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Beyond Design: Skewed Again

When skew needs to be adjusted, it is best done at either end of the differential pair. In this way, the coupling and signal quality of the remainder of the pair is maintained.

Differential signaling is a method of transmitting serial, high-speed, complementary signals down a pair of coupled transmission lines. The equal and opposite nature of the differential pair means that demand on the power distribution network (PDN) is less than for a similar, single-ended data path. And, since external interference tends to affect both signals equally, the noise is cancelled providing high immunity to common mode electromagnetic interference compared to single-ended transmissions. But that is assuming the pair are perfectly balanced and terminated correctly. Generally, on-die termination (ODT) compensates for this, which may otherwise have a significant impact on signal quality and power dissipation.

Differential skew refers to the time difference between the two single-ended signals in a differential pair. Any mismatch in delay (skew) will result in changing part of the differential signal power into common-mode power.

Differential skew has become a performance limiting issue for high-speed SERDES links. The operation of such links involves significant amounts of signal processing to recover clocks, reduce the effects of high-frequency losses, reduce inter symbol interference (ISI), and improve signal-to-noise ratio. Skew limits the bandwidth of these links, adds data-dependent jitter, and limits the possibility of equalizing links to compensate for high-frequency skin effect and dielectric losses.Read the full column here.Editor's Note: This column originally appeared in the June 2013 issue of The PCB Design Magazine.