AN-5017 Datasheet, PDF (3/5 Pages) Fairchild Semiconductor – LVDS Fundamentals

Differential Signaling (Continued)
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
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