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LVDS interfaces for industrial applications

Feature

By Thomas Brand, Field Applications Engineer, Analog Devices

In this era of automation, digitisation and Industry 4.0, the transmission of signals and data are taking on an ever greater role. The networking of machines, systems and even individual sensors in these applications both on and off the production floor requires not only a stable infrastructure but also secure, fast, high-precision, disturbance-free, high-bandwidth transmission paths that may need to withstand extremely harsh environmental conditions, depending on the application. There are several different methods used for data transmission: wired, wireless, serial and parallel. All approaches have their own advantages and disadvantages, which need to be weighed carefully against each other for each application case.

The wireless route

In contrast to wired systems, wireless technologies depend on several critical parameters. An important aspect of radio communication is radio range, which is dependent on the location of the transmission and reception antennas, transmission power, the condition of the local environment and frequency range. Thus, in the planning of radio transmission applications, the corresponding criteria, especially in relation to the range and thus the quality of the received signal, must be considered.

In the past, bandwidths of a few Mbps were adequate for applications such as interface converters or industrial backplanes. The partly necessary isolation of the different interfaces, such as the serial peripheral interface (SPI) or RS- 485, can still be realised with standard solutions. However, current trends such as Industry 4.0 and the Internet of Things (IoT) call for copious measurements and extremely complex control systems with higher data rates or bandwidths that, in turn, must be borne by the interfaces. Through these and other rising demands on the interfaces, such as additional safety in the form of higher dielectric strength, greater reliability and lower space requirements, conventional solutions are no longer sufficient. Here, too, digital isolators represent the best solution because they not only satisfy the heightened demands for safety, performance and reliability, but also offer integrated isolation and multiple inputs and outputs.

LVDS

A common method used in signal transmission applications involves low voltage differential signaling (LVDS). This involves an established interface standard (TIA/EIA-644) for serial data transmission, which features a high noise immunity, apart from its extremely energy-saving properties and its high data rate potential of up to a few Gbps. These positive characteristics can be attributed to the internally-used current control or limitation of the driver modules to a maximum of 3mA. The signal’s differential voltage is a mere 20mV. However, it is subsequently amplified back to the logic level of 300mV (differential) on the receiver side, which among other things, yields extremely low electromagnetic interference (EMI) and extremely fast switching speeds.

LVDS interfaces are often used with control and regulating systems in which large data volumes must be sent between the electronic circuits or between short cable lengths. They are also used for synchronisation of different components in a complete application by distributing the clock signals very rapidly to the respective components. Analogue front ends (AFEs) in industrial measurement applications and control systems count among the typical applications for LVDS. However, they are also commonly used for realisation of digital interfaces between multiple data nodes and in the transmission of video signals, for example via HDMI. .

Another aspect that should not be neglected is the possibility of galvanic isolation that LVDS circuits provide. Hence, they are also used wherever isolation of communication interfaces, such as in electronic circuits or backplanes, is required. Backplanes are circuit boards with multiple connectors for accommodating various add-on board modules, allowing for easy plug-and-play extension of the base system to include additional assemblies. However, the add-on modules are often subjected to high voltage transients because, in many applications they are in direct contact with components connected to the power distribution grid. The add-on modules are accordingly vulnerable to external events such as lightning strikes. Electrostatic discharge caused by human contact or internal capacitors that are suddenly charged, charged with reverse polarity or discharged when the add-on modules are plugged in or unplugged can also lead to high transients. Thus, a safely-isolated interface is indispensable for the system. Otherwise, the connected assemblies could easily be damaged or users endangered if voltage transients occur.

Functionally-isolated communication interfaces are also favourable for industrial measuring instruments because isolated interfaces offer a floating ground, for example, between analogue-to-digital converters (ADC) and microcontrollers. This allows the measured signals to be isolated from influencing and interfering with or influences and interference from the rest of the application.

Realising LVDS interfaces

Diverse products for realising isolated LVDS interfaces are already on the market. Analog Devices offers a very efficient and reliable solution with its isolated LVDS family consisting of ADN4650, ADN4651 and ADN4652, which supports data rates of up to 600Mbps and simultaneously also corresponds to the standard values of nonisolated LVDS interfaces. In comparison, standard digital isolators can reach just 150Mbps. Very-high data rates, despite existing isolation, are possible thanks to the iCoupler technology, which involves a microelectromechanical systems (MEMS) reproduction of a transformer that enables simple, space-saving isolation of digital signals.

The LVDS family also offers very precise time characteristics, as well as extremely low jitter, also known as timing jitter. Jitter describes the variations in the rising and falling edges of the digital signals with respect to the ideal time reference. At high data rates, low jitter is extremely important because it takes just 1.6ns to transmit one bit at 600Mbps. Any jitter in the rising or falling edges of the signals must allow the ADC enough time for the actual high or low level so that sampling can be performed correctly. For the ADN465x family, jitter is typically 70ps.

The LVDS modules also offer two isolated LVDS channels, which form the transmission and the reception channel in the ADN4651. The channels in the ADN4652 are arranged inversely to the channels in the ADN4651, whereas the ADN4650 only offers transmission or reception channels, depending on the wiring. The ADN465x family works internally with a supply voltage of 2.5V; industrial systems often, unfortunately, do not have this supply voltage but, often, only 3.3V. For this reason, low dropout voltage regulators (LDOs) enabling an external supply voltage of 3.3V at the inputs are integrated into the ADN465x family. The supply to the module or its inputs and its isolated output side can, for example, be accomplished with the isolated ADuM5000 DC-DC converter. This can selectively generate an isolated output voltage of either 5V or 3.3V, with a maximum power output of 500mW; see Figure 1.

In combination with the ADuM5000, this device family can meet the numerous demands placed on isolated LVDS interfaces in today’s industrial applications. This highly-integrated solution also meets all prerequisites for standardised bus communications. LVDS interfaces are frequently used in energy-saving applications. For this, the combination of the ADN4651 and the ADuM5000 represents an extremely power-saving alternative to traditional optocoupler solutions. The simultaneous isolation of several channels is often also required.

Figure 1: Isolated LVDS interface with ADN4651 and ADuM5000

In LVDS applications, channels are used in parallel to maximise the throughput and, with it, the baud rate. The described circuit with the above-mentioned modules from Analog Devices offers one four-channel isolator, and two transmission and two reception channels. This permits signal transmission by means of two complete transmission and reception channels on only one electronic assembly with simultaneously very-high transmission rates.

The data rates of almost DC to 600Mbps can easily be reached with the ADN465x family, provided that the specifications for the maximum pulse-width distortion are adhered to. In addition, a few factors necessary for transmission of differential signals at high speeds must be taken into consideration in the layout. Thus, the input- and output-side traces should be matched and exhibit an approximate impedance of 50Ω with respect to ground, or 100Ω between the signal lines. Furthermore, it is advisable to attach 100Ω terminators to the LVDS inputs, as shown in Figure 2.

Figure 2: Circuit for isolated LVDS wiring with the ADN4651

The cable length and the connector type also affect the maximum data rate. Lower data rates of up to 200Mbps in combination with connectors for high data rates and shielded wire pairs even enable cable lengths of several meters.

The ADN4650/ADN4651/ADN46521 are signal-isolated LVDS buffers that operate at up to 600Mbps with very low jitter. This combination with the ADuM5000 makes it ideal for high-speed signal transmissions by being able to achieve 600Mbps for short distances and 200Mbps for up to several meters.

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