Continuous-time analogue signals entering conventional digital signal processors must first be sampled and quantised. But is this necessary? A team at Columbia University believes that maybe it is not. It is investigating systems that quantise but do not sample inputs, and believes the resulting readings could be processed directly with corresponding continuous-time digital hardware. The technique promises to eliminate aliasing that is introduced by traditional techniques and reduce the number of in-band quantisation errors.rnThe idea of using continuous-time digital circuitry has some history. One form, the continuous-time sigma-delta converter was first floated in a research paper some 15 years ago. But National Semiconductor has just started supplying samples of the first commercial implementation.rnimmunity to electrostatic discharge as crucial to its development.rnNational has stuck with the 130nm process from TSMC as the basis for the ADC 12EU050 that Xignal chose originally for its first part. In principle, the technology should port down to 90 or 65nm processes more easily than pipelined ADCs. Napolitano said the sigma-delta architecture is not a direct replacement for its pipelined converters despite the power savings.rn"The key market is ultrasound imaging: most medical with some non-medical applications," says Napolitano. "They are mostly going to portable and smaller systems, so they require long battery lifetime and the ability to squeeze more into a smaller space."
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