8.2.3.9 Interconnecting Media

The physical communication channel between the driver and the receiver may be any balanced paired metal

conductors meeting the requirements of the M-LVDS standard, the key points which will be included here. This

media may be a twisted pair, twinax, flat ribbon cable, or PCB traces.

The nominal characteristic impedance of the interconnect should be between 100 Ω and 120 Ω with variation no

more than 10% (90 Ω to 132 Ω).

8.2.3.10 PCB Transmission Lines

As per SNLA187, Figure 8-4 depicts several transmission line structures commonly used in printed-circuit boards

(PCBs). Each structure consists of a signal line and a return path with uniform cross-section along its length. A

microstrip is a signal trace on the top (or bottom) layer, separated by a dielectric layer from its return path in a

ground or power plane. A stripline is a signal trace in the inner layer, with a dielectric layer in between a ground

plane above and below the signal trace. The dimensions of the structure along with the dielectric material

properties determine the characteristic impedance of the transmission line (also called controlled-impedance

transmission line).

When two signal lines are placed close by, they form a pair of coupled transmission lines. Figure 8-4 shows

examples of edge-coupled microstrips, and edge-coupled or broad-side-coupled striplines. When excited by

differential signals, the coupled transmission line is referred to as a differential pair. The characteristic

impedance of each line is called odd-mode impedance. The sum of the odd-mode impedances of each line is

the differential impedance of the differential pair. In addition to the trace dimensions and dielectric material

properties, the spacing between the two traces determines the mutual coupling and impacts the differential

impedance. When the two lines are immediately adjacent; for example, if S is less than 2 × W, the differential

pair is called a tightly-coupled differential pair. To maintain constant differential impedance along the length, it is

important to keep the trace width and spacing uniform along the length, as well as maintain good symmetry

between the two lines.

SN65MLVD203B

SLLSFG7 – AUGUST 2020

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