[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

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.


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