[SI-LIST] Re: Current Return Vias

  • From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
  • To: "Ihsan Erdin" <erdinih@xxxxxxxxx>, giovanni.guasti@xxxxxxxxxx
  • Date: Mon, 24 Jul 2006 15:34:43 -0700

Ihsan,

Well put.


> [Original Message]
> From: Ihsan Erdin <erdinih@xxxxxxxxx>
> To: <giovanni.guasti@xxxxxxxxxx>
> Cc: <si-list@xxxxxxxxxxxxx>
> Date: 7/23/2006 7:36:40 AM
> Subject: [SI-LIST] Re: Current Return Vias
>
> Giovanni,
> The EMC justification of a ground via in the close proximity of a
switching
> via is to minimize radial waves to the edges of the card by providing a
> "return path" (I hate this phrase...) A two-wire kind of transmission line
> -as you put it- would be another rationalization against the impedance
> discontinuity for high-speed signals. In practice, however, the placement
of
> a ground via close enough to a switching via in order to provide a
matching
> impedance to -say 50 ohm- or to mitigate radial wave propagation is in
most
> cases -if not all- physically impossible. I think this kind of SI/EMI
> rule-of-thumbs are based on a qualitative understanding of electromagnetic
> theory rather than rigorous research results. In this context, I share
Lee's
> stance to debunk these recommendations because they have significant
effect
> on the design cost by closing routing channels and eating up on the
valuable
> board real-estate. If anybody has come across any research that states
> otherwise in a peer-refereed publication I'd like to hear that.
>
> Regards
>
> Ihsan
>
>
> On 7/22/06, Giovanni Guasti <giovanni.guasti@xxxxxxxxxx> wrote:
> >
> > Kenny,
> > GND vias near the signal are not dedicated to return currents, but they
> > are often used to optimize the impedance of the via.
> > As the via is a short transmission line, only the higher speed signals
> > can benefit of the difference between an optimized via and a "usual"
> > via.
> >
> > You have to compare the higher frequency component of your signal, its
> > wavelength and the via length. This will give you an idea of the
> > effective needing to optimize this short transmission line.
> >
> > Of course you could have a 133MHz signal with very sharp edges and high
> > frequency components, even if it seems very unusual... In this case it
> > would be wise to choose a slower transmitter!
> >
> > The rule is to understand if the via behaves like a transmission line
> > for your signal or not, and in the first case to do the best to reduce
> > impedance discontinuities.
> > Best regards,
> >         Giovanni
> >
> >
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> > On Behalf Of Lee Ritchey
> > Sent: 22 July 2006 18:36
> > To: Kenny Frohlich; si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: Current Return Vias
> >
> > Kenny,
> >
> > It is not true that you need a "return current" via next to each layer
> > changing signal  via.  I continue to be amazed that engineers who are
> > looked upon as SI experts say such things.
> >
> > Imagine you have a 4 layer PCB, such as the mother  board in a PC, where
> > there are only two planes, one Vdd and one ground, where would such vias
> > connect?  There have been billions of these made to date that work just
> > fine and have very fast signals on them.  The return currents you are
> > concerned about find their way from plane to plane through the
> > collection
> > of decoupling capacitors and interplane capacitance that you had to
> > engineer into the power delivery system in order to make it stable.
> > Focus
> > on this and the return currents take care of themselves.  EMI is
> > minimized
> > he same way..
> >
> >
> >
> >
> > > [Original Message]
> > > From: Kenny Frohlich <kenny_frohlich@xxxxxxxxx>
> > > To: <si-list@xxxxxxxxxxxxx>
> > > Date: 7/22/2006 6:45:56 AM
> > > Subject: [SI-LIST] Current Return Vias
> > >
> > > Dear Experts,
> > >   I understand that I need to provide ground vias next to via
> > explictly
> > for the purpose of letting return currents jump between layers.  I know
> > it's a requirement for high speed signals, especially differrential
> > signals.  Is this also required for low speed single-ended signals
> > (133Mhz
> > or slower)? =20
> > >   If this is a requirement, what would be a good signal via to ground
> > via
> > ratio? For example,  there are five signal vias within a 1 inch area,
> > how
> > many ground vias do I need?
> > >   =20
> > >   Thank you
> > >   Kenny
> > >   =20
> > >  __________________________________________________
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