Re: [SI-LIST] : Parallel Plate Capacitance for Bypass

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From: Ron Miller (rmiller@Brocade.COM)
Date: Mon Mar 27 2000 - 10:39:10 PST


Thanks Steve, I knew it was somewhere around there.

So, by extension, 2.2 mil dielectric, available at very little if any
extra cost should be about 1000pf/in sq for FR4
and eliminatee the need for discrete capacitors below 1000 pf.

ron

"S. Weir" wrote:

> Ron,
>
> 5 mils should yield about 200pF / in sq, before taking away for vias.
>
> Regards,
>
>
> Steve.
> At 02:30 PM 3/24/2000 -0800, you wrote:
>
>> > Chris
>>
>> Go to the AVX webpage and the American Technical Ceramics web
>> pages. They have S-parameters which show that the smaller profile
>> ceramics are good to above 1 Ghz.
>>
>> Small profile usually means smaller capacitance so you may need
>> a small one of .01 uf and a larger one of about .47 uf.
>>
>> For frequencys 3 Ghz and up use a capacitive layer in your
>> board. Recently I heard that with 5 mil FR4 dielectric you get about
>> 100pf per square inch. So, using a 2 mil layer would give about 7.5 X
>> this value(per square inch).
>>
>> Ron Miller
>>
>>
>>
>>
>> > According to specifications, X7R type chip capacitors used for power
>> > subsystem bypass seem to become ineffective above 150 to 200 Mhz. For
>> > frequencies higher up, you are reliant upon the internal parallel plate
>> > capacitance. I have two questions. 1. Is the calculation for the required
>> > capacitance that is needed for the parallel plate capacitance the same as
>> > that used for a discrete chip capacitor bypass network, or is there a
>> > conservation of charge situation where the real answer is Cpp*dVnoise =
>> > Cload*dVchange? (where Cpp = parallel plate capacitance, dVnoise =
>> > tolerable change in power supply voltage, Cload = sum of load capacitance
>> > being switched simultaneously, dVchange = voltage change through driver
>> > output switch). 2. If you have a power plane sandwiched between 2 ground
>> > planes, does the parallel plate capacitance double for a given area?Thank
>> > you,Chris HansenSr. Design EngineerDPT & Adaptec Companies
>>
>> --
>> Ronald B. Miller _\\|//_ Signal Integrity Engineer
>> (408)487-8017 (' 0-0 ')
>> fax(408)487-8017
>> ==========0000-(_)0000===========
>> Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose,
>> CA 95131
>> rmiller@brocade.com, rbmiller@sjm.infi.net
>>
>>
>

--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
     ==========0000-(_)0000===========
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net

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