Re: [SI-LIST] : bypass cap question (long, simple)

Jeff Seeger (jseeger@appliedcad.com)
Tue, 03 Feb 1998 14:41:12 -0500

Nirmal Jain wrote:
>
> Dear Jeff,
>
> > Just to add a question to this fine discussion, what role
> > does the capacitor formed by the laminate between reference
> > planes play? A great many designs are organized in strip-
> > line pairs, with signals contained in such a cavity.
>
> The capacitor formed by the GND/PWR plane is generally
> referred as "Integrated Decoupling Capacitor" and the
> frequency response of the integrated decap (very high Q)
> is generally much better than the discrete decap.
> snip <
> For a 10 cm sq. board with 5 mil (127micron) thick laminate
> (diel constant of approx 4) between the GND/PWR plane would
> give you approximately 2787 pF.
> snip <

Nirmal,

All very true. But my question is really about capacative
mechanisms in laminate providing a path for return currents.
If we take the example layup, with two signal layers in a
stripline configuration, conventional layout technique is
to contain a net within this stripline cavity. If this is
done, it appears to me that the capacitor formed by the
three sheets of laminate between the reference planes may
provide a part or all of the path for return currents of
signals changing layers within that cavity.

------------- reference plane
--- wiring, vertical
--- wiring, horizontal
------------- reference plane

The reason a typical layout will take these inner two layers
as a pair is that the outer layers are usually too congested
with components to make a viable pairing with a clear layer.
(if your horizontal layer is 50% covered by component lands
then a 100% clear vertical layer doesn't do you much good)

So my question remains, True (no other d-caps needed)?

Poor (rely on this path and I'll step on your signal)?

False (too high a capacitance to accomodate return currents -
discrete caps needed)?

Best regards,

-- 
 
      Jeff Seeger                             Applied CAD Knowledge Inc
      Chief Technical Officer                      Tyngsboro, MA  01879
      jseeger "at" appliedcad "dot" com                    978 649 9800