[SI-LIST] Re: (no subject)
- From: Alan Hilton-Nickel <ahilton@xxxxxxxxxxxxx>
- To: Goutham.S@xxxxxxxxxx
- Date: Mon, 11 Mar 2002 09:59:51 -0800
Goutham,
Good questions.
Goutham.S@xxxxxxxxxx wrote:
> 1.0 2.5V signals should refer only to 2.5V or GROUND PCB plane and not 3V/5V
> plane.
>
> >-----If 2.5V signals on signal layer are having 5V reference plane
> adjacent to the signal layer then what is the problem?
The problem with running the 2.5 V signals beside a 5V palne is this -
Controlled impedance implies a relationship between the conductor that carries
the signal and the conductor that is used for the return path. In order for a
plane to be a return path for a signal, the plane *must be connected* to the
circuitry for the signal. The 2.5V drivers and receivers are not *connected* to
the 5V plane, except through what is likely to be a very high-inductance path.
If you really must do this, you could put a decoupling capacitor from 2.5V to 5V
at both ends of the signal to provide a lower-inductance path to the plane. For
very high speed signals with tight timing parameters, this inductance may still
be too high.
> 2.0 Ethernet PHY signals should refer to only transformer secondary supply
> voltage PLANE.
>
> >-----If System designer ignores the above point what will happen?
I'll pass on that one, since I'm not sure I understand the physical arrangement
you are discussing. If the signal is also on the secondary side of the
transformer, then the return path issue I mentioned in (1) would likely apply.
Alan Hilton-Nickel
Transmeta Corp
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