SAR Converter with Muscle Offers Speed and Accuracy

Norwood, MA — Analog Devices Inc., a global provider in high-performance semiconductors for signal processing applications and the market leader in data converters, today introduced a 16-bit SAR (successive-approximation register) ADC (analog-to-digital converter) that provides design engineers with 50 percent more speed and more than twice the accuracy of competing products, all in packaging that is 40 percent smaller than the competition.

The new SAR converter, the AD7621, operates at 3 MSPS (mega samples per second) while providing one LSB (lease significant bit) INL (integral non-linearity) and DNL (differential non-linearity). In addition to delivering the highest combination of speed and accuracy, the AD7621 enables lower power consumption and better INL performance with no missing codes. The AD7621 also reduces the price per channel by allowing engineers to design systems that multiplex to a greater number of channels. A general purpose ADC, the AD7621 is suitable for applications for which resolution is a critical performance requirement, such as high-end data acquisition, CT scanners, spectrum analyzers, ATE (automatic test equipment) and general purpose test equipment.

The AD7621 joins Analog Devices' industry leading PulSAR family of SAR converters, which is based on an ADC architecture that provides the added advantage of zero data latency, a critical factor in data acquisition systems. SAR converters represent the majority of the ADC market and are often the most cost-effective technology for applications that require very precise digital modeling of analog signals. Other PulSAR family converters include the AD7677, a 16-bit, 1 MSPS ADC, and the AD7674, an 18-bit, 800 kSPS (kilo samples per second) ADC.

“When designers are considering SAR converters, speed alone is not the only performance criteria,” said Mike Britchfield, product line director, precision converters, Analog Devices. “Our competition has compromised accuracy for speed. With the AD7621 there are no compromises” for Analog Devices products, high-performance must mean speed and accuracy, combined with low power consumption, standard packaging and a competitive price.”

The AD7621 is available in a 7 mm x 7 mm 48-lead LQFP (low profile quad flat pack) or a 7 mm x 7 mm 48-lead LFCSP (lead frame chip scale) package. True 16-bit accuracy provides clear imaging for scanner applications. The device features three different conversion rate modes to optimize performance for individual applications.

The AD7621 features low power consumption with typical power dissipation of 100 mW. It can be operated from a single 5-V supply and interfaced to either 5-V or 3.3-V digital logic. The SAR does not exhibit any pipeline delay, making it ideal for multiple multiplexed channel applications. The AD7621 offers designers 16 bits of no missing codes, SNR (signal-to-noise ratio) of 90dB, and pin-for-pin compatibility with the Analog Devices' AD7674, an 18-bit, 800 kSPS SAR ADC. The AD7621 features include an internal conversion clock, an internal reference buffer, error correction circuits and both serial and parallel interface ports. To bridge the gap for many high-performance data acquisition applications, the AD7621 can be paired with several of Analog Devices' operational amplifiers, including the AD8021, a custom-compensated amplifier that solves the gain/bandwidth performance challenge.

Analog Devices is a manufacturer of precision high-performance integrated circuits used in analog and digital signal processing applications. The company is headquartered in Norwood, Massachusetts, and employs approximately 8,450 people worldwide. It has manufacturing facilities in Massachusetts, California, North Carolina, Ireland, the Philippines and the United Kingdom.

Analog Devices, Inc., Ray Stata Technology Center, 804 Woburn Street, Wilmington, MA 01887. Tel: 800-ANALOGD (800-262-5643)

In the world of competitive specs and bragging rights for the best in a category, it is similar to a game of leap frog where the leader is only the leader until the lagging frog becomes the leader again with a new leap into the limelight. And so it goes with Successive Approximation Register (SAR) converters and this latest leap by Analog Devices into the impressive but momentary lead for the title of fastest SAR converter.

Realizing that fast and furious is not the only spec that keeps customers coming back; ADI pushed its process technology to also provide accuracy. The 7621 not only has speed but accuracy. For example, the DC accuracy or the integral nonlinearity (INL) and differential nonlinearity (DNL) error is typically +/-1 LSB. ADI also says it guarantees the SNR at a typical 90 dB, which is outstanding for an A/D converter, and guarantees 88 dB in the industrial temperature range of -40 to +85 C. The 7621 is 50% faster and 3.5x more accurate than competitive products currently on the converter landscape. The message to the market is that ADI is not sacrificing the accuracy to increase the speed to 3MSPS.

The 7621 uses a technique called charge redistribution that uses binary-weighted capacitors. It uses an array of capacitors and each successive cap has half the capacitance of the previous device to divide-down the voltage. This helps capture a charge across the capacitor array then methodically moves through the caps and switches them to the reference voltage, which stores the charge (bit one), or the cap is grounded and discharged, which translates to a bit zero. This sequence approximates all 16 bits and the only delay is the time it takes to do a conversion. An advantage of the charge redistribution technique is that it doubles as a sample and hold amplifier and as an A/D converter. So when you flip the switch from sample to hold on the converter it is disconnected from the analog input and the converter is not affected by the changing analog input.

Compared to other converter technologies like sigma-delta, the SAR architecture is much better at measuring discrete versus continuous signals. For example, the sigma-delta is perfect for weigh-scales, temperature measurements, pressure and flow meters, and any sensor measurement that has continuous almost DC-like signals. The SAR converter, however, works best with inputs like sinusoidal waveforms and noise peaks, or any signal riding on a sine wave for which you want to capture a signal in a specific point in time.

The SAR A/D converter is not as accurate as the sigma delta and not as fast as the pipeline converter but it gives you a nice blend of both. It provides 16-bits of accuracy and 3 MSPS speed. There is a 5 MSPS sigma delta converter on the market with 16-bit accuracy but if you look at the DC performance it is only typical and not guaranteed. The AC performance or signal to noise ratio is poor and that makes it similar to a converter with12-bit accuracy rather than the 16-bits indicated in the data sheet.

ADI expects manufacturers of MRI equipment, medical instrumentation, and CT imaging products to require hundreds to thousands of channels, which all need A/D converters. Now these customers can put a 3 MSPS part where previously they used a 500 kSPS or 1 MSPS part. That means the customer can will see a 3x to 6x improvement in speed with fewer ADCs per channel and a subsequent reduction in cost-per-channel. Instrumentation manufacturers will also want to consider these converters for similar reasons as medical equipment makers.

This is a significant introduction in the world of converters and it will do well in the market but I do hope that ADI backs up its spec claims and fills in all the performance characteristic graphs on the data sheet.

The AD7621 is sampling now and will be in full production in June 2004. The device is priced at $29.95 per unit in 1,000-piece quantities.

For more information, please visit AD7621.

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