Toshiba TDP-D1 Service Manual page 84

Many ADCs support SOG separation internally. If an external sync separation circuit is required, a
National LM1881 (or equivalent) circuit works well up to XGA resolutions. Use a high-speed
comparator for higher resolutions.
Power
The type of analog circuitry used for these high-speed interfaces can be sensitive to power
supply variations. It has been determined through empirical experimentation that ripple on a given
supply of only 5mV p-p can be seen in the display image. This ripple is typically at a resonant point
for the RGB subsystem, such as the vertical, horizontal, or pixel frequency rates, or a sub-harmonic
or harmonic frequency. To remove these effects, it is mandatory that you use low-ripple supplies
with good L/C isolation decoupling circuits. These decoupling circuit grounds must be returned to
the power supply subsystem and kept isolated from the input analog subsystem. Also, remember that
the display processor has a large digital processing subsystem. These power supplies and their
associated grounds also need to be isolated from anything analog. Many display designers use
separate linear regulators for analog subsystems. These linear regulators can remove switching noise
from the power planes. In general, using separate ground planes is not a proper way to isolate analog
circuitry from the power supply or digital sections of a design. Instead, it is best to direct the
decoupling return through the shortest possible path to its source. Power planes, however, should be
kept out of areas where the power is not needed. For example, do not allow a power plane to lie
below or above the RGB signal subsystem. This reduces stray capacitance while reducing possible
noise injection. The critical power design considerations are:
Isolate the power supplied to the ADC and PLL (analog power) from the power supplied to the
digital portions of the circuit board. Isolate the power planes for these portions of the circuit board
from the digital power plane, although they all can be on the same layer. Use a linear regulator to
supply these voltages to reduce the risk of switching noise being present on the plane.
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