Since the very dawn of the computer and communications industries, one of the basic driving forces behind the advancement of technology has been the quest for smaller and smaller end-user devices. Early on the motivation was simple: Computers that took up several football fields were not too useful or accessible for more than just a few users. Now, though, the quest for mobility and portable systems has continued this trend towards smaller, handheld devices that cost less and consume less power. In the asymmetrical digital subscriber line (ADSL) market designers are continually looking for areas or functions to integrate so their devices can meet the growing demand for cheaper and smaller end equipment.

ADSL is a DSP-based communications technology that can dramatically increase the speeds of data communications over the typical copper wiring that connects most homes and businesses to the public telephone network. ADSL is particularly well suited to Internet-related communications since its performance capabilities mirror the usage pattern of the Internet. ADSL provides speeds of up to 8 Mbit/s for downloads from the network and 800 Kbit/s for uploads from the user to an Internet service provider.

When it comes to ADSL analog front-ends, integrating ancillary functionality on one semiconductor device can have certain advantages and disadvantages. Every decision an integrated circuit designer must make comes with a tradeoff but, by and large, the decisions to integrate greater functionality on ADSL codecs has reduced ADSL modem costs by reducing board space and component count. That does not make this series of decisions any easier to make, but it provides much of the motivation that continuously moves the process toward greater levels of integration.

Converter Performance

The issue in this area is to integrate the DACs and ADCs into an ADSL codec, and what level of resolution should the integrated converters have? Typical codecs available today have 12-bit converters, but several newly-introduced models include 14-bit converters. This higher resolution carries a price tag in terms of the increased silicon area and power consumption, but most modem designers find that the benefits of integrating 14-bit converters far outweigh the marginal cost difference.

By incorporating 14-bit converters instead of 12-bit converters, an ADSL modem achieves an extra 12 dB of headroom in its signal-to-noise ratio, which gives the modem higher data communications speeds over longer distances. The additional resolution afforded by a 14-bit converter extends the modems reach by 4000 ft. downstream and 2000 ft. upstream, but the designer faces the challenge of obtaining 14-bit performance in a mixed-signal environment that has high-speed digital switching noise present.

Analog Signal Processing

If all signal processing was to be done in the digital domain of the DSP, the converter resolution required would be very high. In order to obtain 14-bit performance in the ADC, the signal should occupy the full dynamic range but, because of the attenuation characteristic of the telephone line and other interference, the signal received will be weak. So filtering and gain programming need to be done in the analog domain to ensure that the signal uses the full dynamic range.

Analog Filters

Filtering is the main component in analog signal processing, and looking to integrating the analog filters into the codec, designers must weigh the advantages and disadvantages of each. Integration gives the modem designer less control over the accuracy or performance of the filters. With discrete filters, the designer can use components that have tighter tolerances, while resistance and capacitance values can change by up to 20 percent because of variations in the fabrication processing of integrated circuits. In addition, using discrete passive filters can reduce the systemęs power consumption over active integrated filters.

Codecs with integrated filters can make up for any process variations by using laser-trimming techniques and self-tuning approaches. The integration results in significant board space savings. For example, in an earlier generation of ADSL line cards using discrete components, two-thirds of the card was devoted to the analog portion of the system; consolidating the number of analog devices on-board and reducing the real estate requirements helps implement plug-in cards with much smaller form factor. Integrating the analog filters can also reduce the number of board layers needed for the modem, which in turn reduces the costs associated with the system.

Amplifiers

Another area of integration is the amplifier. Integrating programmable gain amplifiers onto an ADSL codec presents a challenge to the circuit designer. With discrete gain amplifiers, the modem design can use devices that are produced with a high-performance, high-voltage (around 12 V) process. Integrating gain amplifiers onto the codec implies realizing the same performance with a 3-V CMOS process. PGAs with comparable noise performance have been achieved in integrated codecs. The resulting modem is smaller and less costly because fewer discrete devices are used and it consumes less power, which is a very real concern as ADSL systems become deployed. While providing these advantages, an integrated solution conforming to the ADSL specification provides the same reach or effective transmission distances as modems that have discrete PGAs.

Further Integration

Some ADSL codecs available today have integrated various circuitry, such as general-purpose I/O circuits, auxiliary amplifiers and a VCXO DAC, which is used for controlling the VCXO. The codecs also have a glueless interface to the DSP, which means that no additional interface logic is required between the codec and the DSP.

General purpose I/Os on the codec can be used to monitor and control other processes that are taking place on the board. The auxiliary amplifiers can be used for any on-board analog buffering or filtering without having to use additional discrete amplifiers. Having these features already available on the codec simplifies the modem designeręs task and shortens the design cycle, because the designer will not have to design this functionality into the system.

Putting It All Together

A highly-integrated ADSL codec, such as the one illustrated in the block diagram below, has many of the advantages that designers are looking for today. These features include reduced board space, fewer components, lower costs and outstanding reach performance.

Whatęs Next?