[SI-LIST] anlog and digital power plane isolation with ferrite bead good idea?

  • From: Dong Kim <kimdongsik_us@xxxxxxxxx>
  • To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>, steve weir <weirsi@xxxxxxxxxx>, si-list@xxxxxxxxxxxxx
  • Date: Mon, 27 Dec 2010 16:03:19 -0800 (PST)

My colleague, a PDN engineer, insist he has to use ferrite bead to isolate an 
analog power plane from digital plane for an large FPGA I am using on a PCB 
I have been trying to convince him it would be better to have good decoupling 
filter by adding inner-plane capacitance and decoupling caps. 
But, he is insisting I should prove it either by theory or simulation before he 
can change his mind to remove the isolation ferrite bid and add planer cap in 
my board stack-up.  I could not really prove by methodical thory nor have time 
and tool to simulate.
With my shallow  knowledge, it will just end with arguing even if he agree to 
do what I tell him to do.
My knowledge is  from my past board design with similar characteristics, I did 
not use the ferrite to isolate the analog and digital power planes.   It worked 
I also has shown him Lee Ritch's 2nd volume of "Right the first time".  
But, I guess it did not convince him.
Please someone explain why analog and digital power isolation by using may be 
not a good idea.
Please help. 
Dong S. Kim

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