Ian:

Thanks for the refresher lesson in "how real world parts you can actually
get your hands on perform" as opposed to "theoretically, a smaller case size
will always perform better".

Best regards,
Ivan Baggett
Bagotronix Inc.
website:  www.bagotronix.com


----- Original Message -----
From: "Ian Wilson" <[EMAIL PROTECTED]>
To: "Protel EDA Forum" <[EMAIL PROTECTED]>
Sent: Wednesday, June 04, 2003 7:37 AM
Subject: Re: [PEDA] Re[2]: six or eight-layer (or more?) stackups -
Capacitance


> On 07:14 PM 4/06/2003, John Sheahan said:
> >On Wed, Jun 04, 2003 at 09:34:18AM +0200, Norbert Hoppe wrote:
> > > When selecting parallel caps, it is important to remember that as the
> > larger
> > > value capacitor goes inductive, the smaller value cap is still
capacitive.
> > > At a particular frequency, a LC circuit is developed between the 2
caps.
> > > An infinite impedance could be generated with no decoupling benefit
> > provided.
> > > When this occurs, single-capacitor decoupling is all that one can use
> > > for this application.
> >
> >actually - if you look at ESL graphs for multilayer SMD caps - you will
> >see it depends much more on case size than on capacitance.
> >So the 100n tends to win.
> >This quote I think may be  older wisdom for thruhole components.
> >john
>
> The first resonance (at least) of a cap is series, so looks like a short
> circuit. By adding a number of different valued caps you can scatter a
> number of these nice AC shorts around your board and around your
> frequencies of interest.
>
> Above resonance the reactive impedance starts to rise as the impedance
> characteristic is now inductive.  In many cases this is not an issue, as
> the effective reactance is still low in the frequencies of interest.  In
> other situations, though, it is a critical issue and hence designers have
> used, and will continue to use, a variety of values in parallel - very
> common in RF environments.
>
> However, big small caps, or is that small big caps, you know ... large
> capacitance in small volume, have pretty cruddy material, X7R if you are
> lucky or Z5U if capacitance is big.  These materials have pretty poor, and
> frequency dependent, ESR which decrease their value as decouplers.  Due
> largely to the effects of the lossy material, a Kemet, for example, X7R
> shows a sloppy self resonance and the following series impedance at 100
MHz:
>
> Value   Size  Impedance
> 103      0603    ~1 ohm
> 103     0805    ~0.5 ohm
> 103     1206    ~0.3 ohm
> 104     0805    ~1 ohm
> 104     1206    ~1 ohm
>
> So if you spec a 10nF 0603 you have a resistor, not a decoupler, at 100
> MHz.  According to Kemet, the 0603 only performs better than the other
> sizes at over the narrow freq range of about 10 to 30 MHz.  The lossy
> dielectric, and the need to use thinner metal in the large capacitances
(to
> keep the pkg the same), is killing the performance.  Maybe that is an
> overstatement - but compare the self resonance curves of a COG/NPO
material
> to that of a X7R, or worse Z5U,  you can see the dramatic effect the
losses
> have on the resonance shape.  COG/NPO has dissipation factors in the order
> or 0.1% while the other materials are between about 2.5 to 5% or more.
(In
> fact, the lower Q of the high capaciatnce devices is partially a good
> thing.  Having high Q resonances around a board is a shocker when you find
> you fail EMC.)
>
> Note also that the 100n 0805 has roughly *twice* the impedance @ 100 MHz
> than the cap *one tenth* the value in the same pkg!  In this case, if you
> are operating above 30 MHz the 1206 10n wins, followed closely by the 0805
> 10n. 100n in any pkg and 10n in 0603 have about twice the impedance.
>
> See figures 4, 5 & 6 of:
>
http://www.kemet.com/kemet/web/homepage/kechome.nsf/vapubfiles/F3102Gce/$fil
e/F3102GCe.pdf
>
> There is always progress in material science so the small-packaged, larger
> capacitance devices get better over time.
>
> For modern high speed decoupling - I use lots of 10n devices, a few bulk
> devices and good (hopefully) layer stackup and split plane
> arrangement.  Specific devices operating at speed will have special
> treatment.  Currently, I side with the get to the plane fast crowd and
have
> my supply via close to the power pads and then decoupling caps strung to
> these same vias with nice fat tracks.  I don't, usually, have a via,
track,
> cap, then component pad arrangement - though my guess is, with the right
> sort of component selection either arrangement can be done well.
>
> Ian Wilson
>
>
>



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