# [SI-LIST] UltraCAD ESR and Bypass Capacitor Caculator

*From*: "Abe Riazi" <ariazi@xxxxxxxxxxxxxxx>*To*: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>*Date*: Thu, 31 Jul 2003 11:18:35 -0700

Dear Lee, Please recall that when I was attending your course, we had a brief discussion regarding the UltraCAD's ESR and Bypass Capacitor Calculator. I have purchased a license of the program (it costs $75.00). The program can help to determine the optimum number (and values) of bypass capacitors required for a high-speed design board. Douglas Brooks has written a paper, "ESR and Bypass Capacitor Self Resonant Behavior How to Select Bypass Caps" which describes how to apply the calculator. One point emphasized by his paper is that to achieve optimum decoupling one needs to use a range of values of decoupling caps (example, some 1uF caps, some 0.1uF caps, some 0.01uF caps with low ESL) rather than just a single value of capacitor. The objective being to produce a flat low PDS impendace over a specified broad frequency bandwidth. On the other hand, I have noted that many engineers have a different view in that they believe a SINGLE value of capacitor (such as 0.1uF 0603 (or 0805) package with X7R dielectric ceramic chips) should be used for ALL high frequency decoupling capacitors on the PCB. The UltraCAD calculator does not provide any information related to the best required location for the decoupling caps. The calculator also does not account for the distributed nature and frequency dependent loss of power planes (which can be important at frequencies exceeding 100 MHz.) Subsequently, it seems to me that UltraCAD ESR and Bypass Capacitor Calculator can be a useful tool for learning and experimenting with various decoupling strategies, but probably lacks the accuracy that the Cadence PDS tools can offer (of course, the Cadence PDS tools are also significantly more expensive). I also have two questions: 1. In your opinion, in order to achieve optimum high frequency decoupling, is it necessary to use several different values of ceramic caps (e.g. some 1.0 uF, some 0.1uF some 0.01uF ceramic chips) or is it preferable to use just a single value (such as 0.1uF) for ALL the decoupling caps on the PCB ? 2. Is it sufficiently accurate to determine the optimum location of decoupling caps by applying rules of thumb or is it necessary to simulate? Thank you in advance for your reply. Abe -- Binary/unsupported file stripped by Ecartis -- -- Type: application/ms-tnef ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: http://www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: http://www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu

**Follow-Ups**:**[SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator***From:*Paul Levin

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