitter. Each output can be protected by current limiting
with a small series resistor, like 100Ω.
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