[SI-LIST] Re: Capacitors and Anti-resonance

Well I did my homework
in "High-Speed Digital Design" by H. Johnson & M. Graham. (12th printing)
It is said p. 260...
" a rising edge of 1ns propagating in FR-4 material has a length of about l=
6 in.
No benefit will be derived in this example from a capacitor grid spaced
further apart than l/12 = 0.5 inch"

Now if I follow the same logic...
for a 1 ns system I can use different values of caps for every sections 
if
they are spaced apart by more than 0.5inch
without risking any antiresonance effect.

Does it make sense?
Anybody?


>  -----Original Message-----
> From:         Paradis, Daniel  
> Sent: Tuesday, July 29, 2003 9:58 AM
> To:   'si-list@xxxxxxxxxxxxx'
> Subject:      Capacitors and Anti-resonance
> 
> I started to use only 0.1uF decoupling caps on our latest designs.
> This is to avoid the anti-resonance effect that occurs when using 2
> different caps near each other.
> The rule of thumb is to use the largest cap available for the package. 
> 
> So far I got good results, but I was wondering....
> 
> Is-there a simple way to determine the distance between 2 different
> decoupling caps beyond which there is no anti-resonance effect?
> 
> In other words..
> Say that I stick to this general rule of using only one cap.
> It seems to me that there could be exceptions to that rule.
> In the case of a high speed clock buffer for instance; 
> I could need 0.0047uF instead of 0.1uF
> If this clock buffer is located far from other parts of the design. 
> Can I avoid anti-resonance?
> 
> 
> Daniel Paradis
> Staff Electrical Engineer
> Digital Subscriber Networks
> 
> Scientific-Atlanta, Inc.
> 5030 Sugarloaf Parkway, ATL 1.3468
> Lawrenceville, GA 30042
> Tel: (770) 236-7896
> Fax: (770) 236-2449
> 
> 


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