[SI-LIST] Re: Ground, the preferred reference plane

  • From: Chris Cheng <chris.cheng@xxxxxxxxxxxx>
  • To:
  • Date: Wed, 11 Dec 2002 13:59:27 -0800

Actually there are definite preference on reference plane depending on which
I/O technology you choose.
For those old enough to worked on the original ECL (which I am not), the
prefered reference is the -ve plane not the ground plane. There are a lot of
open drain I/O's (GTL, ECL to name a few) where the prefer return is the
voltage the driver's source is tied to. It just happened when Bill Gunning
started working on GTL he chose to use open drain pull down to ground (to
save power) and from that point on most of the computers in this world start
using GTL i/o and thus it makes sense to use ground plane as a reference for
i/o. Most of the currently so called PECL circuits are shifted up open
emitters and technically they should be referenced to the power plane rather
than ground for their return. We are just looking the other way and assuming
they are perfect differential signals and thus only need to reference
themselves (+ve vs. -ve), in reality that is not completely ture.



-----Original Message-----
From: Loyer, Jeff [mailto:jeff.loyer@xxxxxxxxx]
Sent: Wednesday, December 11, 2002 12:49 PM
To: Anand.Kuriakose@xxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Ground, the preferred reference plane



This has been discussed in great length in this forum.  The archives =
should have tons of information on it. =20

My short take: the most prevalant cause of problems is changing the =
reference plane without providing a suitable means for the return =
current to transition between the planes.  There is much question about =
what a "suitable means" is - at what frequencies do "stitching caps" =
become useless, for instance. =20

Since most chips have the signals referenced to ground internally, it is =
a safer thing to have your signals likewise referenced to ground =
externally.  This generally applies to cards, also.

In short, it is possible to reference your signals to any plane, but =
using ground as the reference plane is a safer bet.  An exception might =
be if a chip specified that their signals were referenced to a power =
plane, but I haven't seen this.

Jeff Loyer



-----Original Message-----
From: Kuriakose, Anand [mailto:Anand.Kuriakose@xxxxxxx]
Sent: Wednesday, December 11, 2002 12:25 AM
To: 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] Ground, the preferred reference plane



Hi All,

In "High Speed Digital System Design" by Stephen Hall, it is mentioned  =
that
the ground-referenced signals have cleaner signal integrity when =
compared to
power-referenced signals.=20

Chipset design guides (not all) also recommend to have the high speed
signals like processor signals routed over ground plane rather than over
power plane. Also similar statements are made in a few other docs.

I'd like to understand how does it improve the signal integrity of the
signal when routed over GND plane rather than over power plane. In
otherwords, what makes GND plane the preffered reference plane?

One other point is that when signals are routed over power planes, the
return current can get back to where it started without passing through =
any
decoupling caps, making the return loop smaller (assuming that signals =
do
not cross splits in the plane and no return path discontinuity due to =
layer
changes). However, if the same signal is routed over GND plane, the =
return
current will have to pass through a decoupling cap to complete the loop.
Correct me if i am wrong in my above point.=20

Regards,
Anand.









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