2.3.1 Decoupling capacitors

Voltages from traces on a printed circuit board can couple to each other in places where it is not desired, (like power supply planes). To decouple the traces, we add capacitors to absorb some of the voltage and help reduce this effect. For more information on how to correctly place decoupling caps, see the data sheet section for power-supply decoupling.

PLL and some analog supplies benefit from filters or ferrite beads to keep the noise from causing clock jitter. The minimum recommendation is a ferrite bead with a resonance at 100 MHz along with at least one capacitor on the device side of the bead. Additional recommendation is to add one capacitor just before the bead to form a Pi filter. The filter needs to be as close as possible to the device pin, with the device side capacitor being the most important thing to be close to the device pin. PLL pins close together can be combined on the same supply. PLL pins farther away from each other may need their own filtered supply.

For more information, see General Hardware Design/BGA PCB Design/BGA Decoupling (SPRABV2).