[SI-LIST] Re: A question about reference plane in gigabit ethernet design!
- From: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>
- To: <JackWCLin@xxxxxxxxxxxx>,"Si-List (E-mail)" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 3 Oct 2003 08:17:24 -0400
Jack,
Pieces to this scenario were discussed separately in the past on this list:
you
may want to search the archives for more details.
The starting point is that even if you use differential traces for
signaling, it
is still a good idea to count on a reference plane (which can be either
ground
or supply plane, dependent on how the bypassing is done) to carry a
noticeable portion of the return current. The reason is purely practical:
this way you can use narrower traces (so that you can achieve higher
routing density) and you need to worry less about stray coupling to
other nearby traces and components.
I hope this helps you to get started.
Regards,
Istvan Novak
SUN Microsystems
----- Original Message -----
From: "Jack W.C. Lin" <JackWCLin@xxxxxxxxxxxx>
To: "Si-List (E-mail)" <si-list@xxxxxxxxxxxxx>
Sent: Friday, October 03, 2003 1:45 AM
Subject: [SI-LIST] A question about reference plane in gigabit ethernet
design!
> Dear All:
> I have one question about Ethernet Design and hope someone who is familiar
> with this field can provide your idea. For better impedance control in
> differential signaling, we route differential signal traces over specified
> reference plane, power or ground planes. Image plane (image plane) is a
> return current path for high speed single ended signal, but may not needed
> for differential signal pair because most return current will return back
> through opposite polarity signal. In my belief, power or ground reference
is
> not a concern. But, I found different statements in different design
> guideline for gigabit chip. one said that differential pair should be
> reference to ground, the other said they should be reference to power
plane.
> The later statement has further explain, it said: center tap of
transformer
> will use the power which is the plane be referenced by differential pairs,
> so return current can be withing on the power plane. I confused about this
> statement!! The purpose of center tap is to remove unwanted common mode
> noise on differential pairs. If the return current mean this common mode
> current, I can understand. If it means others, I really don't know waht
are
> they? Sorry, let me organize my question as following:
> (1)Why center tap capacitor pull up to power? not ground?
> (2)If I provide power to transformer through trace, not plane; then I
don't
> need force differential pair
> routed over power reference, right?
> (3)Do we need really concern about return current on reference plane for
> differential signal?
> Reguads
>
> Jack
>
>
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