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 > > > * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/[EMAIL PROTECTED] * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
