LVDS技术详解：入门指南与设计挑战

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LVDS用户手册是一份全面的指南，旨在帮助读者深入了解低电压差分信号(LVDS)技术，这是一种高速接口技术，特别适合于数据传输和信号处理应用。该手册第四版包含了丰富的内容，从LVDS技术的物理层特点开始，深入探讨了其在实际设计中的关键要素。 1. **LVDS技术概述** (Chapter 1): 这一部分首先定义了什么是差分信号技术，强调了LVDS作为低电压版本的重要性，它能够在低功耗和噪声敏感的环境中提供高数据速率。接着，CML（电流模式逻辑）和LVPCECL（低电压正发射极耦合逻辑）这两种相关技术也被提及，让读者了解它们与LVDS的区别以及选择最佳技术时需要考虑的因素。 2. **网络拓扑** (Chapter 2): 拓扑设计包括点对点连接、多点或多路连接，以及混合不同信号技术的网络结构。这章节提供了设计者在构建复杂系统时如何规划LVDS信号流的关键指导，同时讨论了如何选择合适的接口技术以适应不同的应用场景。 3. **串行时钟数据编码（SerDes）架构** (Chapter 3): SerDes是序列到并行数据转换的核心部分，对于LVDS系统至关重要。章节详细解释了并行时钟方式的工作原理，以及如何设计高效的串行通信链路，确保信号的准确传输和降低抖动的影响。 **设计与布局指南** (Chapter 39-45)：这部分介绍了设计LVDS电路时必须遵循的原则，包括信号线的布局、终端匹配和信号调理等，这些都是保证信号质量、减少串扰和反射的关键步骤。 **信号条件与媒体** (Chapter 47-58): 高速信号在不同介质上的传播会受到诸如衰减、反射和噪声等因素的影响。这部分内容涵盖了如何通过信号调理技术来优化这些影响，确保信号的完整性。 **半导体输入/输出模型** (Chapter 77-82): 为了实现高性能的LVDS接口，理解器件内部的工作原理至关重要。这一部分介绍了半导体元件在LVDS信号传输过程中的行为模型，帮助设计者预测和解决潜在问题。 **设计挑战的解决方案** (Chapter 83-101): 最后，手册提供了应对设计过程中常见挑战的策略和方法，如噪声抑制、电源管理、温度补偿等，帮助设计师优化他们的LVDS设计，使之满足严格的性能标准。 LVDS用户手册是一个综合的资源，涵盖了从基础概念到高级设计实践的各个方面，无论你是初次接触LVDS技术的工程师，还是经验丰富的设计师寻求深入理解，都能从中获益匪浅。

2.2 Multipoint / Multidrop

Unlike point-to-point topologies, multipoint topologies have multiple signal drivers and receivers all sharing a single

interconnect. A variant of the multipoint topology where there is a single driver and multiple receivers is referred to as

a “multidrop’ topology (Figure 2-2).

Figure 2-2. A Multidrop Topology

Figure 2-2 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 “half-duplex” topology (Figure 2-3) which consists of

two driver/receiver pairs that transmit and receive signals between two points over a single interconnect.

Figure 2-3. A Half-Duplex Topology

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Physical connection of multiple drivers and receivers to a common signal bus presents unique challenges to successful

multidrop-topology designs. e major challenge that system designers encounter are the impedance discontinuities that

device loading and device connections (stubs) introduce on the common bus. Matching the impedance of the loaded bus

and using signal drivers with controlled signal edges are the keys to error-free signal transmissions in multipoint topologies.

Two versions of LVDS have been optimized for multipoint: Bus LVDS (B-LVDS) and Multipoint LVDS (M-LVDS).

B-LVDS shares many of the characteristics of LVDS but has much higher current drive (10 mA typical) and controlled

(slower) edge rates. B-LVDS is designed speciﬁcally for driving multiple loads and in a well-designed backplane can support

up to 32 loads. B-LVDS-controlled edge rates help reduce reﬂections in multidrop conﬁgurations with multiple loads and

associated stubs. e slower edges limit B-LVDS to slower data rates, typically below 1 Gbps.

e growth of B-LVDS in multidrop applications prompted the creation of an industry standard for an optimized version

of LVDS intended for such networks. National Semiconductor co-authored the standard that became TIA/EIA-899 and the

technology Multipoint LVDS (M-LVDS). M-LVDS includes all of the desirable attributes for multidrop including increased

drive, slow controlled edges, tighter input thresholds, and a wider common mode. M-LVDS is capable of driving 32 loads at

speeds up to 250 Mbps.

2.3 SerDes Architectures

Two obvious ways to add bandwidth are to either increase the bus speed or add parallel channels. Prior to the advent of

high-speed interface, the latter approach was often selected, resulting in very wide Low-Voltage Transistor-to-Transistor

Logic (LVTTL) buses and backplanes. e technique of combining slower LVTTL signals into a single-bit stream (serial-

izer), transporting the data at high speed and then redistributing at the receiver (deserializer), is very common and often

referred to as Serializer/Deserializer (SerDes). e diﬀerent SerDes architectures and advantages of each are covered in the

SerDes Architectures Section on page 19.

2.4 Mixing Signaling Technologies

It is quite common for hardware platforms to have multiple signaling requirements for clocks, data, and control signals.

In modular systems, card-to-card or box-to-box communication may involve diﬀerent vendors and technologies. For

these reasons, LVDS, LVPECL, and CML often coexist in the same platform, and translation between diﬀerent signaling

technologies is a necessary component of system design.

One approach to simplify translation is to AC couple the two networks using capacitors in each of the diﬀerential channels.

is technique eliminates DC oﬀsets and threshold diﬀerences, however AC coupling requires DC-balanced data. For

non-DC-balanced data, various termination networks allow translation between technologies. For a full discussion on

translation networks, see Termination and Translation, page 31.

2.5 Selecting an Interface Technology

e selection of an interface device or technology is an important one. Error-free and reliable signal transmission is a critical

component of many systems. e ﬁrst decisions are often the topology: Will the network be point-to-point or multidrop?

Will SerDes be required? Bandwidth, power, and jitter budgets then need to be considered.

LVDS is the most common high-speed interface and has the dual advantages of low power and wide common mode.

LVPECL generally uses additional power but supports very high data rates with excellent jitter. CML devices oﬀer similar

performance to LVPECL and have the advantage of a simple and usually integrated termination.

Network Topology

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