[SI-LIST] Re: Bypass vs Decoupling capacitors
- From: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>
- To: <weirsp@xxxxxxxxxx>, <cpad@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 27 Aug 2004 23:14:03 -0400
Steve,
Actually the opposite is true. As it was reported and published,
for the RC termination to be effective, the cumulative inductance
of RC termination parts for a wide
range of board geometries has to be approximately 20% of the
plane inductance or less. If you do a Vcc / Sig / Sig / Gnd sandwich,
the plane inductance becomes higher, and it will be proportionally
easier to satisfy the inductance requirement from the RC termination
components.
Regards,
Istvan
----- Original Message -----
From: "steve weir" <weirsp@xxxxxxxxxx>
To: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>; <cpad@xxxxxxxxx>;
<si-list@xxxxxxxxxxxxx>
Sent: Friday, August 27, 2004 9:01 AM
Subject: [SI-LIST] Re: Bypass vs Decoupling capacitors
> Istvan, the assumption, is that mounted inductance will make discrete
> devices ineffective at HF. That assumption then leads to dedicated Vcc /
> Gnd pairs as necessary with lossy material to damp the cavity, as opposed
> to Vcc / Sig / Sig / Gnd sandwiches. So we lose the second surface of the
> GND layer as an image return.
>
> Regards,
>
>
> Steve.
>
> At 08:55 AM 8/27/2004 -0400, Istvan NOVAK wrote:
> >Steve,
> >
> >I am curious, what do you mean by "it is plane
> > termination that requires not only the extra
> >parts, but extra layers"?
> >
> >Regards,
> >Isatvan
> >
> >
> >----- Original Message -----
> >From: "steve weir" <weirsp@xxxxxxxxxx>
> >To: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>; <cpad@xxxxxxxxx>;
> ><si-list@xxxxxxxxxxxxx>
> >Sent: Friday, August 27, 2004 2:03 AM
> >Subject: Re: [SI-LIST] Re: Bypass vs Decoupling capacitors
> >
> >
> > > Istvan, well we do agree on both the points that it requires judgment
and
> > > cost is a big issue, especially if it means adding layers. I can see
a
> > > number of scenarios where it works the other way, and it is plane
> > > termination that requires not only the extra parts, but extra layers.
The
> > > cost of placements needs to be accounted as well. But then, we get
paid
> >to
> > > figure out the appropriate compromises.
> > >
> > > Regards,
> > >
> > >
> > > Steve.
> > > At 08:48 PM 8/26/2004 -0400, Istvan NOVAK wrote:
> > > >Steve,
> > > >
> > > >Eventually it is a judgement call of the designer. If you have to
> > > >increase the number of layers by one (and to keep the stackup
> > > >symmetry, it is never just one), the board cost may go up by
> > > >$5-15, depending on the size and complexity. For that money
> > > >one can buy many hundred resistors and capacitors to quiet
> > > >the planes.
> > > >
> > > >Regards,
> > > >Istvan
> > > >
> > > >
> > > >
> > > > > Istvan, yes on most of the points we agree. There are always
> >exceptions
> > > >to
> > > > > any hard rule. A large Vcc plane that does not have thin / lossy
> > > > > dielectric is subject to resonance at pretty low frequencies. So
in
> >the
> > > > > cost equation, we have to figure out which costs more:
> > > > >
> > > > > 1) Dealing with the increased route complexity of an islanded
scheme
> > > > > 2) Buying lossy dielectric, or special parts / materials to edge
> >terminate
> > > > > the planes.
> > > > >
> > > > > I am very biased towards 1)
> > > > >
> > > > > Regards,
> > > > >
> > > > >
> > > > > Steve.
> > >
> > >
>
>
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