Low-Voltage Differential Signaling (LVDS)
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21-09-2008, 11:12 AM


Low-Voltage Differential Signaling (LVDS) is a new technology addressing the needs of today's high performance data transmission applications. The LVDS standard is becoming the most popular differential data transmission standard in the industry. This is driven by two simple features: "Gigabits @ milliwatts!" LVDS delivers high data rates while consuming significantly less power than competing technologies. In addition, it brings many other benefits, which include:

a)Low-voltage power supply compatibility
b)Low noise generation
c)High noise rejection
d)Robust transmission signals
e) Ability to be integrated into system level ICs

LVDS technology allows products to address high data rates ranging from 100's of Mbps to greater than 2 Gbps. For all of the above reasons, it has been deployed across many market segments wherever the need for speed and low power exists.

Consumers are demanding more realistic visual information in the office and in the home. This drives the need to move video, 3D graphics and photo-realistic image data from cameras to PCs and printers through LAN, phone, and satellite systems to home set-top boxes and digital VCRs. Solutions exist today to move this high-speed digital data both very short and very long distances, on printed circuit boards (PCB) and across fiber or satellite networks. Moving this data from board-to-board or box-to-box however, requires an extremely high-performance solution that consumes a minimum of power, generates little noise, (must meet increasingly stringent FCC/CISPR EMI requirements) is relatively immune to noise, and is cost-effective.
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i need full report of lvds. please send me.
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can i get a full report
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.doc   1CR06EC002-LVDS.doc (Size: 683 KB / Downloads: 108)


INTRODUCTION
Recent growth in high-end processors, multi-media, virtual reality and networking has demanded more bandwidth than ever before. But the point-to-point physical layer interfaces have not been able to deal with moving information at the data rates required. Some of today’s biggest challenges that remain to be solved include: the ability to transfer data fast, lower power systems than currently available, and economical solutions to overcome the physical layer bottleneck.

Data Transmission standards like RS-422, RS-485, SCSI and others all have their own limitations most notably in transferring raw data across a media. Not anymore. Low Voltage Differential Signaling (LVDS) is a high speed (>155.5 Mbps), low power general purpose interface standard that solves the bottleneck problems while servicing a wide range of application areas.

This application note explains the key advantages and benefits of LVDS technology. Throughout this application note the DS90C031 (LVDS 5V Quad CMOS Differential Line Driver) and the DS90C032 (LVDS 5V Quad CMOS Differential Line Receiver) will be used to illustrate the key points. Over 50 LVDS devices are offered currently (1998) from National, please refer to the LVDS device datasheets for complete specifications.

Low-voltage differential signaling, or LVDS, is an electrical signaling system that can run at very high speeds over inexpensive twisted-pair copper cables. It was introduced in 1994, and has since become very popular in computers, where it forms part of very high-speed networks and computer buses.

Low-voltage differential signaling (LVDS) is a signaling method used for high-speed transmission of binary data over copper. It is well recognized that the benefits of balanced data transmission begin to outweigh the costs over single-ended techniques when signal transition times approach 10 ns. This represents signaling rates of about 30Mbps or clock rates of 60MHz (in single-edge clocking systems) and above. LVDS, as documented in TIA/EIA-644, can have signal transition time as short as 260ps turning a printed circuit board trace into a transmission line in a few centimeters. Care must be taken when designing with LVDS circuits, such as the SN65LVDS31 quadruple line driver and SN65LVDS32 quadruple line receiver. This document provides some guidelines for the basic application of LVDS.

What is Differential Signaling?
Differential signaling is a method of transmitting information electrically by means of two complementary signals sent on two separate wires. The technique can be used for both analog signaling, as in some audio systems, and digital signaling, as in RS-422, RS-485, Ethernet (twisted-pair only), PCI Express and USB. The opposite technique, which is more common but lacks some of the benefits of differential signaling, is called single-ended signaling.

There are plenty of choices when selecting a high-speed differential signaling technology. Differential technologies generally share certain characteristics but vary widely in performance, power consumption, and target applications. Table 1-1 lists various attributes of the most common differential signaling technologies.

