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FIGURE 4. Termination

In a point-to-point system configuration, the termination

resistor should be placed within 2 cm of the receiver. For a

multi-drop configuration, the termination resistor should

also be located within 2 cm of the last receiver.

Fast Switching Speeds

Typical slew rates for LVDS are under 1 ns when measured

from 10% to 90% of the edge. When edge rates approach

less than half the time of the distance to the load, the load

can no longer be thought of as a lumped load and trans-

mission line effects must be considered. Because LVDS is

most often used in driving cables and in backplanes, trans-

mission line effects are a concern for the system designer.

One of the largest contributors to bit error in medium to

long cable and bus driving systems is reflections. Reflec-

tions are caused by mismatches in line impedance which

cause inductive and capacitive ripples in the signal which,

in turn, reduce the drivers ability to provide a clean signal to

the receiver. For this reason, it is essential for the imped-

ance of all cables, connectors, busses, and termination

resistors to be closely matched. The LVDS common mode

rejection feature helps to minimize reflections caused by

mismatched transmission lines.

Jitter

There are many ways that digital jitter can effect a system

operation. A transmission channel typically passes signals

at a specific bit rate or within a range of bit rates. Jitter has

the effect of shortening some bits, while lengthening oth-

ers. This shortening of bits can increase the signal speed

and cause dropped bits in the transmission. Additionally,

excessive jitter can cause dropped bits due to the system’s

internal timing correction system not having the ability to

track the signal.

Jitter can be defined as a type of line distortion caused by a

random variation in a signal’s reference timing position.

The deviation can either be leading or lagging the ideal

position. Jitter is usually expressed in picoseconds (ps), as

a percent (%), or as a unit interval fraction (UI) and can be

caused by a number of factors including reflections, noise

and crosstalk.

Jitter is divided in to three basic categories: Deterministic

jitter, random jitter, and frequency dependent jitter. Deter-

ministic jitter is typically a result of phase changes which

are correlated to specific events like data path bandwidth

limitations. Random jitter is often caused by thermal noise

and other random variables that are not necessarily related

to specific events. Frequency-dependent-jitter is typically

caused by things such as power supply noise and

crosstalk.

Jitter is most easily shown by the use of an eye pattern.

Figure 5 shows an example of an eye pattern. The size of

the eye opening determines the quality of the signal, and

jitter can be measured at the switch point. The eye pattern

is useful for much more than to measure jitter. It is also

beneficial for measuring Intersymbol Interference (ISI) -

signal attenuation caused by such things as high frequency

overlapping and dispersion - crosstalk, skew, and reflec-

tions.

FIGURE 5. Eye Pattern at Driver Outputs