[SI-LIST] A question about reference plane in gigabit ethernet design!
- From: "Jack W.C. Lin" <JackWCLin@xxxxxxxxxxxx>
- To: "Si-List (E-mail)" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 3 Oct 2003 13:45:30 +0800
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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