Industry standards bodies define LVDS and M-LVDS technologies in specifications ANSI/TIA/EIA-644A and ANSI/TIA/EIA-899, respectively. Some vendor datasheets claim LVDS I/Os (or pseudo-LVDS) but in fact may not meet the required common mode or some other important parameter. Therefore, compliance to the LVDS specification TIA/EIA-644A is an important consideration.Current-Mode Logic (CML) and Low-Voltage Positive-Emitter-Coupled Logic (LVPECL) are widely used terms throughout the industry, although neither technology conforms to any standard controlled by an official standards organization. Implementations and device specifications will therefore often vary between vendors. AC coupling is used extensively which helps resolve threshold differences that might otherwise cause compatibility issues. Note that all of the technologies listed are differential and thus share the advantages common to differential signaling such as excellent noise immunity and low device-generated switching noise.


A typical LVDS driver – receiver pair is shown in Figure 1. A (nominal) 3.5mA current source is located in the driver. Since the input impedance of the receiver is high, the entire current effectively flows through the 100Ω termination resulting in a (nominal) 350 mV voltage across the receiver inputs. The receiver threshold is guaranteed to be 100 mV or less, and this sensitivity is maintained over a wide common mode from 0V to 2.4V. This combination provides excellent noise margins and tolerance to common-mode shifts between the driver and receiver. Changing the current direction results in the same amplitude but opposite polarity at the receiver. Logic ones and zeros are generated in this manner. CML and LVPECL have a similar architecture but with different strength current sources and termination schemes.



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03-02-2011, 09:58 PM

[b]LOW VOLTAGE DIFFERENTIAL SIGNALLING TECHNOLOGIES
SEMINAR REPORT
by

UNNIKRISHNAN K
Roll No.24, S7, ECE B.Tech
DEPARTMENT OF ELECTRONICS & COMMUNICATION
COLLEGE OF ENGINEERING
TRIVANDRUM-695016
SEPTEMBER 2010

LOW VOLTAGE DIFFERENTIAL SIGNALLING
TECHNOLOGIES


.pdf   LOW VOLTAGE DIFFERENTIAL SIGNALLING.pdf (Size: 798.66 KB / Downloads: 131)


ABSTRACT

Recent growth in high-end processors, multi-media, virtual reality and networking has
demanded more bandwidth than ever before. But the point-to-point physical layer interfaces
have not been able to deal with moving information at the data rates required. Some of
today’s biggest challenges that remain to be solved include: the ability to transfer data fast,
lower power systems than currently available, and economical solutions to overcome the
physical layer bottleneck. Data Transmission standards like RS-422, RS-485, SCSI and
others all have their own limitations most notably in transferring raw data across a media. Not
anymore. Low Voltage Differential Signalling (LVDS) is a high speed (>155.5 Mbps), low
power general purpose interface standard that solves the bottleneck problems while servicing
a wide range of application areas.

LVDS technology uses differential data transmission. The differential scheme has a
tremendous advantage over single-ended schemes as it is less susceptible to common mode
noise. Noise coupled onto the interconnect is seen as common mode modulations by the
receivers and is rejected. The receivers respond only to differential voltages. LVDS
technology is not dependent on a specific power supply, such as +5V. This means there is an
easy migration path to lower supply voltages such as +3.3V, +2.5V or even lower while still
maintaining the same signalling levels and performance. Technologies like ECL or PECL are
more dependent on the supply voltage. This feature is highly desirable in any application that
foresees moving to lower supply voltages without substantial redesign or worrying about
mixed voltage operation (+5V/+3.3V) on system boards.
ii

List of Figures

1. Approximate Signalling Rate vs Transmission Distance for Various Interfaces 2
2. Single-Ending Interface Circuit Schematic Diagram 4
3. Differential Interface Circuit Schematic Diagram 5
4. LVDS driver and receiver 7
5. Differential signalling 7
6. Typical Data Rates and Cable Drive Strength 8
7. Typical LVDS transmitter driver 9
8. Typical LVDS receiver driver 9
9. Typical CML Implementation 10
10. Typical LVPECL Implementation 11
11. Typical point to point Implementation 13
12. Typical multidrop Implementation 14
13. Typical halfduplex Implementation 14
14. Typical multipoint Implementation 15
15. Driver output voltage 16
16. Differential input voltage threshold 17



1


Chapter 1
Introduction


Highspeed transmission of binary data has converged to a general class of low
voltage differential signalling (LVDS) that presents unique challenges to the designer. This
seminar and presentation report is a compilation of application notes for data transmission above 30 Mbps
using an LVDS electrical layer. What is LVDS? Where should it be used? What are its
benefits? What are its limitations? Answers to these and other questions are the subject of
this report. This guide examines the details of lowvoltage differential signalling (LVDS)
and provides applications information to help designers use these devices. However, before
exploring the details of LVDS, step back first and look at data transmission in general, and
see where LVDS fits in. Transmitting data from one location to another, whether it takes
place between integrated circuits on a printedcircuit board, or between networked
computers located halfway around the world, has resulted in the development of numerous
transmission technologies






