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Big Pay-off for Small Trade-Off

AC Performance Characteristics of the LTC2248
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Milpitas, Calif. — A new family of high-speed analog-to-digital converters (A/D converters) from Linear Technology combines outstanding AC performance with extremely low power, making them ideal for a range of wireless and cable communications systems.

The LTC2220 family includes A/D converters ranging from 10 mega-samples per second (MS/sec) up to 170 MS/sec at 12-bit and 10-bit resolution, as well as 14-bit converters up to 80 MS/sec. This new A/D converter family complements Linear's previously announced LTC1750 family of 5-V A/D converters, which achieves unmatched AC performance.

The LTC2220 family of 3.3-V converters feature significantly lower power than competing devices without sacrificing AC performance. These low power A/D converters excel at undersampling, making them well suited for communications applications such as WCDMA cellular basestation transceivers, digital predistortion power amplifier linearization, and cable modem termination systems.

“With the introduction of this extensive family of high speed, low power ADCs, Linear Technology has demonstrated its commitment to this key product area,” stated Bob Reay, vice president and general manager of Linear Technology's Mixed Signal Business Unit. “We have raised the performance bar by providing our customers with a broad family of ADCs that delivers both outstanding AC performance and low power.”

The LTC2224, a 12-bit 135 MS/sec A/D converter, exemplifies the industry-leading performance of the entire family. At just 630 mW, in a 7mm x 7mm QFN package, it has the lowest power at this speed, is almost half the size of its nearest competitor, yet achieves over 67 dB signal-to-noise (SNR) up to 170-MHz input — better than even higher-powered competitors. Further, the system footprint is small because few external bypasses are needed. Ideal for low power basestation designs and optimized for undersampling, the LTC2224 features a 775-MHz bandwidth and achieves 77-dB spurious-free dynamic range (SFDR) up to 250-MHz input. Pin-compatible versions at sample rates of 105 MS/sec and 80 MS/sec feature even lower power. Pin-compatible 10-bit versions at each speed are also in full production.

The entire LTC2220 A/D converter family, which includes 24 devices, will be introduced by the fourth calendar quarter. Linear's new A/D converter family features industry-leading AC performance at every sample rate, the lowest power of any such A/D converters in the industry, and pin-compatibility, allowing easy migration from 10-bit to 12- or 14-bits, or to different sample rates. In addition to the LTC2224, Linear is also announcing today five additional high-performance, low-power A/D converters — LTC2222, LTC2223, LTC2234, LTC2232 and LTC2233. All are now in production and are packaged in 7mm x 7mm QFN packages.

Summary of Features: LTC2220 Family

* Sample Rate: 135 Msps/105 Msps/80 Msps

* 68dB SNR up to 140MHz Input

* 80dB SFDR up to 170MHz Input

* 775 MHz Full Power Bandwidth S/H

* Tiny Board Footprint

* Single 3.3-V Supply

* Low Power Dissipation: 630 mW/475 mW/366 mW

* Selectable Input Ranges: +mn0.5 V or ±1 V

* No Missing Codes

* Optional Clock Duty Cycle Stabilizer

* Shutdown and Nap Modes

* Data Ready Output Clock

* Pin Compatible Family
135 Msps: LTC2224 (12-Bit), LTC2234 (10-Bit)
105 Msps: LTC2222 (12-Bit), LTC2232 (10-Bit)
80 Msps: LTC2223 (12-Bit), LTC2233 (10-Bit)

Pricing for the 3-V devices ranges between $6.70 for the LTC2233, 10-bit device and $33.92 for the LTC2224, 12-bit device in 1,000-piece quantities. For an overview of the entire LTC2220 family, including pricing, availability and data sheets visit: www.linear.com

Typically, lowering power consumption results in a dramatic drop in AC performance — but it's expected and it's a small price to pay, said Todd Nelson, product marketing manager for Linear Technology's mixed signal products. However, in Linear's case, AC performance degradation is hardly noticeable in the company's latest high-speed A/D converter family.

Looking at Linear's AC performance characteristics, primarily the SNR and SFDR, which are generally traded off with power, there was only a slight reduction in AC performance compared to a three or four fold reduction in power (see slide). When comparing the LTC2248 A/D converter in Linear's newer 3-V A/D converter line to its older 5-V A/D converter (LTC1750), the SNR dipped from 75.8 dB to 73.5 dB and the SFDR actually improved, according to Nelson.

Linear was also able to deliver more dramatic undersampling to its customers. A typical input frequency for cellular basestation applications is 70 MHz. Therefore, the Nyquist theory would dictate sampling at 140 MS/sec. Most customers, however, sample at 80 MHz, which is considered normal in this market, Nelson said. But, what customers in the basestation and other communications markets have always wanted is more dramatic undersampling, he said. They want the input frequency to go up — perhaps to 150 MHz or 250 MHz while the sampling rate stays relatively the same. “Our sample and hold can accept input frequencies up to 575 MHz without inducing any distortion, while most competing products in this range can only accept frequencies up to 250 MHz or 400 MHz,” Nelson explained.

Reducing the number of external power supply bypass capacitors, the required size of the external power supply bypass capacitors, and reducing the value of external reference capacitors so smaller package sizes could be used, enabled the company to cut its package size (7mm x 7mm QFN package) in half. Linear achieved this by incorporating the bypass function on the chip itself and designing the reference circuitry to be stable with low noise with smaller capacitors.

While reducing the number of external capacitors from as many as 20 down to 8 hasn't really affected Linear's package size, it affects the customer's solution size on the circuit board, Nelson said.

Linear is in the process of developing different speeds and resolutions to address different applications. The company plans to develop dual-version A/D converters as well. The company's LTC2220 3-V A/D converters are single-version devices.

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