[SI-LIST] Re: placement of decoupling caps (again)
- From: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
- To: leeritchey@xxxxxxxxxxxxx
- Date: Wed, 28 Apr 2004 10:34:12 -0700
The other solution to combat high frequency noise, due to fast switching
transients, is by continuous ground referencing of all high speed
signals. Ground referencing eliminates most of the mode conversion
noise in via transitions, with ground plane to ground plane vias
providing the additional shorts necessary to suppress propagation of
noise throughout the PCB.
If you keep a continuous ground reference on all high speed signals,
from package to package, then you do not need thin power/ground
dielectric layers, as Chris Cheng has pointed out many times.
Decoupling capacitors have a maximum effective useful range of
approximately 100 MHz due to mounted inductance, above that the silicon
and package are responsible for power delivery. For 100 MHz and below,
capacitor placement is relatively unimportment if, and only if, there
are no appreciable power plane resonances below about 200 MHz. It is
when power plane resonances occur that placement of capacitors becomes
extremely critical, since capacitors can be used to alter the resonance
pattern with careful placement. The solution to this problem is to
eliminate structures which cause mid-frequency (50 to 200 MHz) resonances.
If ground referenced signaling is used, along with power planes without
mid-frequency resonances, and good capacitor mounting/breakout
practices, then thin dielectric power/ground layers are not needed, and
capacitor placement is not critical.
best regards,
scott
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
Teraspeed is the registered service mark of
Teraspeed Consulting Group LLC
Lee Ritchey wrote:
>I should have qualified my statement by observing that all of these tests
>have been done with power planes spaced closely together, <10 mils, in
>order to produce a good plane capacitor. When the planes are far apart,
>
>
>>15 mils, it is true that one can observe changes in behavior as capacitors
>>
>>
>are moved about. Under these conditions, what is also true is that high
>frequency ripple, >200 MHz, is large and doesn't get very good no matter
>what one does with the capacitors.
>
>As a result, EMI is likely to be high, ripple on Vdd is going to be high
>and the circuit is likely to be "flaky" if there are any wide buses with
>fast rise times. This problem is fixed by making sure there is a good
>plane capacitor. This is done by placing the planes close together. Soon
>as the planes are close together, doesn't matter where you put the caps.
>
>
>Lee W. Ritchey
>Speeding Edge
>P. O. Box 2194
>Glen Ellen, CA 95442
>Phone- 707-568-3983
>FAX- 707-568-3504
>
>I just took the energy it took to be angry and wrote some blues.
>Count Basie
>
>
>
>
>>[Original Message]
>>From: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
>>To: Eric Steimle <steimle@xxxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
>>Date: 4/14/2004 3:38:41 PM
>>Subject: RE: [SI-LIST] placement of decoupling caps (again)
>>
>>UMR did a paper in 1995 that determined location is not important as long
>>
>>
>as you don't cut up the power planes. Others have repeated those same
>tests, as have I, and come up with the same answers.
>
>
>>The proponents of location matters need to do some testing to show that
>>
>>
>their position is valid.
>
>
>>Lee
>>
>>Lee W. Ritchey
>>Speeding Edge
>>P. O. Box 2194
>>Glen Ellen, CA 95442
>>Phone- 707-568-3983
>>FAX- 707-568-3504
>>
>>I just took the energy it took to be angry and wrote some blues.
>>Count Basie
>>
>>
>>
>>
>>>[Original Message]
>>>From: Eric Steimle <steimle@xxxxxxxxxxxxx>
>>>To: <si-list@xxxxxxxxxxxxx>
>>>Date: 4/14/2004 1:59:37 PM
>>>Subject: [SI-LIST] placement of decoupling caps (again)
>>>
>>>I know this has been discussed many times before and so far I have:
>>>
>>>
>>>Read all the Sun papers on the yahoo list
>>>
>>>Read through many papers from AVX on decoupling
>>>
>>>Spent some time searching through the list archives
>>>
>>>And read through many papers suggested in the list archives
>>>
>>>
>>>
>>>So far I've come up with a lot of information on how to choose bypass
>>>caps, how many to use, what kinds, ESL, ESR, but not much on the correct
>>>placement of these bypass caps in a normal system (normal to me would be
>>>a processor some ram and maybe some DAAs or other peripherals). While
>>>combing through the list archives I've seen some people saying location
>>>doesn't matter, while others arguing that it does. So I was hoping that
>>>some people could suggest some further sources of information about
>>>decoupling cap placement that would help me make more informed
>>>decisions.
>>>
>>>
>>>
>>>Thanks in advance
>>>
>>>Eric Steimle
>>>
>>>
>>>
>>>
>>>
>>>------------------------------------------------------------------
>>>To unsubscribe from si-list:
>>>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>>
>>>or to administer your membership from a web page, go to:
>>>//www.freelists.org/webpage/si-list
>>>
>>>For help:
>>>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>>
>>>List FAQ wiki page is located at:
>>> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>>>
>>>List technical documents are available at:
>>> http://www.si-list.org
>>>
>>>List archives are viewable at:
>>> //www.freelists.org/archives/si-list
>>>or at our remote archives:
>>> http://groups.yahoo.com/group/si-list/messages
>>>Old (prior to June 6, 2001) list archives are viewable at:
>>> http://www.qsl.net/wb6tpu
>>>
>>>
>>>
>
>
>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>//www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
>List technical documents are available at:
> http://www.si-list.org
>
>List archives are viewable at:
> //www.freelists.org/archives/si-list
>or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
>
--
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
Teraspeed(SM) is the service mark of Teraspeed Consulting Group LLC
------------------------
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