2
Chapter 2
Data transmission basics

Data transmission, as the name suggests, is a means of moving data from one
location to another. Choosing the best transmission standard to accomplish this requires
evaluation of many system parameters. The first two considerations encountered are How
fast? and How far? How fast? refers to the signalling rate or number of bits transmitted per
second. How far? is concerned with the physical distance between the transmitter and
receiver of the data. Consideration of these two primary system parameters usually results in
a significant narrowing of the possible solutions. Figure1 shows the speed and distance
coverage of some familiar data transmission choices.



Fig.1 Approximate Signalling Rate vs Transmission Distance for Various Interfaces

As can be seen in Figure1, signalling rate eventually decreases as transmission
distance increases. While steadystate losses may become a factor at the longest
transmission distances, the major factors limiting signalling rate, as the distance is 3
increased, are time varying. Cable bandwidth limitations, which degrade the signal
transition time and introduce intersymbol interference (ISI), are primary factors reducing the
achievable signalling rate when transmission distance is increased. Figure 1 also shows that
generalpurpose, singleended logic, including backplane transceiver logic (BTL: IEEE
1194.1), gunning transceiver logic (GTL), and gunning transceiver logic plus (GTLP)
provide satisfactory interface solutions when the transmission distance is short (< 0.5 m) and
the signalling rate is moderate. When transmission distance is increased, standards with
higher voltage swings or differential signalling often move the data. If data transmission
over about 30 m and less than 50 Mbps is required, differential signalling standards
TIA/EIA422 and TIA/EIA485 should be considered. High differential outputs, sensitive
receivers, and the capability to operate with up to 7 V of ground noise make these interfaces
ideal for long direct connections between equipment. TIA/EIA422 and TIA/EIA485 use
similar voltage levels but differ in the bus topologies they can support. TIA/EIA422 is used
for multidrop (one driver and many receivers) operation, while TIA/EIA485 allows for
multipoint signalling (many drivers and receivers).
For signalling greater than 50 Mbps or in lowpower applications, lowvoltage
differential signalling (LVDS) or multipointlowvoltagedifferential signalling (MLVDS)
provides an attractive solution. Introduced in 1996, LVDS, specified in TIA/EIA644 offers
high signalling rates and low power consumption for pointtopoint or multidrop buses. M
LVDS, specified in TIA/EIA899, was introduced in 2002 and offers similar benefits for the
multipoint application. The benefits, features, and application of LVDS and MLVDS are
the subject of this handbook. When the signaling rate requirements exceed the capabilities
of LVDS, currentmode logic (CML) circuits are used, principally the emittercoupled logic
(ECL) and positive ECL (PECL) families. Signaling at 10 Gbps is possible with ECL/PECL
devices. The high speeds are achieved at the cost of high power consumption.

SingleEnded vs Differential
Singleended transmission is performed by using one signal line for each
information channel and a common ground return path shared among numerous information
channels. Figure2 shows the electrical schematic diagram of a singleended transmission
system. Singleended receivers interpret the logical state at their inputs based upon the
voltage at the single input line with respect to ground.
4


Figure 2. Single-Ending Interface Circuit Schematic Diagram

The advantages of singleended transmission are simplicity and low cost of
implementation. A singleended system requires only one line per signal. It is therefore ideal
when cabling and connector costs are more important than the signaling rate or transmission
distance. You will find this tradeoff in lowspeed PC applications such as a parallel printer
port or serial communication with many handshaking lines. The main disadvantage of the
singleended solution is its relatively poor noise performance at high signaling rates or long
distances. Because the noise coupled to the circuit adds to the signal voltage, it is
susceptible to data errors. The signal line of a singleended circuit acts as an antenna to
radiate and receive electric fields while the area formed around the circuit path is an antenna
for magnetic fields. The voltage source VN, as shown in Figure 1–5, represents this high
frequency electromagnetic coupling. Since singleended interface circuits generally share
the return path with other circuits, there is also a component of conducted susceptibility as
changes in the ground current create the ground noise voltage represented by VG. This noise
is generally low in frequency (i.e., 50 Hz from main power).

