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AM335x - PCB layout decoupling and ferrite beads on VDDSHx rails?

AnBer
Mastermind 30120 points
Other Parts Discussed in Thread: AM3358, AM3359

Team,

Looking at the decoupling guidelines it seems that Ferrite beads are a good practises for some of the supplies like PLL, ADC supplies:
http://processors.wiki.ti.com/index.php/General_hardware_design/BGA_PCB_design/BGA_decoupling#Ferrite_Bead_Recommendations
Indeed the AM3358 Starterkit schematics does use one for the ADC supply at page 4.

- But there do not seems to be guidelines to use ferrite bead specifically for all I/O rails (like VDDSHx). On AM3358 EVM and starter kit ferrite beads are used on VDDSHX rails and on the AM3359 ICE not. Are there some specific reasons why ferrite beads have been used in one case and not in the other?

- Are ferrite bead needed as a precaution because of the voltage level detection circuit of the dual IO technology?


Thanks and best regards,

Anthony

  • 0
    TI__Guru**** 393215 points
    Hi Anthony,
     
    I don't think dual-voltage I/O cells have anything to do with the ferrite beads. From what I see ferrite beads have been added on all boards where the VDDSHVx rails are exposed on an external layer of the PCB (test points or current sensing resistors). Ferrites together with decoupling capacitors form a low-pass LC filter that should reduce eventual noise emissions from these additional exposed components.
     
    For custom design my opinion is that burying the supply rails on internal PCB layers should provide enough noise immunity if everything is properly routed and shielded with GND. Of course recommended quantity of decoupling capacitors and their placement as close as possible to processor power balls is very important too.
  • 0 in reply to Biser Gatchev-XID
    TI__Mastermind 30120 points

    Biser,

    Thanks for your comment.