SN65LVDS1, SN65LVDS2, SN65LVDT2

SLLS373L – JULY 1999 – REVISED DECEMBER 2014

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Figure 20. Controlled-Impedance Transmission Lines

10.2.1.2.7

Termination Resistor

As shown earlier, an LVDS communication channel employs a current source driving a transmission line which is

terminated with a resistive load. This load serves to convert the transmitted current into a voltage at the receiver

input. To ensure incident wave switching (which is necessary to operate the channel at the highest signaling

rate), the termination resistance should be matched to the characteristic impedance of the transmission line. The

designer should ensure that the termination resistance is within 10% of the nominal media characteristic

impedance. If the transmission line is targeted for 100-

Ω impedance, the termination resistance should be

between 90

Ω and 110 Ω.

The line termination resistance should be located as close as possible to the receiver, thereby minimizing the

stub length from the resistor to the receiver. The limiting case would be to incorporate the termination resistor

into the receiver, which is exactly what is offered with the SN65LVDT2. The SN65LVDT2 provides all the

functionality and performance of the SN65LVDS2 receiver, with the added feature of an integrated termination

load.

While we talk in this section about point-to-point communications, a word of caution is useful when a multidrop

topology is used. In such topologies, line termination resistors are to be located only at the end(s) of the

transmission line. In such an environment, SN65LVDS2 receivers could be used for loads branching off the main

bus, with an SN65LVDT2 used only at the bus end.

10.2.1.2.8

Driver NC Pins

NC (not connected) pins are pins where the die is not physically connected to the lead frame or package. For

optimum thermal performance, a good rule of thumb is to ground the NC pins at the board level.

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