[SI-LIST] Re: Power plane coupling
- From: Istvan Novak <istvan.novak@xxxxxxx>
- To: ludovic.levieil@xxxxxxxxxxx
- Date: Thu, 20 Oct 2005 20:42:48 -0400
Ludovic,
Coupling between supply rails is not a problem as long as noise is
properly kept under control .
Vertical coupling between the Vcc1 and Vcc2 layers will be strong in the
frequency range where connected capacitors loose their effectiveness.
Assuming that you treat each supply rail properly within its own domain, a
potential complication arises when the noise signatures are significantly
different among the coupled rails. Suppose you have a 3.3V rail, feeding
generic glue logic, vertically coupled to a supply puddle of an analog
supply rail. Probably you could tolerate 100s of mV noise on the 3V
rail, whereas a few mV may be the limit on a sensitive analog rail.
Spectral signature of the noise and noise sensitivities have to be
factored in,
but this scenario would be risky unless you quiet the 3V rail to an
unrealistic (and otherwise unnecessary) low noise level. The solution:
avoid coupling supply rails with very different noise requirements and
noise characteristics.
Regards,
Istvan novak
SUN Microsystems
Ludovic Levieil wrote:
>Hello All,
>In my current board design I have the following stack up:
>
> .......
>---------------- GND (solid plane)
>------ ----- --- VCC1 (splitted plane)
>--- ----- ------ VCC2 (splitted plane)
>---------------- GND (solid plane)
> .......
>
>4 mils separate GND and VCC planes
>5 mils separate VCC1 and VCC2 planes
>
>Both VCC planes are splitted in different power domains and I am wondering
>:
> - if having two coupled VCC planes is good/acceptable when
>thinking about noise ??
> - if there is a problem in having one power domain on on plane
>overlapping at least two power domains on the other plane ??
>
>Thanks
>
>Ludovic Levieil
>
>------------------------------------------------------------------
>
>
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