[SI-LIST] Re: DDR2 2-slot design preference...

Jabori, the issue in each case will be the resonant frequency of the GND 
- GND cavity.  Stitch the cavity with an adequate via density to get the 
resonance well above your signal energy and either topology can be made 
to work well.

Steve.
Jabori, Monji wrote:
> Hi Experts,
>  
>
> I am looking at two designs for a 2-slot DDR2-800 memory system. One is
> a butterfly design and the other has the slot on opposite sides of the
> motherboard (top and bottom).
>
> I am looking at the GND reference for both designs and have the
> following question knowing that I have to use GND referenced layers for
> each DDR2 channel.
>
>  
>
> For a butterfly design, my traces on an 8-layer design will flow from
> the memory controller via 2 internal layers with 2 different GND layers,
> i.e., one slot will have to change GND reference while the other will
> not.
>
>  
>
> In an opposite 2-slot design, however, we can make the traces that go to
> each internal layer have the same GND reference as the DIMM that is
> closer to it. For example, the Top DIMM will have the same GND reference
> (L2) as its channel traces coming on L3 while the Bottom DIMM will have
> its GND reference (L7) as its channel traces coming on L6.
>
>  
>
> Having said the above, would you guys prefer one design over another
> from an Signal Integrity point of view??
>
>  
>
> Thanks in advance.
>
>  
>
> Monji
>
>  
>
>  
>
>
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