When to Use a Line Driver for Specialty Logic and Analog PCBs

MCUs, FPGAs, CPUs, SoCs, and any other acronym for a digital component includes standardized interfaces like SPI, I2C, UART, or high-speed interfaces like USB. On the industrial side, there are CAN bus and industrial-grade versions of the common high-speed digital protocols. What about specialty logic applications that don’t use any of these protocols or that need to operate with mixed signals? There are also analog signals to consider, which may not have any standardized interface but may require amplification and transmission over long distances.

Getting signals over long distances or fanned out to a larger number of receivers requires some additional components that can deliver sufficient power to multiple receivers (such as a buffer) or that can overcome losses on long interconnects, such as transfer over a cable. A line driver is one type of component used in standardized protocols like LVDS, as well as in mixed or specialty logic applications requiring data transfer over long distances. Line drivers are sometimes mentioned alongside clock distribution or fanout buffering, both of which are important points in developing specialty logic circuits as well as lower frequency analog systems.

Because high-speed digital protocols generally don’t use separate line driver components, it’s important to know when to use these in other types of systems (whether digital or analog). In this article, we’ll explore the relationship between line drivers and buffering, and we’ll present some line driver options you can find on the market.

Line Driver Applications

A line driver is basically a buffer or amplifier that can take a low-level input and provide a high-level output. These components also provide isolation between a low-level driver and a receiver circuit with a high impedance element as seen from the output side of the line driver. In effect, a line driver increases the signal level in logic applications, meaning more power can be delivered to load components. This gives us two possible applications for a line driver:

1. Amplifying an input signal and driving a long transmission line

2. Amplifying an input signal and routing it to multiple loads (fanout)

If you have a single line driver and a group of receivers connected on long transmission lines, then you’re effectively performing both functions simultaneously. Line drivers that perform this function may be named “fanout buffers” or something similar. These components are essentially performing the functions shown in the diagram below.

Although these components came onto the market before there were many smaller processors or SoCs with integrated digital interfaces, they are still useful in many specialty applications. Some of the common cases are outlined below.

Digital Channels With High Data Rate

Line drivers are available that support a variety of fanout ratios and data rates. The data rate specification (assuming a binary protocol like NRZ is used) is equivalent to the clock rate specification, meaning there will be some maximum clock rate that is compatible with these components. To support higher data rates, some line drivers apply pre-emphasis to the output bitstream in order to suppress intersymbol interference.

For digital applications, the point of having a line driver is to give the driving signal enough gain to overcome the total input capacitance of having many components on a bus, as well as to overcome losses on long lines. When multiple components and their input transmission lines are arranged in parallel on a bus, the arrangement will have some parasitic capacitance to ground. These capacitances add and increase the driving current required to induce switching in the load within a single clock frame. A common use of these components is in clock trees, or in cases where a system clock is being sent along a high impedance connection to a large number of components on a bus. In some cases, a driver is just too weak to drive a single load, so a line driver boosts the signal so that it can drive the load component.

Line drivers can also be used to reformat an input bitstream to a different line driving standard (either single-ended or differential mode). For example, in the SATA standard, a line driver buffers and retransmits input differential current-mode logic (CML) signals as compensated CML signals. In this example, the line driver compensates signal loss and distortion on PCB traces or a cable so that the correct signal level and rise time is seen at the receiver.

Analog Applications

The amplification provided by a line driver is put to a different use in analog applications, particularly in audio applications. The amplification provided by a line driver increases the effective dynamic range when the line driver is placed close to the signal driver. If noise is received on the output side of the channel, the SNR value will be higher overall thanks to the gain provided by the line driver. This is particularly useful when a low-level analog signal needs to be transmitted over a long cable in a noisy environment. When a differential line driver is used, you get the same benefit as a differential op-amp; common-mode noise is suppressed at the receiver as long as the line lengths are matched.

Long Cable Connections

Long cables that act like transmission lines have their own capacitance, which plays the same role as having many receiver components in parallel on a bus. A line driver can provide a signal boost needed to overcome losses on these lines while also ensuring the downstream receiver can be driven with the proper input signal level. This is basically the function of a differential line driver in RS485, which is normally integrated into an RS485 transceiver IC. To ensure signal transfer through the line without reflections, some line drivers incorporate impedance matching circuitry to the cable/connector impedance, both in single-ended and differential line drivers.

Important Line Driver Specifications

Although different line drivers are specialized for particular applications, there are some criteria that are common selection criteria for any line driver:

• Data rate/clock rate: Line drivers have a particular switching speed, which will limit the available data rate. For binary signals, the data rate and clock rate should be equal.

• Differential vs. single-ended: High data rate line drivers will use differential outputs. Lower speed/frequency line drivers or clock fanout buffers may use single-ended signals, and controlled impedance might not be required. Make sure to check your interface specifications.

• Interface conversion: Some line drivers incorporate interface level conversion. This is usually required when converting to an LVDS interface.

• Output skew: High data rate line drivers with multiple outputs will always have some skew between the outputs. This is important if precise timing is required across the fanned out components.

• Harmonic distortion: This is important for line drivers used in analog applications, and it will determine the level of filtering needed to remove any undesired harmonics generated during amplification.

Some example line driver component options are shown below.

ON Semiconductor, NB3L8504S

The NB3L8504S from ON Semiconductor is a quad-channel differential line driver that can provide fast differential clock buffering or conversion of input data streams into LVDS signal levels. This component provides transmission of bit streams or clock pulse streams up to 700 MHz with 50 ps maximum output skew. Each output can be toggled using an output enable pin (OE), which can be toggled with a simple GPIO pin from a processor. ON Semiconductor offers multiple sets of line driver products in SMD packages, some of which have high fanout and driver/receiver pairs.

Renesas, ISL1557IRZ-T7

The ISL1557IRZ-T7 from Renesas is a differential line driver with dual op-amp topology in an SMD package. This component can drive loads up to 750 mA from a 12 V supply, making it useful in industrial applications. In addition, this component has up to 300 MHz bandwidth for relatively high-frequency analog applications. For lower frequency applications, harmonic distortion is rated at -80 dBc at 150kHz.

Microchip, SY89474UMG

The SY89474UMG from Microchip combines functions of a differential multiplexer and a 1:2 fanout buffer in a single package. This component offers fanout buffering for clocks above 2.5 GHz or NRZ bitstreams exceeding 2.5 Gbps. The input supports multiple interfaces with AC or DC coupling, as shown in the application circuits below. This component is superior to the ON Semiconductor option shown above when higher data rates are required in multi-gig links; the rise/fall times of the LVDS signals from this component are ~170 ps, but with only 20 ps part-to-part skew.

Other Components for Systems with Line Drivers

Line drivers are a general class of components that find their use in a range of applications, but they need other supporting components for total system functionality. Some other essential components needed to support digital and analog functions in line driver circuits include:

• MCU for processing data

• Specialty amplifiers

• ADCs for mixed signal capabilities

• Voltage regulators

When you need to find a line driver and other supporting components for your next mixed signal system, use the complete set of advanced search and filtration features in Octopart. When you use Octopart’s electronics search engine, you’ll have access to updated distributor pricing data, parts inventory, and specifications, and it’s all freely accessible in a user-friendly interface. Take a look at our integrated circuits page to find the components you need.

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