Replace four A/D converters with one tiny A/D converter

Milpitas, Calif. — Linear Technology Corp. is expanding its low-power, high-speed analog-to-digital converter offering with six new devices.

The showstopper of the group, which is aimed at cellular basestations, is a 125 Msamples/s, 14-bit converter (LTC2255) that boasts strong AC performance and extremely low power.

Outperforming its nearest 14-bit competitor, the A/D converter consumes 49 percent less power at just 395 mW, significantly lowering the power budget and thermal considerations required for multiple channel devices, said Todd Nelson, product marketing manager of Linear Technology's mixed-signal products. “This provides a significant advantage in applications where efficiency and cooling is critical, such as satellite receivers, wireless basestations and portable electronics,” he said.

The A/D converter comes in a 5mm x 5mm QFN package with integrated bypass capacitors — requiring only a small number of tiny external components. The device eliminates the need for large and costly decoupling capacitors, affording the smallest solution size available, which eases printed-circuit board space constraints and allows for more compact, cost effective designs. “With its small dimensions, low power and reduced external component requirement, designers can easily fit four of these A/D converters where just one competing solution would fit,” Nelson said.

The LTC2255 is aimed at 3G and emerging 4G technologies, Worldwide Interoperability for Microwave Access, Inc. (WiMAX) and other wideband wireless applications. In addition, the combination of high sampling rate, low-current and 14-bit resolution make it suited to battery powered, high performance test and instrumentation equipment.

The A/D converter offers exceptional low-level input signal performance due to its high linearity, and it is designed with good margin relative to the sample rate for reliable performance over a wide temperature range. At 125 Msamples/s sampling rate, it achieves excellent AC performance with 72.1dB signal-to-noise ratio (SNR) and 85dB spurious-free dynamic range (SFDR) at 70 MHz.

The LTC2255 is one of six new A/D converters that fall within Linear Technology's pin- compatible family of low-power, high-speed A/D converters — with higher sampling rates of 105 Msamples/s and 125 Msamples/s in resolutions of 10-, 12- and 14-bits. Each device is available in both commercial and industrial temperature grades.

Pricing is $49.00 for the LTC2255 (14-bit, 125 Msamples/s), $41.00 for the LTC2254 (14-bit, 105 Msamples/s), $27.50 for the LTC2253 (12-bit, 125 Msamples/s), $23.00 for the LTC2252 (12-bit, 105 Msamples/s), $12.00 for the LTC2251 (10-bit, 125 Msamples/s), and $7.50 for the LTC2250 (10-bit, 105 Msamples). Click here for the LTC2255/LTC2254 data sheets. Click here for the LTC2253/LTC2252 data sheets. Click here for the LTC2250/LTC2251 data sheets.

Linear Technology , 1-800-454-6327,

Linear Technology's new A/D converter family is similar to its older dual 14-bit A/D converter family (see Related Stories link below), which came out in January. “These parts offer solid performance, smaller size and lower power. The customer isn't giving up anything. We're just extending these features to A/D converters that operate at faster rates,” Nelson said.

Of the six new A/D converters, the company is most proud of its LTC2255 device. This one offers the highest resolution (14-bit) and fastest speed (125 Msamples/s) of the lot.

Most designers look at AC performance and power consumption when considering an A/D converter, Nelson said. Generally, one spec is more important than the other, depending on the application. While there may be competing devices with stronger AC performance, Nelson believes the LTC2255 offers the lowest power (about half of competing devices). “You may find similar A/D converters with very good dynamic range, but at just under 400 mW, our part will be the preferred choice,” he said.

Basestations are the biggest market for converters at this speed and resolution. “The power level and size (5mm x 5mm) of the LTC2255 offers a win/win solution for basestation designers,” Nelson said.

Cellular basestation units are getting smaller so designers need to pack more channels into a tighter space. Doing this generates additional heat and there is no room for airflow and heat sinks around the parts, which is why smaller parts that produce less heat are very enticing to basestation makers right now, Nelson said. Also, power supplies in the same unit have to be sized smaller so the power consumption of each product must be lower, he said.

In response to size constraints, Linear Technology integrated a lot of the capacitance necessary for supply and reference bypassing. “In addition, the internal reference is designed so it needs less capacitance as well,” Nelson said.

Adding capacitance takes up a lot of unnecessary space, which Linear Tech has minimized. “We're probably being conservative when we say that you can replace four A/D converters with one of ours,” Nelson added.

Last month, Analog Devices Inc. (Norwood, Mass.) unveiled a similar device. Linear Tech's new A/D converters are available now in volume. ADI's A/D converters won't be available in production quantities until September.

ADI's AD9445 is optimized to capture weak signals in noisy environments while simplifying basestation receiver designs. It provides SFDR above 80 dBc and a SNR of 72.5 dBfs at input frequencies up to 300 MHz.

Click here to read more about ADI’s AD9445 A/D converter.

Texas Instruments Inc. (Dallas) appears have been the first company on the scene with a 14-bit, 125 Msamples/s A/D converter in 2003! TI's ADS5500, with total power dissipation of 750 mW, featured 70-dB SNR and 82-dB SFDR at 100-MHz input frequency.

Click here to read more about TI’s ADS5500 A/D converter.

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