Differential Transmission

Differential transmission addresses many of the shortcomings of singleended
solutions by using a pair of signal lines for each information channel. Figure3 shows an
electrical schematic diagram of a differential transmission system. The differential driver
uses a pair of complementary outputs to indicate the state transmitted. The differential
receiver detects the voltage difference between the signal pair, rather than relative to
ground, to determine its output state.
5


Figure 3. Differential Interface Circuit Schematic Diagram

This mode of transmission has several important advantages over single ended. You
can see the fundamental advantage in the derivation of the differential input voltage, VID, in
Figure3. The noise sources VN and VG add to the input signals VIA and VIB, just as with
the singleended circuit, but by taking the difference between the two input voltages, the
common noise terms are cancelled from the desired signal. The differential receiver
accomplishes this and, with small differential input voltage thresholds, maintains high
signaltonoise ratios. There is a presumption that the VN coupled to each signal line are
equal or nearly so. Differential signal pairs that are close together are generally exposed to
the same noise sources. Twisting the signal wires together adds to this advantage. This
ensures similar exposure to electric fields and cancels differential emf from magnetic field
coupling by reversing the polarity in adjacent loops created by the twist.
In addition to noise immunity, differential circuits radiate substantially less noise to
the environment than singleended circuits. This is primarily due to the complementary
current in each line of the signal pair cancelling each other’s generated fields. Conducted
noise is also lower because there is little commonmode current to circulate through the
signal return path. Differential signalling adds cost and complexity in silicon and
interconnecting hardware where it is roughly double that for a singleended interface. As
signalling rate or the number of circuits increase, this becomes less of a disadvantage as you
add ground wires to make the singleended signalling work. Indeed, you can find few data
interfaces above 10 Mbps or longer than a half meter or so that are not differential.



6
Chapter 3
Differential signaling technology
There are plenty of choices when selecting a highspeed differential signaling
technology. Differential technologies generally share certain characteristics but vary widely
in performance, power consumption, and target applications. Table1 lists various attributes
of the most common differential signaling technologies.


Industry standards bodies define LVDS and MLVDS technologies in specifications
ANSI/TIA/EIA-644A and ANSI/TIA/EIA-899, respectively. Some vendor datasheets claim
LVDS I/Os (or pseudoLVDS) but in fact may not meet the required common mode or some
other important parameter. Therefore, compliance to the LVDS specification TIA/EIA-644A
is an important consideration. CurrentMode Logic (CML) and LowVoltage Positive
EmitterCoupled Logic (LVPECL) are widely used terms throughout the industry, although
neither technology conforms to any standard controlled by an official standards
organization. Implementations and device specifications will therefore often vary between
vendors. AC coupling is used extensively which helps resolve threshold differences that
might otherwise cause compatibility issues. Note that all of the technologies listed are
differential and thus share the advantages common to differential signalling such as
excellent noise immunity and low devicegenerated switching noise.
A typical LVDS driver – receiver pair is shown in Figure-4. A (nominal) 3.5 mA
current source is located in the driver. Since the input impedance of the receiver is high, the
entire current effectively flows through the 100D termination resulting in a (nominal) 350
mV voltage across the receiver inputs. The receiver threshold is guaranteed to be 100 mV or
less, and this sensitivity is maintained over a wide common mode from 0V to 2.4V. This
combination provides excellent noise margins and tolerance to commonmode shifts
between the driver and receiver. Changing the current direction results in the same
amplitude but opposite polarity at the receiver. Logic ones and zeros are generated in this 7
manner. CML and LVPECL have a similar architecture but with different strength current
sources and termination schemes.


Figure 4. LVDS driver and reciever



Figure 5. Differential signalling

From this simple diagram in Figure-5 the advantages common to all differential
signalling technologies can be seen. First, note that the current source is always on and
routed in different directions to drive logic ones and zeros. This always on characteristic
eliminates the switchingnoise spikes and EMI resulting from turning highcurrent
transistors on and off (as required in singleended technologies). Secondly, the two lines of
the differential pair are adjacent to each other providing a considerable amount of noise
immunity. Noise from crosstalk or EMI that is absorbed in one of the pair will also appear in
the adjacent line. Since the receiver responds to the difference between the two channels,
“commonmode” noise that appears on both lines of the pair will cancel at the receiver.
Also, as the two adjacent lines carry equal current, but in opposite directions, EMI
generation is minimized. 8
Chapter 4

LVDS – LowVoltage Differential Signalling
The 350 mV typical signal swing of LVDS consumes only a small amount of power
and therefore LVDS is a very efficient technology, delivering performance at data rates up
to 3.125 Gbps. The simple termination, low power, and low noise generation generally
make LVDS the technology of choice for data rates from tens of Mbps up to 3 Gbps and
beyond.


