[SI-LIST] Re: Bypass vs Decoupling capacitors
- From: steve weir <weirsp@xxxxxxxxxx>
- To: istvan.novak@xxxxxxxxxxxxxxxx, <cpad@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 27 Aug 2004 22:06:29 -0700
Istvan, we agree that the phase angle needs to stay near zero. The
difference in viewpoint here is that via inductance, unless we put a whole
lot of vias in parallel is going to be a problem. Whereas a lossy material
effects a resistive shunt.
Regards,
Steve.
At 11:14 PM 8/27/2004 -0400, Istvan NOVAK wrote:
>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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