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AN-5017 LVDS Fundamentals
© 2005 Fairchild Semiconductor Corporation
AN500463
www.fairchildsemi.com
Fairchild Semiconductor
Application Note
December 2000
Revised June 2005
AN-
5017 L
VDS Fundament
al
s
AN-5017
LVDS Fundamentals
Introduction
With the recent developments in the communications mar-
ket, the demand for throughput is becoming increasingly
more crucial. Although older differential technologies pro-
vide significant signal integrity benefits compared to single-
ended technologies, many of them consume much more
power at lower throughput than LVDS.
The LVDS standard was created to address applications in
the data communications, telecommunications, server,
peripheral, and computer markets where high-speed data
transfer is necessary. LVDS offers a low cost, high speed,
low power solution when compared to the standards of the
past.
What is LVDS?
LVDS is defined in the TIA/EIA-644 standard. It is a low
voltage, low power, differential technology used primarily
for point-to-point and multi-drop cable driving applications.
The standard was developed under the Data Transmission
Interface committee TR30.2. It specifies a maximum data
rate of 655 Mbps although some of todays applications are
pushing well above this specification for a serial data
stream.
Compared to other differential cable driving standards like
RS422 and RS485, LVDS has the lowest differential swing
with a typical voltage swing of 350 mV with a typical offset
voltage of 1.25V above ground. See Figure 1.
FIGURE 1. Signal Level Comparison
LVDS features a low swing differential constant current
source configuration which supports fast switching speeds
and low power consumption. Figure 2 shows this configu-
ration. This allows for other features not found in single
ended technologies such as Common Mode Rejection and
Failsafe, which will be discussed later in this application
note.
FIGURE 2. Driver/Receiver Schematic www.fairchildsemi.com
2
AN-5017
Differential Signaling
Differential signaling offers many advantages over single
ended technologies. LVDS signaling centers around 1.25V
with a 350 mV swing and is not dependent on power supply
voltage. Not only does this result in a faster, more stable
signal, it also makes migration to lower power supply volt-
ages much easier.
Another advantage to differential technology is that the bal-
anced differential lines have tightly coupled equal but polar
opposite signals which reduce EMI. The magnetic fields
radiated by each of the conductors are drawn toward each
other and cancel much of the magnetic fields.
Common Mode
Differential signaling also offers common mode rejection.
The receiver ignores any noise that is coupled equally on
to the differential signals and only considers the difference
between the two signals. The receiver has a common
mode voltage range of 0.05V to 2.35V. LVDS receivers will
operate with as much as a
r
1V ground shift between the
driver and receiver. This is shown graphically in Figure 3.
Low swing differential signaling can also improve signal
integrity concerns at higher speeds. As throughput
demands increase throughout the information industry,
higher frequencies and wider bit widths cause transmission
line reflections and crosstalk. As system loading increases,
the characteristic impedance of a system can change and
cause impedance mismatches which will, in turn, send
reflective signals across the transmission line. These
reflections can cause bit errors or increase settling times
making timing budgets more difficult as speeds increase.
Differential signaling technologies like LVDS solve this by
accepting common mode noise on the differential line.
Additionally, lower swing differential technologies reduce
reflections by having small voltage swings which limit the
energy supplied to the transmission line.
FIGURE 3. Common Mode Noise Range
Failsafe
Failsafe is a feature offered in LVDS that will help system
reliability by preventing errors. Failsafe guarantees that the
outputs are in a known state (HIGH) when the receiver
inputs are under certain fault conditions. Without the fail-
safe feature, any external noise above receiver thresholds
could trigger the output to an unknown state.
According to the TIA/EIA-644 standard, when the receiver
inputs are open, not connected to the generator, or if the
generator is powered off, the failsafe feature will drive the
outputs high. If the receiver inputs are shorted, the outputs
will be in failsafe mode (HIGH State). The standard also
states that the receiver outputs will also go in to a failsafe if
the differential inputs remain within the threshold region for
an abnormal period of time.
This protection feature has many benefits for a system
designer. For instance, some applications may dictate that
not all of the LVDS receiver inputs are used. With the fail-
safe feature, the receiver outputs will always be in a known
state as long as the inputs are not receiving a valid signal.
Termination
Termination of LVDS is necessary at the receiver input to
generate the Output Differential Voltage (V
OD
). The TIA/
EIA-644 specification stipulates an internal termination
resistor value between 90
:
and 132
:
. Fairchild recom-
mends a termination resistor value between 90
:
and 110
:
depending on the characteristic impedance of the cable.
Termination of LVDS is much easier than most other tech-
nologies. ECL and PECL both use a 220
:
pull-down resis-
tor on each driver output as well as a 100
:
resistor across
the driver outputs. GTLP, due to the open drain configura-
tion, must have a termination resistor (usually 50
:
double
terminated) to a 1.5V pull-up voltage in order to generate a
GTLP signal. (See Figure 4) 3
www.fairchildsemi.com
AN-
5017
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 systems
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 signals 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 I