Figure 6. Typical Data Rates and Cable Drive Strength

For higher data rates, technologies such as CML or LVPECL are required. As shown
graphically in fig6, CML and LVPECL are capable of very high data rates in excess of 10
Gbps. Achieving these very high data rates requires extremely fast, sharpedge rates and
typically a signal swing of approximately 800 mV. For these reasons, CML and LVPECL
generally require more power than LVDS.

9
LVDS Transmitter & Driver schematics



Figure 7. Typical LVDS transmitter driver


Figure 8. Typical LVDS receiver driver

10
CML – CurrentMode Logic
CML is a highspeed pointtopoint interface capable of data rates in excess of 10
Gbps. As shown in Figure 9, a common feature of CML is that termination networks are
integrated typically into both drivers and receivers. CML uses a passive pullup to the
positive rail, which is typically 50D. Most implementations of CML are AC coupled, and
therefore require DCbalanced data. DCbalanced data tests require data coding that contain,
on average, an equal number of ones and zeros.



Figure 9. Typical CML Implementation


LowVoltage PositiveEmitterCoupled Logic

LVPECL and PECL are both offshoots of the venerable ECL technology first
introduced in the 1960s. ECL is powered commonly between ground and 5.2V. Because of
the negative rail requirements and ECL’s incompatibility with other logic families, a
positive rail technology was introduced known as PositiveEmitterCoupled Logic (PECL).
ECL, PECL, and LVPECL all require a 50D termination into a termination rail that is about
2V less than the most positive rail. ECL drivers are lowimpedance openemitter outputs
that generate typically 700 mV to 800 mV. The output stage remains in the active region,
preventing saturation, and results in very fast and balanced edge rates. Positive features of 11
LVPECL are the sharp and balanced edges and high drive capability. Drawbacks of
LVPECL are relatively high power consumption and sometimes the need for a separate
termination rail.


Figure 10. Typical LVPECL Implementation

Selecting an Optimal Technology

With the existence of various differential technologies, a need for some guidance in
selecting an optimal signalling technology for an application is obvious. The following are
the factors under consideration when selecting an optimal technology for a given
application:
• Required bandwidth
• Ability to drive cables, backplanes, or long traces
• Power budget
• Network topology (pointtopoint, multidrop, multipoint)
• Serialized or parallel data transport
• Clock or data distribution
• Compliance to industry standards
• Need or availability of signal conditioning

LVDS is the most common differential signalling interface. The low power
consumption, minimal EMI, and excellent noise immunity are the features that have made
LVDS an interface of choice for many applications. In addition, the LVDS wideinput 12
common mode makes LVDS devices easy to interoperate with other differential signalling
technologies. The latest generation of LVDS operates from DC to as high as 3.125 Gbps,
allowing many applications to benefit from LVDS. These multigigabit LVDS devices
feature preemphasis and equalization that enables signal transmission over lossy cables and
printed circuit board (PCB) traces. Applications requiring data rates greater than 3.125 Gbps
will likely require CML signalling. In addition, certain communication standards (e.g. PCIe,
SATA, and HDMI) mandate the use of specific signalling technologies or describe a set of
conditions such as signal amplitude and reference to VCC, consistent with CML. For
applications with data rates between 2 Gbps and 3.125 Gbps, the optimum choice will
depend on the desired functionality, performance, and power requirements. For relatively
short distance transmission where signal conditioning is not required the device power and
jitter dominate, with CML generally having the lowest jitter and LVDS the lowest power.
For longreach requirements, losses in the media dominate and the best choice is generally
the device with the best signal conditioning solution for the data rate and media.













