Decoupling Capacitors & Ground Planes - Analog Devices ADSP-2106x SHARC User Manual

• The use of 3.3V components and power supplies will help transmission
line problems significantly because the receiver switching voltage of
1.5V is close to the middle of the voltage swing. In addition, ground
bounce and noise coupling will be less. The ADSP-2106x is available in
a 3.3V version.
• Experiment with the board and isolate crosstalk and noise issues from
reflection issues. This can be done by driving a signal wire from a pulse
generator and studying the reflections while other components and
signals are passive.
11.5.6 Decoupling Capacitors & Ground Planes
Ground planes must be used for the ground and power supplies. A
minimum of eight bypass capacitors (0.02 µF ceramic) should be used,
placed very close to the VDD pins of the package. See Figure 11.11.
Use short and fat traces for this. The ground end of the capacitors
should be tied directly to the ground plane outside the package
footprint of the ADSP-2106x, not within the footprint (i.e. underneath
it, on the bottom of the board). A surface-mount capacitor is
recommended because of its lower series inductance. Connect the power
plane to the power supply pins directly with minimum trace length. The
ground planes must not be densely perforated with vias or traces as
their effectiveness will be reduced. In addition, there should be several
large tantalum capacitors on the board.
Figure 11.11 Bypass Capacitor Placement
ADSP-2106x
Place bypass capacitors outside of the package.
11.5.7 Oscilloscope Probes
When making high-speed measurements, be sure to use a "bayonet"
www.BDTIC.com/ADI
System Design
ADSP-2106x
Do not put the bypass capacitors within the package footprint
(on the underside of the board) if signal feedthroughs
perforate the ground plane. Return current must flow to the
outside ground plane, which may be poorly connected to the
inside ground plane if this is perforated by feedthroughs.
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