DS92001

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SNLS147F – JUNE 2002 – REVISED APRIL 2013

Match electrical lengths between traces to reduce skew. Skew between the signals of a pair means a phase

difference between signals which destroys the magnetic field cancellation benefits of differential signals and EMI

will result. Do not rely solely on the auto-route function for differential traces. Carefully review dimensions to

match differential impedance and provide isolation for the differential lines. Minimize the number of vias and

other discontinuities on the line.

Avoid 90° turns (these cause impedance discontinuities). Use arcs or 45° bevels.

Within a pair of traces, the distance between the two traces should be minimized to maintain common-mode

rejection of the receivers. On the printed circuit board, this distance should remain constant to avoid

discontinuities in differential impedance. Minor violations at connection points are allowable.

TERMINATION

Use a termination resistor which best matches the differential impedance or your transmission line. The resistor

should be between 90

Ω and 130Ω for point-to-point links. Multidrop (driver in the middle) or multipoint

configurations are typically terminated at both ends. The termination value may be lower than 100

Ω due to

loading effects and in the 50

Ω to 100Ω range. Remember that the current mode outputs need the termination

resistor to generate the differential voltage.

Surface mount 1% - 2% resistors are the best. PCB stubs, component lead, and the distance from the

termination to the receiver inputs should be minimized. The distance between the termination resistor and the

receiver should be < 10mm (12mm MAX).

PROBING LVDS TRANSMISSION LINES

Always use high impedance (> 100k

Ω), low capacitance (< 2 pF) scope probes with a wide bandwidth (1 GHz)

scope. Improper probing will give deceiving results.

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