[SI-LIST] Re: diff signaling

  • From: "Mohammad Tabatabai" <mohammadt@xxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Fri, 2 Apr 2004 10:11:24 -0800

It actually does help in near-field emmissions as the flux field
couples more strongly.

Mohammad

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Jon Powell
Sent: Friday, April 02, 2004 10:13 AM
To: mohammadt@xxxxxxxxxxxx; zabinski.patrick@xxxxxxxx;
chris_landrum@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: diff signaling


I wouldn't count much on the EMI help. The vast majority of the return
current is still in the return plane and thinking that enough jumps into the
diff signals at a gap crossing is generally not a good analysis.

jon


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Mohammad Tabatabai
Sent: Friday, April 02, 2004 9:55 AM
To: jonpowell@xxxxxxxxxxxx; zabinski.patrick@xxxxxxxx;
chris_landrum@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: diff signaling


Also Routing diff-pairs closely coupled preserves edge rates
which will help greatly in reducing ISI jitter (for high-speed
data). Another added benefit is as the diff pairs couple more
strongly together the impedance matching also becomes a function
of their field coupling and not entirely to the return ground
below hence providing less susceptibility to impedance mismatch
when a diff pair crosses over boundaries which do not have ground
plane for return currents not to mention an added benefit for
reduction in EMI as well.

-Mohammad

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Jon Powell
Sent: Friday, April 02, 2004 9:04 AM
To: zabinski.patrick@xxxxxxxx; chris_landrum@xxxxxxxxxx;
si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: diff signaling


In addition, by routing the next to each other they are more likely to
receive similar amounts of crosstalk noise (from the same sources) than if
you routed them, say, across different parts of the board, even if you did
length matching. Also you can get some timing benefits and additional
dynamic impedance control from the mutual crosstalk between the adjacent
differential signals.


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Zabinski, Patrick J.
Sent: Friday, April 02, 2004 6:32 AM
To: chris_landrum@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: diff signaling


Chris,

I  believe there's a bit a confusion...

I agree that the traces/interconnect do not necessarily
reject common mode noise any better than single ended
traces.  If radiated energy is applied to a copper
trace (or a pair of traces in the case of differential),
there will be energy transferred to the traces.  So,
a pair of differential traces do not have any special
properties that reject common mode noise.

However, what's more important is how the receiver electronics
react to this noise.  In a good differential receiver,
the circuit only considers the difference in voltage
between the two input nodes and does not care what the
common mode voltage is (within the limits of the power
rails).  Thus, if there is common-mode noise on
both traces, then a the receiver will effectively
ignore (i.e., reject) the common-mode noise and
only consider the difference in voltage
between the two traces.

Accordingly, I don't believe it's the interconnect
that provides good common-mode rejection, but it's
the receiver where you get the most benefit.

Taking this a step further, the line driver also benefits
from differential signals by providing a better,
more controlled path for the return signal, where
a single ended driver must often rely on decoupling
elements to provide the return path.

With respect to line length matching, this becomes
important from the perspective of common-to-differential
mode conversion.  If an external source applies
common-mode energy to a differential pair (say,
through crosstalk), a difference in interconnect
lengths between the true and complement signals will
effectively convert the common-mode noise to diff-mode
noise, and the receiver will not be able to reject
this.

Accordingly, it's important to consider common-to-diff
conversion and line-length matching to maintain the
benefits of differential signaling.

Hope this helps,
Pat

>
> I have been searching through the archive for information
> regarding this but
> I have not come up with anything concrete.
> Many people mention using differential signaling in an effort
> to reduce
> common mode noise. Looking through Lee Ritchey's book "Right the First
> Time", his discussion implies that this is not the case. For
> there to be
> common mode noise rejection between two traces, the fields
> would have to be
> equal and opposite. In a PCB structure with a diff pair
> referencing GND
> planes (say in a stripline topology) this seems impossible!
> Routing signals
> next to eachother is convenient for length matching (skew
> management) but I
> don't see much else.... Any thoughts.
>
> And I do apologize if this has been discussed in detail before.
> Thanks
> Chris
>
>
>
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