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.