4
4.0 Applications
4.1 PC Board Considerations
The layout for the Bt860/861 should be optimized for the lowest noise possible
on the power and ground planes by providing good decoupling. The trace length
between groups of power and ground pins should be as short as possible to
minimize inductive ringing. A well-designed power distribution network is
critical for elimination of 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.
4.1.1 Component Placement
Components should be placed as close as possible to the associated pin so traces
can be connected point-to-point. The optimum layout enables the Bt860/861 to be
located close to both the power supply connector and video output connectors.
4.1.2 Power and Ground Planes
Separate digital and analog power planes are recommended as illustrated in
Figure 4-1. 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 pins, protection diodes, and COMP decoupling. There should be at least an
1/8-inch gap between the digital and analog power planes, connected by a single
point through a ferrite bead. The ground plane should be a single unified plane
overlapping both analog and digital power planes. The path back to the power
supply should have 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.
The bead separating the digital and analog power planes should be located
within three inches of the Bt860/861. The bead provides impedance to switching
currents and high frequency noise. Use a low-resistance (<0.5 Ω) bead, such as
Ferroxcube 5659065-3B, Fair-Rite 2723021447, or TDK BF45-4001.
D860DSA
Conexant
4-1
CONEXANT [ CONEXANT SYSTEMS, INC ]
