Re: [SI-LIST] : Differential Impedance Effects on Diff. Return Current

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From: Chris.H.Simon@gd-is.com
Date: Thu Jan 04 2001 - 06:27:05 PST


Franz,

This response doesn't directly answer your question, but there is a related issue which
sometimes gets lost by "newbies" as well as a few old timers. This also greatly affects
the amount of return current in nearby conductors.

There is a distinction between a "differential" signal pair and what I like to call
"complementary single-ended". Many people who see a pair of traces routed in parallel
make the assumption that these are differential. To me that implies that the currents in
the two traces are, for the most part, equal and opposite. Whether these traces are
differential or not is completely dependent on what's connected to the ends.

If the driver ends of the traces are connected to complementary outputs of a Positive ECL
driver and the receiver ends are each terminated through a resistor to ground, then they
are not differential (at least not by my definition.) In this situation there is a
positive and unequal current flowing in each trace, from the driver end toward the
termination resistor. There are definitely some significant currents flowing in whatever
ground planes are around, or in some unintentional ground path if there is no ground
plane. So the currents in what many people call "differential PECL" are not equal and
opposite.

If the driver ends are connected to the two outputs of an LVDS driver, and the receiver
ends are terminated with a resistor from one trace to the other, then they *are*
differential. In this situation the current flowing in one trace towards the termination
resistor returns in the other trace back to the driver. It has no choice since this is
the only path, at least for the "low" frequency components of the current.

In both cases I suspect there will be some high frequency transient currents flowing in
whatever return path is nearest at the instant of time when the signals actually switch.
The magnitude of these transient currents is dependent on the signal slew rate, driver
timing skew, path length differences, imbalance in driver characteristics, characteristic
impedance discontinuities, etc. This needs to be taken into account when designing the
transmission path.

There are many instances where someone will use the term differential when they are really
talking about a signalling system that is complementary single-ended. I'm not suggesting
that one needs to do anything different with the routing or trace coupling for one vs. the
other, only that they have different implications for return currents.

Chris

*******************************************************

Please excuse my 'newbie-like' questions, my inclusion in this mailing list
is more for curiosity and personal advancement/understanding than as a
profession.

First, is this statement valid?
- In a differential pair, one 'leg' of a signal's return current path is
through the complementary 'leg' of a differential pair and not through the
ground or power planes (Assuming equal trace lengths, Zo=50 single-ended,
Zo=100 diff. impedance - using ECL logic as an example).

Now, assuming the above statement is true:
If the differential impedance is NOT 100 Ohms (Differential traces NOT
routed differentially) how does this effect the return current path? Does
the return current begin to flow through the ground and power planes rather
than through the differential pair?

Thanks SI gurus!

Franz.

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