Re: gEDA-user: High speed decoupling (was: Simulation of ceramic capacitors, pairs and groups)

[Karel Kulhavy <clock@xxxxxxxxxxxxx>]

On Tue, Mar 07, 2006 at 09:53:15AM +0000, Evan Lavelle wrote:
> Karel Kulhavy wrote:
> >I have made a simulation to see how the capacitor pairs and groups actually
> >behave:
> >http://ronja.twibright.com/technotes/cercap.php
> 
> Thanks; very nice. However, I'm not sure that the numbers are right. The 
> figure shows that a 10nF//100nF pair gives a low impedance (< ~1 Ohm) 
> over a range of about 1.5MHz to 200MHz, which would make it ideal for 
> decoupling high speed parts. I compared this with an old Cypress HOTLink 
> app note where 22nF//100nF pairs were analysed, and the author's opinion 
> was that this wasn't suitable for high-speed decoupling, because of a 
> high resonant peak (~ 100 Ohm) at 150MHz. Your results show a peak of 
> about 0.4 Ohm (at 30MHz), so are much better.
> 
> The differences are:
> 
> 1) The appnote assumes real parasitics of 5nH for a surface-mounted MLC 
> cap, compared to your manufacturer's figures of 1.6 - 1.9nH. Your 
> figures also seem to be for leaded caps: the SM figures are even lower.
> 
> 2) The appnote gives a lower ESR of about 30 mOhm, while you're using 
> 100 - 150 mOhm. The peak height varies inversely with the ESR.
> 
> Anyone have any other thoughts on real in-system ESL and ESR numbers?

I think the ESR and ESL doesn't depend on if the capacitor is in system
or alone. What we need is just a reliable source of numbers or someone
measuring it. I don't know how I can measure ESR and ESL on a scope.

CL<