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January 2007

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Subject:	Re: Straddling the Return Path
From:	Charles Gervasi <[log in to unmask]>
Reply To:	Designers Council Forum <[log in to unmask]>, Charles Gervasi <[log in to unmask]>
Date:	Fri, 5 Jan 2007 07:31:55 -0800
Content-Type:	text/plain
Parts/Attachments:	text/plain (58 lines)

Jack,

In many cases the planes' capacitance is much less than 2nf, yet I still
think we are getting our money's worth from it because it doesn't roll
off as fast a component's capacitance does. 

Even low value capacitors' impedance starts rising when you get to 100s
of MHz.  If you have fast edge rates, it's easy to have strong spectral
components into the GHz.  

Here is a formula for plate capacitance from Howard Johnson's book:
C = 2.249 * 10^(-13) * Er * A / h.
A = Area in sq in.  h = separation in inches.  

I can't find any formulas that show at what frequencies plane
capacitance stops working, but I believe it's much higher than for
components you buy and put on the board.

Regards,

CJ

-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Jack
Olson
Sent: Thursday, January 04, 2007 8:39 PM
To: [log in to unmask]
Subject: Re: [DC] Straddling the Return Path

2nF ???

Is that all?

Have I been getting RIPPED OFF all these years?

sheesh!

J


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