Sigma-delta (S?) analog-to-digital converters (ADCs) have traditionally been used for data conversion in low-speed applications such as weigh scales, temperature controllers and pressure monitors. But a new breed of S? ADCs is carving out another niche. Known as continuous time S?, these converters are serving in applications where high dynamic range and wide bandwidth are needed. Applications include portable ultrasound systems, wireless base stations and industrial instruments, among others. To be sure, engineers found solutions for those challenges in the past. The problem was, however, that those earlier solutions required the addition of extra components, such as anti-aliasing filters, that took up board space. The continuous time S? ADCs don't do that. "We can now save the customer from needing to use a discrete low-pass filter in front of the ADC," says Scott Kulchycki, product marketing manager for mixed-signal products at National Semiconductor, which pioneered the technology in early 2008. "This is a huge benefit, especially in portable applications where you're trying to achieve smaller size."
National's Sigma-Delta Continuous Time Targets Portable Ultrasound National Semiconductor's ADC12EU050 is an eight-channel, 12-bit, 50-megasample per second ADC that offers an alias-free sample bandwidth up to 25 MHz, while consuming 30% less power (350 mW) than competing pipeline devices. The device features 68 dB of signal-to-noise-and-distortion (SINAD) and a signal-to-noise ratio of 70 dB full scale (dBFS). Operating from a 1.2V supply, it consumes just 44 mW per channel at 50 megasamples per second. National claims that the low-power nature of the device enables manufacturers to extend battery life and reduce heat in portable medical ultrasound applications and industrial imaging equipment.
Analog Devices Targets Wireless Infrastructure, Medical Instruments Analog Devices (ADI) claims that its AD9261, AD9262 and AD9267 are the lowest noise, widest bandwidth family of continuous time S? ADCs available today. ADI engineers say they deliberately targeted the product at a gap that exists between pipeline and SAR ADC architectures. "The SARs have a problem achieving the bandwidth and the pipelines have the bandwidth but have a hard time achieving the noise performance," Sharma says. "We've designed this part to serve right in the middle of that product gap." The new ADC family's combination of speed, accuracy and bandwidth are said to be ideal for wireless infrastructure (such as base stations), medical devices and industrial instruments.
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