13
Chapter 5
LVDS standards [/b]
The main three standards defining low voltage signaling are
• LVDS (TIA/EIA644)
• LVDSRev A (EIA644A)
• MLVDS (EIA899)
LVDS (TIA/EIA644)
This standard defines a point to point topology. As pointtopoint topologies involve
only a single driver and receiver pair, interconnecting media can be very simple and
typically require a minimal number of transitions from one medium to the next (i.e. a
transition from a printed circuit board to a cable via a connector). A minimal number of
transitions usually means the signal path will likely have wellcontrolled impedance. A
controlledimpedance environment allows very high signalling rates. While all differential
signalling technologies may be used in links configured as a pointtopoint topology, LVDS,
CML, and LVPECL are designed for pointtopoint signal transmission. Interface devices
that feature LVDS, LVPECL, or CML have driveroutput signals with fast edges that allow
multigigabit transmission.




Figure 11. Typical point to point Implementation


• It is a point to point simplex configuration.
• Parallel Data can be transmitted using LVDS pair per bit. (Parallel LVDS). 14
LVDSRev A (EIA644A)
This standard defines a multidrop implementation. Unlike pointtopoint topologies,
multidrop topologies have multiple signal drivers and receivers all sharing a single
interconnect. A variant of the multidrop topology where there is a single driver and multiple
receivers is referred to as a “multipoint’ topology

Figure 12. Typical multidrop Implementation
Figure 12 illustrates a typical multidrop topology. Terminating the signal bus on the
far receiver side is advisable only when the signal driver is on the opposite end of the bus
from the terminated receiver. In all other cases (e.g. driver connected to the middle of the
bus), the bus needs to be terminated at both ends of the bus. Another frequently
implemented variant of the multipoint topology is “halfduplex” topology (Figure 13) which
consists of two driver/receiver pairs that transmit and receive signals between two points
over a single interconnect.

Figure 13. Typical halfduplex Implementation

• It is a simplex or half duplex , multipoint system.
• Usually called as BLVDS(Bus LVDS)
15
MLVDS (EIA899)
The final architecture to be discussed is multipoint. In a multipoint configuration
many transmitters and many receivers can be interconnected on a single transmission line.
The key difference here is the presence of two or more drivers. Such a situation creates
contention issues that need not be addressed with pointtopoint or multidrop systems.
Multipoint operation allows for bidirectional, halfduplex communication over a single
balanced media pair. To support the location of the various drivers throughout the
transmission line, double termination of the transmission line is now necessary. Figure 14
shows a multipoint configuration.






Figure 14. Typical multipoint Implementation








16
Chapter 6
LVDS & MLVDS Line Circuit Characteristics

The chapter presents specific requirements of the TIA644A and 899 standards
along with nonstandard characteristics that affect overall performance.

1. Driver Output Voltage(VOC)
The driver’s primary function is to launch an electromagnetic wave into one or two
transmission lines. As explained earlier, this is equivalent to generating a voltage across a
resistor, albeit a lowvalued resistor. Most device specifications designate the differential
output voltage as VOD and referenced to the inverting output. Both the minimum and
maximum output voltages are significant parameters, as the minimum sets the minimum
noise margin for the interface and the maximum establishes the input voltage range of
circuits connected to the bus.



Figure 15. Driver output voltage

Definition: It is the arithmetic mean of two complementary o/p voltages.


STANDARD MIN
VOC
(mV)
MAX
VOC
(mV)
LOAD
RES(Ω)
LVDS  150 100
MLVDS  150 50 17

2. Differential Input Voltage Threshold(VID)
The input signal to the transmission line is the driver output voltage; the differential
receiver must detect what comes out. The first parameter to consider for a receiver is the
differential input voltage threshold, VIT, as it defines the voltage needed to change the
receiver output state. The specified maximum and minimum VIT establish the limits for
VIT, and difference voltages above or below them assure that the receiver indicates a valid
logic state at its output. The EIA/TIA644A standard specifies a maximum VIT of 100 mV
and a minimum of –100 mV under all operating conditions and over the input common
mode voltage range. With a minimum driver differential output voltage of 247 mV, this
provides a worstcase differential noise margin of 147 mV. Although this may seem like a
small value, interference in a wellbalanced differential transmission system introduces
virtually no differential noise. In addition to a higher minimum output voltage for TIA/EIA
899 drivers, compliant receivers must exhibit a maximum VIT of 50 mV and a minimum of
–50 mV for Type1 receivers, for a minimum 400 mV differential noise margin. TIA/EIA
899 also specifies Type2 receivers with a maximum VIT of 150 mV and a minimum of 50
mV. The offset VIT of a Type2 receiver allows a 0V difference to be a valid bus state and
is useful for wiredlogic signalling or failsafe provisions. Figure 2–3 shows a graphical
summary of the input voltage thresholds.
Definition: It is the voltage needed at the receiver to change its state.


