bt865a Conexant Systems, Inc., bt865a Datasheet - Page 55

• Manufacturer Part Number: bt865a
• Description: Ycrcb To Ntsc/pal Digital Video Encoder
• Manufacturer: Conexant Systems, Inc.

3.1
3.2
100138C
02/17/03
3
Component Placement
Power and Ground Planes
PC Board Considerations
A simplified schematic is shown in
are listed in
The layout should be optimized for lowest noise on the power and ground planes by
providing good decoupling. The trace length between groups of VAA and GND pins
should be as short as possible to minimize inductive ringing.
A well-designed power distribution network is critical to eliminating digital switching
noise. The ground plane must provide a low-impedance return path for the digital circuits.
A PC board with a minimum of four layers is recommended, with layers 1 (top) and 4
(bottom) for signals and layers 2 and 3 for ground and power, respectively.
Components should be placed as close as possible to the associated pin. Whenever
possible, components should be placed so traces can be connected point to point.
The optimum layout enables the Bt864A/Bt865A to be located as close as possible to the
power supply connector and the video output connector.
Separate digital and analog power planes are recommended. The digital power plane
should provide power to all digital logic on the PC board, and the analog power plane
should provide power to the VAA power pin, protection diodes, RF modulator, VREF,
VBIAS, and COMP decoupling. There should be at least a 1/8-inch gap between the
digital and analog power planes, connected by a single point through a ferrite bead, as
illustrated in
overlapping both analog and digital power planes. The path back to the power supply
should be with the lowest impedance possible with only one possible return path. This
layout eliminates noise on the analog signals caused by cross-currents from digital
switching.
This bead should be located within 3 inches of the Bt864A/Bt865A. The bead
provides impedance to switching currents, which provides increased impedance at
high frequencies. A low-resistance (<0.5 Ω) bead should be used, such as Ferroxcube
5659065-3B, Fair-Rite 2723021447, or TDK BF45-4001.
Table
Figure
3-1.
3-2. The ground plane should be a single unified plane
Conexant
Figure
3-2. The recommended component values
3-1