Electronic Design

Tiny Footprints, Sweet Sounds, And Higher IFs

Two major trends will dominate digital-to-analog converters (DACs) this year. First, competition will intensify to even greater heights thanks to strides made in consumer audio in your home, car, and pocket. Second, DACs for communications systems, where they operate in the transmit chain, are virtually evolving into modulation subsystems while eliminating a few upconversion stages in the process.

AUDIO
As in other types of analog and mixed-signal development, suppliers are adding and consolidating functionality. This certainly makes life a bit easier for systems customers who can't find analog engineering talent.

Audio/video receivers (AVRs) and home-theater systems (plus high-end computer gaming systems) embody as many as eight separate audio output channels, with all sorts of combinations. (see the figure). High-end audio extends to automobiles as well.

There is an industry-wide push toward making life easier for OEMs. Late last year, Cirrus announced three multichannel surroundsound codecs. The codecs enable audio-system designers to add six or eight single-ended or six differential input sources, and six or eight single-ended or differential output sources. Each has eight DACs with a complete serial port that supports all standard audio data formats as well as time-division multiplexing mode.

PERSONAL SOUND
Maxim bypassed the multispeaker race in favor of DACs for personal media like cell phones, MP3 players, and portable DVD players. The company introduced a stereo audio DAC, with built-in headphone amplifiers, that operates from any 1.8- to 3.6-V single supply. It also delivers 30 mW of output power from a 1.8-V supply.

For professional audio applications like digital mixing consoles, digital effects processors, multitrack recorders, and broadcast studio equipment, Texas Instruments introduced the industry's first four-channel, 24-bit audio DAC. It boasts 118-dB dynamic range, 2100-dB total harmonic distortion, and sampling rates up to 200 kHz. Of particular importance for audio mixer boards and similar products is that all four channels together consume only about 200 mW of power.

COMMUNICATIONS
Toward the end of last year, Linear Technology introduced a direct-conversion I-Q modulator with output capabilities from 1.5 to 2.4 GHz. This effectively eliminated one or two stages of frequency upconversion in the transmit path.

Intended for W-CDMA, TD-SCDMA, GSM, and PHS cellular, broadband fixed wireless access, and point-to-point radio links, the modulator incorporates a matched pair of double-balanced mixers in which the I and Q inputs are directly upconverted to RF. It's also targeted at other high-performance radios, as well as 3G wireless basestations.

Also for comm applications, Analog Devices recently announced 10-, 12-, and 14-bit high-frequency DACs that provide sample rates of up to 1.2 Gsamples/s. Thus, multicarrier generation is possible up to their Nyquist frequency. They share a serial port interface (SPI) for programming many internal parameters and read-back of status registers. Output current can be programmed from 10 to 30 mA.

Taking its own all-in-one approach to making wireless-system design less painless, Intersil introduced a programmable wideband quadrature amplitude modulator/upconverter. It combines shaping and interpolation filters, a complex modulator, timing and carrier numerically controlled oscillators (NCOs), and dual DACs into a single package. The device accepts up to 16-bit I and Q samples to generate virtually any quadrature AM or PM modulation format.

An NCO interpolation filter allows the input and output sample rate to have a non-integer or variable relationship. This makes it easy to deal with sample rates that don't have harmonic or integer frequency relationships to the input data rate with SFDR of 70 dB at 100 Msamples/s for input sample rates as high as 25 Msamples/s. Digital output spectral purity exceeds 50 dB at the maximum output sample rate.

Another entrant comes from Texas Instruments. The ocmpany's low-power, dual, 12-bit, 40-Msample/s DAC is slated for use in the transmit section of wireless PC modem cards, wireless network cards, and handheld radios. Other applications for the DAC include portable test equipment, direct digital synthesis (DDS), portable medical instrumentation, and arbitrary waveform generators (AWG).

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