RE: [SI-LIST] : Test Interface board routing - Diff vs. Single

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Tue Jan 30 2001 - 04:52:24 PST


> In the ATE environment where we have a certain amount of control over the
> environment the signals travel in does differential signal routing have
> any
> advantage over single trace routing (no coupling between traces, just
> coupling
> from trace to ground plane)? Or are they considered to be of equal
> benefit in
> such a controlled environment?
 
Differential signals are usually beneficial. Many input receivers are
actually differential amps, whether or not they have a second input pin (if
not, the "reference" might be a combination of Vdd and Vss), and receive
accuracy and timing can be enhanced by giving it a true differential signal
to work with. Also the effective amplitude is double that of a single ended
signal.

Now if you mean how does a truly differential route compare to a
differential pair routed as *two* single ended wires, I guess it depends on
the environment. It isn't whether the environment is stable or controlled,
but things like how much noise and crosstalk there is. You only get
common-mode cancellation at the receiver when the noise is truly
common-mode, i.e., nearly equal crosstalk into both wires of the pair. I
think it depends a lot on many, many factors (how the rest of the board was
routed, what other signals are on the board, what other chips, where
connectors are, risetimes and switching rates, whether and how often the
signals change reference planes, etc. etc.).

The nature of the terminations and of the drivers may also affect your
choice. The two choices have different even/odd mode characteristic
impedances.

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