9.2.3.4 Interconnecting Media

The backplane and connectors should have a matched differential impedance. 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 helps eliminate reflections and ensure noise is coupled as common-mode. In fact, it has been determined that differential signals which are 1 mm apart radiate far less noise than traces 3 mm 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.

Stub lengths should be kept to a minimum. The typical transition time of the DS92LV040A Bus LVDS output is 0.75 ns (20% to 80%). The extrapolated 100 percent time is 0.75/0.6 or 1.25 ns. 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, 1.25 ns/5 is 250 picoseconds. Let velocity equal 160 ps per inch for a typical loaded backplane. Then maximum stub length is 250 ps/160 ps/in or 1.56 inches. To determine the maximum stub for the backplane, determine the propagation velocity for the actual conditions (refer to application notes AN 905 and AN 808)