Re: High speed capacitors

[Rick Collins - Arius <PCImail@arius.com>]

I expect any of these will do nicely since they are all ceramic caps. You 
will want to stay away from electrolytic caps and even tantalum caps for 
this purpose. Any ceramic cap will do the job.

As it turns out, at the high frequencies that are at issue, the capacitance 
has little impact on the effectiveness of a decoupling cap. Above about 50 
MHz, ceramic caps are inductive as the parasitic inductance swamps the 
designed capacitance. The net effect is still useful up to several hundred 
MHz since the impedance is still low even if it is inductive. But the very 
high frequency characteristics are largely determined by the shape of the 
cap and the way it is connected to the circuit. The most effective parts 
are surface mount parts with virtually no circuit board traces. They should 
be connected directly to the power and ground layers with as short a trace 
as possible.

If you want to optimize the parts, then select caps sizes that are 
relatively wide compared to their length. For example, an 0603 part will be 
about the same as a 1206. But a 1210 will be much better than either of the 
others. This can be seen if you check the impedance curves vs. freq. from 
various manufacturers. The best packages are the wide ones. Any value in 
the range of 0.01 to 0.1 uF used across your board will do the trick for 
general decoupling.


At 10:27 AM 6/6/00, you wrote:
> From the book PCI hardware & software:
>
>"However, the PCI bus specification does state that twelve 0.01 uF high
>speed capacitors must be evenly spaced between the 3.3 volt and ground
>layers."
>
>What does the spec. mean with "high speed" capacitors?
>e.g. the philips databook has the folowing capacitor ranges:
>Class 1 NPO , Class 2 X7R, Y5V, Z5U
>
>types of dielectric EIA/IEC, CECC:
>COG/NPO/CG,
>X7R/ 2R1,
>Y5V/2F4,
>Z5U/2E6
>
>Who can shine a bit of light on this matter?
>
>Eric de Jong.

Rick Collins

rick.collins@arius.com


Arius - A Signal Processing Solutions Company
Specializing in DSP and FPGA design

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