Figure 16. Differential input voltage threshold
18
STANDARD MIN VID (mV) MAX VID (mV) LOAD RES(Ω)
LVDS 100 +100 100
MLVDS 50 +50 50


3. Common Mode Voltage Range (VICR)
Differential signalling works well because noise is common to both signal lines and
is rejected. As with all circuits, there is a limit to the noise voltage magnitude and frequency
that a differential receiver can reject. This leads to the need to specify a receiver’s input
commonmode voltage range, commonly designated as VICR. The commonmode voltage
range required for an interface circuit represents a compromise that takes into account
numerous factors including the available semiconductor technologies, signalling rates, cable
impedances, distance goals, and other relevant environmental factors. The impact of EMI on
a perfectly balanced differential transmission line is to induce a purely commonmode
voltage which is identical on each of the two conductors. The magnitude of the induced
commonmode voltage is a function of several factors: The strength, orientation, and
proximity of the disturbing source; the effects of any shielding that may be present; and the
commonmode impedance characteristics of the transmission system. Predicting the exact
nature of EMIinduced commonmode voltage is difficult at best, but the reader should
assume there is always some amount present.
Definition: It is the maximum common mode voltage the receiver can reject

STANDARD MIN VICR (V) MAX VICR (V)
LVDS  2.4
MLVDS  5.2

4. Fail Safe Feature
Failsafe is a receiver feature that guarantees the output to be in a known logic state
(HIGH) under certain fault conditions. This occurs when the inputs of the receiver are either
open, shorted or terminated
19
Chapter 7
LVDS specification, power consideration, applications


Power consideration



20
As the name implies, LVDS is a lowvoltage signalling standard. An LVDS output
driver provides 350 mV, nominal, into a 100D load. This results in ≈1.2 mW of power
delivered to the load. Figure 1–12 compares the load power of LVDS and other differential
signalling techniques and shows LVDS to be 1.5% to 50% of the presented signalling
approaches. In general, a lower load power correlates closely with lower overall power for the
interface.


Application
• Super computers
• Workstations
• Internet Gateways
• Ethernet switches
• Hi speed Video processing
• Space Industry
• Avionics Industry
21
Typical Application in Car
22
Chapter 8
Conclusion


The most attractive features of LVDS include its high signalling rate, low power
consumption, and electromagnetic compatibility.
We define the number of state changes per unit time as the signalling rate for the
interface. Knowing the unit interval time UI, between state changes, you can derive the
signalling rate as the inverse of the unit interval. TIA/EIA644A and TIA/EIA899 require
that driver output transition times be less than 30% of the unit interval, with a lower limit of
260 ps and 1 ns, respectively. The standards also recommend that the transition time at the
receiver input be less than 50% of the unit interval. The difference between driver output
rise time and receiver input rise time allows for signal degradation through the interconnect
media. Combining the driver transition times required by the standard, and receiver input
rise time recommendations, LVDS supports a theoretical maximum signalling rate of 1.923
Gbps and MLVDS 500 Mbps. Longer transition times, leading to lower signalling rates, are
also allowed by the standards. You should note that the standards define the characteristics
at the interface to the bus and not the line circuit. Line circuits may support signal
transitions faster than 260 ps and even higher signalling rates if standard compliance is not
required. When estimating the minimum signalling rate for a particular line circuit,
designers should use the maximum output transition time
As the name implies, LVDS is a lowvoltage signalling standard. An LVDS output
driver provides 350 mV, nominal, into a 100D load. This result in ≈1.2 mW of power
delivered to the load. In comparison the load power of LVDS and other differential
signalling techniques and shows LVDS to be 1.5% to 50% of the presented signalling
approaches. In general, a lower load power correlates closely with lower overall
power for the interface.
Differential signalling allows LVDS to operate in the presence of electromagnetic
noise, while at the same time minimizes its own radiated emissions. Differential signalling
also rejects externally coupled noise commonmode when within the input range of the
differential receiver

23
Reference

 LVDS Owner’s manual – Fourth edition
 Electrical characteristics of LVDS interface circuits TIA
 LVDS application and data hand book Texas Instruments
 nationallvds
 maximic.com
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jithinrajan
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Posts: 2
Joined: Jun 2011
#6
04-07-2011, 10:47 PM

i need the ppt of the lvds pls send me
i need power point presentation of the topic lvds
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