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Mixed-Signal and Analog Products Texas Instruments Networking Products
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Analog and Mixed-Signal Technologies Pave the Way for xDSLDigital Subscriber Loop (DSL) technologies are a promising bridge between the narrowband, circuit-switched telephone networks of the past and the broadband, packet-switched networks of the future. Internet access, remote office networking, videoconferencing -- DSL will speed up these and many other services and make them more widely available. And DSL technology offers the advantages of high data bandwidth over existing copper wiring, saving installation costs for service providers and promising rapid deployment for customers. ADSL: The Imminent DSL Technology The DSL family of technologies includes Symmetric DSL (SDSL), Asymmetric DSL (ADSL), High-speed DSL (HDSL), Very high-speed DSL (VDSL) and other standards. Among these, ADSL is capturing the most attention today, with its promise of high-speed Internet access for residential and small business users. The basic ADSL standard, known as G.DMT or ANSI T1.413 issue 2, provides data transmission of up to 8 megabits per second (Mbps) downstream and up to 800 kilobits per second (Kbps) upstream, with rate limits varying according to line length. The digital data stream coexists on the same line with voice transmission, so that subscribers can access the Internet and hold phone conversations on a single line simultaneously. While ADSL is the preferred standard today, several other flavors of xDSL are also generating a lot of interest. A trimmed-down ADSL standard known as G.lite will offer up to 1.5 Mbps downstream and 128 Kbps upstream and, unlike G.DMT, can operate without a device that splits the high-frequency digital from the low-frequency voice signals on the phone line. There is also an ADSL version to operate with the Integrated Services Digital Network (ISDN), which is used widely in Europe. Finally, VDSL is of particular importance because it will eventually enable speeds up to 50 Mbps over existing copper wiring. A Comprehensive Line of xDSL Solutions As an industry leader in both digital and analog technology, TI is dedicated to providing solutions that will meet the needs of DSL manufacturers, service providers and end users. These include a comprehensive line of integrated circuit products for both client and central office (CO) equipment. Additionally, TI is committed to an interoperable ADSL solution. TI, Alcatel and Analog Devices recently announced the successful completion of two-way interoperability testing between Alcatel's ADSL CO solution and TI's customer modem silicon solution. In addition, testing has also been completed between TI's CO modem and Alcatel's customer modem. This is the first step in a broader interoperability program The foundation of TI's solutions is leading-edge digital signal processors (DSPs) and the tools and software that surround them. However, a critical element in these solutions is the analog and mixed-signal technology. It's this technology that connects the digital world of the DSP to the real world of analog signals moving over copper wire. Specifically, the analog and mixed-signal circuits required for xDSL technology include coding-decoding (codec) devices, and line drivers and receivers. With its xDSL components, TI is continuing a tradition of providing advanced voiceband POTS (plain old telephone system) codecs for the CO, as well as codecs in voiceband modems up to and including V.90. TI is also building on its history as a provider of data transmission products, including line drivers, transceivers and universal asynchronous receiver/transmitters (UARTs). Leveraging the experience gained from supplying these products, TI is designing its xDSL products to meet the technical challenges inherent in the new communications technologies. Highly Integrated Codecs Codecs are key to successful implementations of xDSL. As the first of TI's ADSL codec offerings, the TLV320AD12 provides a complete CO codec solution, complete with digital filters, that is compliant with T1.413, issue 2. The highly integrated 'AD12 provides speed, low distortion and flexibility for ADSL design. In order to handle the high data rates of full ADSL, the 'AD12 integrates 14-bit resolution converters, which have proven their capability of supporting 8 Mbps plus 800 Kbps bandwidths. A single 3.3 V supply with a power-down mode reduces power consumption. The device offers a selectable 2.2 Mbps or 4 Mbps parallel transfer rate with a serial configuration port for DSP connections. The 'AD12 is unique among full ADSL codecs in its integration of all necessary filters. On-chip digital filtering reduces system complexity. The 'AD12 is designed to minimize space requirements through its high level of integration. On-chip programmable gain amplifiers provide design flexibility and help optimize the system for maximum throughput by providing the full signal range to the analog-to-digital and digital-to-analog converters (ADCs and DACs). The 'AD12 can be connected in a multichannel configuration with multiple lines through one DSP, saving cost and board space in the CO. These features are proving a solid basis for TI's development of ADSL, G.lite and other xDSL codecs that will provide equipment manufacturers with highly integrated solutions for a variety of standards. Low-Distortion Driver/Receivers Twisted-pair copper wiring running thousands of feet outdoors is not an ideal transmission medium. Signal attenuation and interference can be significant problems. For xDSL signals to be transmitted successfully, phone line interface functions must be capable of achieving high current drive with very low signal distortion. TI offers leading-edge line driver/receivers that provide these important operating characteristics. Development is continuing to provide more functionality and give customers greater design flexibility. TI's THS6002 dual differential line driver/receiver integrates two driver and two receiver amplifiers. This 140 MHz device is well suited to a variety of communication systems, especially ADSL applications such as CO switches and multiplexers. Other products include the THS6012, designed with high drive for CO equipment, and the THS6022, designed with lower drive for ADSL client modems and remote terminal applications. Each of these devices integrates two differential drivers. Complementary receiver products are in development. The key to the success of these line drivers is TI's high-voltage complementary bipolar (HVBiCOM) process, which provides the most advanced operating characteristics in the industry for communications line drivers. The HVBiCOM process provides full dielectric isolation, which permits higher bandwidth performance, smaller die sizes and faster slew rates for lower distortion. To overcome signal attenuation in long telephone lines, the THS6002 and THS6012 produce an exceptionally high 500-mA output drive for full-rate downstream transmission. Line drivers and receivers must also have low distortion to overcome noise at low signal levels. A high slew rate minimizes distortion by allowing the output of a driver/receiver to follow more closely the rise and fall of the input signal. The high slew rate of the THS60xx family assures a total harmonic distortion (THD) of only -72 dB at 1 MHz -- the lowest THD in the industry. In all THS60xx devices, each amplifier has an independent power supply to minimize crosstalk. The devices can also operate in wide voltage range, from +/- 4.5 V to +/- 16 V, giving designers flexibility in different CO and client-side equipment. Low power dissipation is especially important in applications such as DSL access multiplexers (DSLAMs), where many ADSL cards may be producing heat in a confined environment. TI's exclusive PowerPAD™ package provides exceptional thermal management and saves space with surface mounting. The thermal pad on the underside of the PowerPAD package is in direct contact with the die to effectively dissipate heat away from the device. PowerPAD packages can dissipate more heat in less board space than larger heat sink or plug type thermal packages. Comprehensive xDSL Solutions xDSL technologies are important links between yesterday's narrowband circuit-switched network and tomorrow's broadband packet-switched network. TI technology is providing leadership in xDSL implementations, not only in the DSPs that enable high-speed signal processing, but also in the analog and mixed-signal products that connect DSPs with the real-world analog signals. TI is developing a comprehensive line of drivers, receivers and codecs that will complement the DSPs at the heart of DSL technology. These products will be key enablers as xDSL technology is deployed for delivery of future high-bandwidth services.
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