2nF ???
Is that all?
Have I been getting RIPPED OFF all these years?
sheesh!
J
.
On 1/4/07, Haldor Husby <[log in to unmask]> wrote:
>
> I agree that interplane capacitance between closely spaced planes is very
> valuable as high-frequency decoupling, but I would add a caution to
> relying
> on it for continuty of the return path. It will work or not depending on
> the
> application, the size of the board and the frequencies of the return
> currents that needs continuity.
>
> In the case of a 100mmX100mm board with 8 mil spacing between a PWR and a
> GND plane, I calculate the parasitic capacitance to be about 2nF. That
> represents an impedance of about 0.8Ohm which would be OK for many
> applications (but not all). At 1MHz the impedance is 80Ohm, and in most
> applications this will force the return current to flow places you don't
> want it. It will work poorly for much of the noise generated by switching
> power supplies, as an example. Smaller layer spacing improves the
> situation,
> smaller boards makes it worse. The designer must in any case calculate the
> capacitance and assure himself/herself that it is adequate in each case,
> not
> rely on it as a general rule.
>
> I would also keep in mind that the interplane capacitance is distributed.
> For very large return currents this will cause the current to flow
> throughout the planes with potential differences across the planes as
> possible results. It is better to use a single net for all AC return as
> far
> as possible. And this is not even thouching on the problems that arise
> when
> power planes with multiple power domaines are used as return
>
> The comment about not using discreet capacitors for return at frequencise
> higher than a few hundred MHz is very good. It is very risky to not have
> an
> explicit return i copper for high-frequency buses. Any use of capacitive
> return must be made with close attention to the inductance in the
> connection, also interplane capacitance.
>
> Med vennlig hilsen/Best regards
> ________________________________________________
> Haldor Husby, Senior Development Engineer
> Data Respons Norge AS
> Kongsberg Næringspark
> P.O. Box 1022
> N-3601 Kongsberg, Norway
>
> Tel: +47 32 29 94 00 Fax: +47 32 29 94 40
> Dir: +47 32 29 94 18 Mob: +47 48 04 83 68
> [log in to unmask]
>
>
> On Thu, 4 Jan 2007 12:35:59 -0600, Susy Webb <[log in to unmask]> wrote:
>
> >Hi Jack....
> >
> >The ideal choice is to reference the signal to opposite sides of the same
> plane when it must change directions, but that cannot alwyas be done.
> Another option as you said, is to reference two different ground planes
> because the return can move very easily through the vias.
> >
> >But returning signals on power layers will work too. You should ALWAYS
> have
> power and ground layers next to each other at least once in your stackup
> for
> medium to high speed boards. If the power and ground are close to each
> other
> <.008" then the signal return can couple from one plane to the other and
> back to the source.
> >
> >As an example, if you consider the source of the energy to be the cap
> near
> an IC, the energy then flows from the power side of the cap to the power
> pin
> of the part, then through the part and out the signal pin and out onto a
> signal layer. Let's say that signal layer references the ground plane, and
> that ground plane is part of a plane pair. When the signal has to change
> direction, it moves to the signal layer on the other side of the plane
> pair
> and the return now references the power plane. The return on the power
> plane
> can capacitively couple to the ground plane because they are so close to
> each other and then find it's return path back to the source... the ground
> side of the cap. This does not increase the inductive loop because the
> return path does not have to separate from the original signal. Remember,
> returnis happening constantly, not just at the end of the signal.
> >
> >If the power and gound planes are separated, this will not work and you
> will be relying on capacitors for return as you mentioned. HOWEVER, the
> faster the frequency, the more limited the usage of the capacitors (even
> high frequency ones), and they are virtually useless above 2-300 MHz. So
> it
> is better to use the inter-plane capacitance of having the two planes
> together.
> >
> >Hope this is clear enough to understand.... it's hard to describe in text
> what a picture would easily draw.
> >Susy
> >
> >>
> >> From: Jack Olson <[log in to unmask]>
> >> Date: 2007/01/04 Thu AM 09:50:41 CST
> >> To: [log in to unmask]
> >> Subject: [DC] Straddling the Return Path
> >>
> >> I have a question about layer stackup and routing.
> >>
> >> Most of our designs have a pair of routing layers
> >> sandwiched between planes, one for vertical and
> >> the other for horizontal traces.
> >> Maybe I haven't had enough coffee yet today, but
> >> I started wondering how the return path energy
> >> gets from one plane to the other (unless there
> >> happens to be a decoupling cap nearby?) If both
> >> planes were GND there wouldn't be a problem,
> >> since they would be stitched together in many
> >> places, but one is GND and the other is PWR.
> >>
> >> So I started wondering.... Wouldn't it be better to
> >> have one routing layer ABOVE the GND plane and
> >> one BENEATH? In this scheme all the routing would
> >> be straddling the SAME return path and it seems
> >> logical the it would be less noisy. There would never
> >> be a discontiuity from the energy trying to get from
> >> one plane to the other, right?
> >>
> >> maybe the gains are not worth the loss in the
> >> overall scheme, though?
> >>
> >> onward thru the fog,
> >> Jack
> >>
> >>
>
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