[SI-LIST] Re: Some reference on reference planes
- From: Rick Collins <gnuarm.2006@xxxxxxxxx>
- To: Mark Grobman <markgrobman@xxxxxxxxx>
- Date: Mon, 07 Nov 2011 16:55:16 -0500
Your stackup only has seven layers. Is that really correct? At
least no one can say is isn't symmetrical. ;-)
I think with a 5 mil spacing of layers you will have plenty enough
capacitance that they will be well coupled and each power layer will
act equivalently as a reference for any signal. As Paul pointed out,
for the lower frequency components you should provide adequate bypass
capacitors. I know this is not a rigorous analysis. But that would
require knowledge of the signal driver, package parasitics and the
power distribution system. But I don't think the power planes will
represent a problem in this case.
BTW, I don't think the total capacitance is the relevant issue. That
will depend on the board size. I think what is important is that
there is enough coupling between the power planes.
Rick
At 04:40 PM 11/7/2011, Mark Grobman wrote:
>Thanks for the quick reply's.
>
>Paul - I read in Bogatain's book that this method is not effective
>-It was mentioned under what happens when you run over a gap in the
>return plane but as I understand the physics is essentilay the same.
>To the best of my understanding the current will "find it's way
>back" in a radiative manner so that as long as the capacitance
>between the relevent planes is suffiecnt it should be ok above a
>certain rise time - I just don't know the numbers. Is this method
>effective from your experince? what's the range of Rt for which it works.
>
>Rick - Your absoulty right. I've been a bit vauge. The setup I'm
>talking about is something like this:
>
>Power 1(Real Ref.)
>Signal 1
>Power 2
>Signal 2
>Power 3
>Signal 3
>Power 4(GND)
>
>And the relevent Signal layer is "Signal 2". The distance between
>different layers is 5 mil on each side.
>
>Mark
>On Mon, Nov 7, 2011 at 11:24 PM, Rick Collins
><<mailto:gnuarm.2006@xxxxxxxxx>gnuarm.2006@xxxxxxxxx> wrote:
>I recall from a course I took that if the plane of the stripline is
>tightly coupled to the reference plane, you should not have a
>problem. But "tightly coupled" may not be what you have. I think
>the context of what I learned was when there was a separation in a
>power plane or even a signal passing across a gap between two
>separate power planes, but in both cases the power planes were
>opposite a ground plane and so were "tightly coupled" acting just
>like the ground plane.
>Where is your driver's "reference plane" that it does not interact
>with the signal? Can you give us a better picture of what you are
>designing rather than talking in the abstract?
>
>At 04:15 PM 11/7/2011, Mark Grobman wrote:
> >Hello experts,
> >I require some help on the subject of reference planes. I'm designing a
> >board and despite my best efforts i'm stuck with a situation where I'm
> >forced to conduct a signal using a stripline neither of whose planes are
> >the reference planes of the signal's driver (not the driver's ground or
> >VCC).
> >
> >Now I know from various App. notes and books that this sort of situation
> >should be avoided and that I have been a bad engineer indeed.
> >
> >Still, assuming the situation cannot be avoided I was hoping to get
> >some quantitative approximation to how bad of an idea this is.Sadly
> >speaking I don't have access to a 3d simulator which can give me exact
> >results so I'm going for best effort design methods. I would love to get
> >your input on the following issues:
> >
> >1. Does the interference caused by not using the correct ref. planes carry
> >throughout the transmission line or does it occur only at the edges where
> >the current "jumps" back to the correct ref. planes?
> >2. Is there a merit figure of RiseTime/planes capacitance/???? for which
> >the situation isn't problematic?
> >3. Will using diff. lines improve the situation?
> >4. Suggested reading on the matter.
> >5. Highly insightful remarks which will blow my mind.
> >
> >Cheers,
> >Mark
> >
> >
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