ADSP-21368 SHARC Processor Hardware Reference 14-35
System Design
Decoupling Capacitors and Ground Planes
Extended copper planes must be used for the ground and power supplies.
Designs should use an absolute minimum of 12 bypass capacitors (four
0.1 μF, four 10 nF and four 1 nF ceramic) for each VDDEXT and VDDINT
supply. More extensive bypassing may be required for some applications.
The capacitors should be placed close to the package as shown in
Figure 14-9. The decoupling capacitors should be tied directly to the
power and ground planes with vias that touch their solder pads. Sur-
face-mount capacitors are recommended because of their lower series
inductances (ESL) and higher series resonant frequencies. Connect the
power and ground planes to the ADSP-21367/8/9 and ADSP-2137x pro-
cessor’s power supply pins directly with vias—do not use traces. The
ground planes should not be densely perforated with vias or traces as this
reduces their effectiveness. In addition, there should be several large tanta-
lum capacitors on the board.
L
Designs can use either bypass placement case shown in
Figure 14-9, or combinations of the two. Designs should try to
minimize signal feedthroughs that perforate the ground plane.
Oscilloscope Probes
When making high speed measurements, be sure to use a “bayonet”-type
ground clip (or similarly short, < 0.5 inch), attached to the tip of the oscil-
loscope probe. The probe should be a low capacitance active probe
with 3 pF or less of loading. The use of a standard ground clip with 4
inches of ground lead causes ringing to be seen on the displayed trace and
makes the signal appear to have excessive overshoot and undershoot.
A 1 GHz or better sampling oscilloscope is needed to see the signals
accurately.
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