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High Frequency Design Considerations
Decoupling Capacitors and Ground Planes
Ground planes must be used for the ground and power supplies. The
capacitors should be placed very close to the
package as shown in
ground end of the capacitors should be tied directly to the ground plane
inside the package footprint of the processor (underneath it, on the bot-
tom of the board), not outside the footprint. 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. A ground plane should be located near the component side of
the board to reduce the distance that ground current must travel through
vias. The ground planes must not be densely perforated with vias or traces
as their effectiveness is reduced.
VDDINT is the highest frequency and requires special attention. Two
things help power filtering above 100 MHz. First, capacitors should be
physically small to reduce the inductance. Surface mount capacitors of size
0402 give better results than larger sizes. Secondly, lower values of capaci-
tance will raise the resonant frequency of the LC circuit. While a cluster of
.1uF is acceptable below 50 MHz, a mix of .1, .01, .001uF and even
100pF is preferred in the 500 MHz range.
Note that the instantaneous voltage on both internal and external power
pins must at all times be within the recommended operating conditions as
specified in the product data sheet. Local "bulk capacitance" (many micro-
farads) is also necessary. Although all capacitors should be kept close to
the power consuming device, small capacitance values should be the clos-
est and larger values may be placed further from the chip.
21-10
Figure
21-4. Use short and fat traces for this. The
ADSP-BF537 Blackfin Processor Hardware Reference
and
VDDEXT
VDDINT
pins of the
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