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

[Evan Lavelle <eml-geda@xxxxxxxxxxx>]

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?

Evan