GARY MELCHIOR /
08.15.2011
Originally featured on BroadcastEngineering.com
The LMH0387
National Semiconductor's bidirectional SDI I/O device simplifies hardware headaches.

Traditionally, SDI ports on video equipment have been fixed as either inputs or outputs. This has led to hardware designers choosing architectures with a limited number of input and output ports based on the number of BNC connectors they can fit into a given form factor. The fixed nature of these SDI ports necessitates many unique board configurations in order to support a variety of applications. For example, one board may require one input and seven outputs, while another requires four in and four out. This leads to higher design costs, as well as higher inventory costs for stocking each unique configuration.

Furthermore, the SDI ports on each of these unique boards must meet stringent design requirements, such as the SMPTE 424M return loss specification. This requires fine-tuning many passive components for each particular board design, a time-consuming and costly process.

New way of thinking about SDI ports

National Semiconductor has developed a single-chip bidirectional SDI I/O — the LMH0387. This configurable I/O device provides the flexibility for SDI ports to be set as either inputs or outputs via software or a switch, allowing one physical board design to be used in multiple configurations. The device includes an integrated return loss network to assure compliance with the SMPTE return loss specification. This saves time by avoiding the need for the often iterative process of fine-tuning passive components to achieve specification compliance, while conserving board area and reducing bill of materials costs.

The LMH0387 configurable I/O is a single-chip solution that integrates the full functionality of an SDI adaptive cable equalizer, SDI cable driver and integrated return loss network in a tiny 7mm × 7mm package. It provides a single pin for both the input and output functions. This pin connects to the BNC connector via a single AC-coupling capacitor. No additional components are required in the SDI signal path.

The bidirectional I/O provides a switch to configure the single pin as either input or output. In the input mode, the equalizer is enabled, and the cable driver is disabled to prevent interference on the shared pin. In the output mode, the cable driver is enabled, and the equalizer is typically disabled for power savings but may optionally be left enabled to provide a loopback path of the signal being driven on the cable.

The bidirectional I/O optimizes BNC usage, which is especially important when the number of available BNCs is physically limited by board size. Suppose you are designing video equipment with five SDI ports, and you would like to provide various options for the number of inputs and outputs. These ports could be configured as four in, one out (4 × 1); 3 × 2; 2 × 3; or 1 × 4. With dedicated equalizers and cable drivers, this would require four unique designs. Using the configurable I/O, this identical functionality can be realized with a single physical design, programmable as four different options. (See Figure 1.) Only one board needs to be designed, optimized, maintained and stocked for a given number of SDI ports. The board's I/O configurations can be preconfigured before delivery, programmed during installation or even changed live post-installation if desired.

By integrating the return loss network, the LMH0387 delivers plug-and-play compliance to rigorous SMPTE return loss specifications and simplifies board design. The SMPTE return loss requirement specifies how close in impedance a network must be to 75Ω across a specified frequency range. This requirement is intended to prevent reflections and increase the power transfer at the BNC. A return loss network, consisting of inductors, resistors and sometimes capacitors, is required to compensate for the input or output capacitance of the receiver or driver circuit. (See Figure 2.) Good return loss is best achieved by placing this network as close as possible to the equalizer or cable driver chips and using physically small components in order to minimize impedance discontinuities. By integrating this network inside the package, the return loss is optimized, and the LMH0387 is able to achieve greater than 5dB of margin above the SMPTE specification.

Summary

The LMH0387 configurable I/O enables a new way of thinking about SDI ports. They are no longer fixed and inflexible, but instead, become adaptable to your particular application. One board using the bidirectional I/O supplants the need for many unique designs. The I/O device can replace an SDI equalizer or cable driver, or do the job of both depending on the need, in addition to simplifying layout and saving board space.

Gary Melchior is an applications engineer with National Semiconductor.



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