Re: tiny inductor with every bypass cap?

BobW wrote:
> "Talal Itani" <titani@[EMAIL PROTECTED]
> wrote in message 
> news:suk7k.3904$JL.2049@[EMAIL PROTECTED]
>>> To be honest 50uH and 0.1uF is a recipe for disaster. At the most a 
>>> ferrite SMT-bead should be used but usually I don't even do that. A
nice 
>>> full ground plane and a nice full VCC plane is usually best. Problem
with 
>>> DSP like this is that you need an additional lower voltage supply so
now 
>>> you are up to three supply planes, meaning you won't get away with
less 
>>> than a 6-layer board.
>>>
>>> If it's super critical you could have the analog supplies come from a 
>>> separate regulator but often converters on a chip with fast digital 
>>> processing going on are quite disappointing. A bond wire affords only
so 
>>> much in RF conductivity.
>>>
>>> Hint: Carefully read up on power supply sequencing. Best case wriobng 
>>> sequencing leads to a locked up DSP, worst case to a dead DSP.
>>>
>>> Oh, and please don't top post.
>>>
>> This DSP has 3.3V and 1.8V supply voltages.  Can the same plane have
both 
>> voltages?
> 
> Talal,
> 
> You really need some consulting help if you're asking questions like
this. 
> The edge rates of your signals are likely to be very fast, and this
requires 
> some real know-how when it comes to designing the circuitry, clock 
> distribution and trace lengths, and the general pcb layout. The previous

> comment regarding supply sequencing should be heeded, too.
> 
> A short answer for you is this:
> 
> If you can arrange it, the controlled-impedance traces should couple to
(be 
> adjacent to) solid (unsplit) GND plane layers. Then, you can split your 
> supply planes as you need. Otherwise, you have to be very careful in
routing 
> your traces so as to avoid coupling gaps which will result in large 
> high-inductance coupling loops, or add extra decoupling to couple these 
> split planes near their splits.
> 
> You should seek some professional help for your design and/or take a
class 
> in high-speed signal integrity.
> 

I'd agree. Best to get some help now, not after there are problems with 
the design, the boss is becoming impatient and it's almost too late. 
Talal, get a consultant that is somewhat local or pay him/her to come to 
your work place. Then you can learn the main tricks via a coaching
process.

Yes, it'll cost money but it is money well spent.

-- 
Regards, Joerg

http://www.analogconsultants.com/

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