DS92CK16

SNAS044C – NOVEMBER 1999 – REVISED APRIL 2013

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DIFFERENTIAL TRACES

Use controlled impedance traces which match the differential impedance of your transmission medium (ie.

backplane or cable) and termination resistor(s). Run the differential pair trace lines as close together as possible

as soon as they leave the IC . This will help eliminate reflections and ensure noise is coupled as common-mode.

In fact, we have seen that differential signals which are 1mm apart radiate far less noise than traces 3mm apart

since magnetic field cancellation is much better with the closer traces. Plus, noise induced on the differential

lines is much more likely to appear as common-mode which is rejected by the receiver.

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. (Note the velocity of propagation, v = c/Er where c (the speed of light) = 0.2997mm/ps or 0.0118

in/ps). Do not rely solely on the autoroute function for differential traces. Carefully review dimensions to match

differential impedance and provide isolation for the differential lines. Minimize the number or 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.

STUB LENGTH

Stub lengths should be kept to a minimum. The typical transition time of the DS92CK16 BLVDS output is 0.75ns

(20% to 80%). The 100 percent time is 0.75/0.6 or 1.25ns. For a general approximation, if the electrical length of

a trace is greater than 1/5 of the transition edge, then the trace is considered a transmission line. For example,

1.25ns/5 is 250 picoseconds. Let velocity equal 160ps per inch for a typical loaded backplane. Then maximum

stub length is 250ps/160ps/in or 1.56 inches. To determine the maximum stub for your backplane, you need to

know the propagation velocity for the actual conditions (refer to application notes AN– 905(SNLA035) and

AN–808(SNLA028)).

TERMINATION

Use a resistor which best matches the differential impedance of your loaded transmission line. Remember that

the current mode outputs need the termination resistor to generate the differential voltage. BLVDS will not work

without resistor termination.

Surface mount 1% to 2% resistors are best.

PROBING BLVDS TRANSMISSION LINES

Always use high impedance (> 100k

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

scope. Improper probing will give deceiving results.

CABLES AND CONNECTORS, GENERAL COMMENTS

Use controlled impedance media. The connectors you use should have a matched differential impedance of

about Zo

Ω. They should not introduce major impedance discontinuities.

Balanced cables (e.g. twisted pair) are usually better than unbalanced cables (ribbon cable, simple coax.) for

noise reduction and signal quality. Balanced cables tend to generate less EMI due to field canceling effects and

also tend to pick up electromagnetic radiation a common-mode (not differential mode) noise which is rejected by

the receiver. For cable distances < 0.5M, most cables can be made to work effectively. For distances 0.5M

≤ d ≤

10M, CAT 3 (category 3) twisted pair cable works well, is readily available and relatively inexpensive.

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