RE: [SI-LIST] : Plane Capacitance

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From: Patterson, Ken ([email protected])
Date: Thu Nov 11 1999 - 12:16:52 PST


Mike,

Placing the power and ground planes can benefit by helping to reduce noise at high frequencies and to reduce the number of bypass caps. By using 2 mils between power and ground plane can give you 483pF per sq. in. when using FR4 with an er of 4.3. This is given by the equation C = 0.225*er*A/d where er is dielectric constant, A is board area in sq.in. and d is the separation between power and ground planes. It turns out to be about 100pf per sq. in. with a separation of 10 mils.

Regards,

Ken Patterson
Sr. Electronic Engineer
ADC/Pathway
777 Beta Dr.
Mayfield Village, Ohio 44143
(440)473-0270

-----Original Message-----
From: Mike Hughes [mailto:[email protected]]
Sent: Thursday, November 11, 1999 1:34 PM
To: [email protected]
Subject: Re: [SI-LIST] : Plane Capacitance

Thanks for all the replies so far, they have been helpful.

To clarify a little: cost is always an issue, but for my application
(mixed-signal IC testing), board costs are a small fraction of the total project
cost. Also the intent of my question is to learn more about the theory, and then
apply what I can.

In my application I do not use signal layers between the planes. I typically use
5 mil spacing between power and ground.

Signal
50ohm Z
GND
5mil
PWR
Fill for required thickness
PWR
5mil
GND
50ohm Z
Signal

I've been wondering if I can use 4 mils or 3 mils to raise the plane
capacitance. I was planning on staying with FR4. As far as 2, 3 or 4 mils being
manufacturable is something that I didn't intend to discuss here, but I
appreciate any replies none the less. My main intention is to find out if
raising capacitance in this way can hurt decoupling. A second intention is to
learn more about determining decoupling requirements (effective area of
capacitance, etc).

Mike

-- 
================================
Mike Hughes               
Product/Test Engineer      
High Speed Conversion       
Analog Devices, Inc. 

Phone 781-937-2370 Fax 781-937-1011 mailto:[email protected] http://www.analog.com ================================

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