[SI-LIST] Re: Power plane coupling
- From: Istvan Novak <istvan.novak@xxxxxxx>
- To: zhang_kun@xxxxxxxxxx, ludovic.levieil@xxxxxxxxxxx
- Date: Tue, 25 Oct 2005 08:10:25 -0400
Zhangkun, Ludovic,
Yes, the vertical coupling between power planes one
above the other at high frequencies is strong. If,
however, the coupled rails are designed to be resonance
free, and they have similar characteristics (impedance,
allowed noise, bandwidth), the coupling does not create
special problems.
As Larry pointed out, a nice feature about the stackup
in question is that the power planes are sandwiched
between grounds, so radiation from the noisy power
plane is less of an issue,and signal traces all
reference ground planes. This building block takes
four layers though, and therefore very seldom can we
afford to have two of these; one close to each surface
of the board; it usually ends up in the middle of the
stackup. If that is the case, be prepared to live with
the increased via inductance connecting the planes to
the surface.
As Zhangkun pointed out, one way to decrease the vertical
coupling is to increase the relative separation between
the two power planes. This can be done by either increasing
the absolute separation vertically between the two power
planes, or reducing the separation between the power
planes and ground planes, or a combination of both.
Increasing the absolute separation between the power planes
is practically free within the maximum board thickness limit
you have on hand. Reducing the power-ground plane separation
to less than 4 mils is also doable; down to about 3 mils is
practically free. Thinner laminates down to 1 mil and 0.5 mil
thicknesses are now also commercially available. Playing with
the vertical separation has its limits. If, say, we assume a
maximum vertical separation between the power planes to be 50
mils and a 0.5 mil power-ground separation, the resulting
capacitive divider at high frequencies results in an about
100:1 noise separation. Even in this extreme case this would
be barely enough to isolate a sensitive analog puddle from a
very noisy logic rail (in this case we assume that the very
noisy logic rail in itself would be OK). In such cases a
better solution is to move the very different supply rails
away from each other horizontally so that vertical coupling
does not occur, or move the sensitive rail (hopefully small
in size) to another (say signal) layer.
Regards,
Istvan
Zhangkun wrote:
>Dear Ludovic
>
>I have made a similar PCB of your stackup. The noise coupling between VCC1 and
>VCC2 is serious. I suggest that the seperation between VCC1 and VCC2 to be
>large.
>
>Best Regards
>
>Zhangkun
>2005.10.24
>----- Original Message -----
>From: "Ludovic Levieil" <ludovic.levieil@xxxxxxxxxxx>
>To: <si-list@xxxxxxxxxxxxx>
>Sent: Thursday, October 20, 2005 4:12 PM
>Subject: [SI-LIST] Power plane coupling
>
>
>
>
>>Hello All,
>>In my current board design I have the following stack up:
>>
>> .......
>>---------------- GND (solid plane)
>>------ ----- --- VCC1 (splitted plane)
>>--- ----- ------ VCC2 (splitted plane)
>>---------------- GND (solid plane)
>> .......
>>
>>4 mils separate GND and VCC planes
>>5 mils separate VCC1 and VCC2 planes
>>
>>Both VCC planes are splitted in different power domains and I am wondering
>>:
>> - if having two coupled VCC planes is good/acceptable when
>>thinking about noise ??
>> - if there is a problem in having one power domain on on plane
>>overlapping at least two power domains on the other plane ??
>>
>>Thanks
>>
>>Ludovic Levieil
>>
>>
>>
------------------------------------------------------------------
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:
