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# Rules for best differential signaling results

Posted: 02 Nov 2007     Print Version

Keywords:differential signals symmetry  symmetry signals  serialised buses

Differential pairs work by making the received signal the difference between two complementary parts referenced to each other, so the effects of their electrically noisy surroundings are minimised. By contrast, single-ended signals work by making the signal the difference between a received signal and power or ground, so noise on the signal or power system does not get canceled out. This is why differential signaling is effective for high-speed signals and why it is used in fast serialised buses and DDR memory.

In a differential pair, the positive and negative sides must be transmitted through identical surroundings all the way along the transmission path. The two sides must stay together, so that the positive and negative signals couple to each other via electric and magnetic fields at the corresponding points on those signals. Differential pairs are symmetrical, so their surroundings must be symmetrical too. Perfect symmetry is—of course—unattainable, not least because of dimensional tolerances. But designers can come close to that ideal to obtain the best differential-signaling results by following a few fundamental rules.

Do

• Make sure that the signals are at the same point on each line at the same time. Equalise trace sections, indicated by the same letter in the figure.

• Preferably route point-to-point, but in any case keep any stubs or diversions (shown as C) to within 0.6Tr inch, where Tr is the driver output rise time.

• Use a field solver to design the trace separation so you know the even- and odd-mode impedance.

• Consider the odd-mode impedance when terminating from either line to ground or a reference voltage to terminate your intended differential signal.

• Also consider terminating even or common mode (half the even-mode value) to terminate unintended noise.

• If you terminate between the two lines, consider the differential-mode impedance.

• Remember that radiated noise picked up from the same source is rejected only if you closely couple the pair, because only when the lines are close together are the surrounding electric and magnetic fields likely to be nearly identical.

• Compensate by lengthening near the driver for any skew between the complementary output signals sourced by the driving device.

• Lengthen only in a differential pattern wherever possible, remembering to balance the number and style of right-hand and left-hand bends.

Don't

• Terminate considering the single-ended characteristic impedance instead of odd- and even-mode impedance: Closely coupled differential pairs are designed to operate in a special way for complementary signals.

• Just equalise overall trace lengths instead of equalising each section.

• Route differential pairs over gaps in power or ground planes.

• Forget to define differential pairs if you are using an autorouter.

• Let test engineers add test lands in different positions on each side of the pair.

• Route other signals too closely parallel to a differential pair.

 Differential pairs work by making the received signal the difference between two complementary parts referenced to each other.

• Route differential pairs above or below unrelated power or ground areas (e.g. a separate analogue power plane).

• Forget to consider where off-board connections go.

• Be hoodwinked by probe or test equipment parasitic inductance and capacitance. If you put a probe on one side of a differential pair, it will probably be thrown out of balance.

- John Berrie
Technology Partner
Zuken UK Ltd

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