AL250 International Power Sources, Inc., AL250 Datasheet - Page 29

• Manufacturer Part Number: AL250
• Description: Video Scan Doubler
• Manufacturer: International Power Sources, Inc.

AL250
9.0 Board Design and Layout Considerations
The AL250/251 contains both precision analog and high-speed digital circuitry. Noise coupling from
digital circuits to analog circuits may result in poor video quality. The layout should be optimized for
lowest noise on the power and ground planes by shielding the digital circuitry and providing good
decoupling.
It is recommended to place the AL250/251 chip close to the VGA output connector, and the video
decoder close to the analog video input connectors if applicable.
9.1 Grounding
Analog and digital circuits are separated within the AL250/251 chip. To minimize system noise and
prevent digital system noise from entering the analog portion, a common ground plane for all devices,
including the AL250/251 is recommended. All the connections to the ground plane should have very
short leads. The ground plane should be solid, not cross-hatched.
9.2 Power Planes and Power Supply Decoupling
The analog portion of the AL250/251 and any associated analog circuitry should have their own
power plane, referred to as the analog power plane (AVDD). The analog power plane should be
connected to the digital power plane (DVDD) at a single point through a low resistance ferrite bead.
The digital power plane should provide power to all digital logic on the PC board, and the analog
power plane should provide power to all of the AL250/251 analog power pins and relevant analog
circuitry.
Power supply connection pins should be individually decoupled. For best results, use 0.1 F ceramic
chip capacitors. Lead lengths should be minimized. The power pins should be connected to the
bypass capacitors before being connected to the power planes. 22 F capacitors should also be used
between the AL250/251 power planes and the ground planes to control low-frequency power ripple.
9.3 Digital Signal and Clock Interconnect
Digital signals to the AL250/251 should be isolated as much as possible from the analog outputs and
other analog circuitry. The high frequency clock reference or crystal should be handled carefully.
Jitter and noise on the clock will degrade the video performance. Keep the clock paths to the decoder
as short as possible to reduce noise pickup.
9.4 Analog Signal Interconnect
The AL250/251 should be located closely to the output connectors to minimize noise and reflections.
Keep the critical analog traces as short and wide (20~30 mil) as possible. Digital signals, especially
pixel clocks and data signals should not overlap any of the analog signal circuitry and should be kept
as far apart as possible. The AL250/251 and the decoder IC should have no inputs left floating.
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July 28, 1999