Modeling and Analysis of Wideband Sigma-Delta Noise Shapers

Abstract: The continuous evolution of CMOS technologies towards deepsubmicron processes has provided new opportunities forintegration of mixed-signal systems on single monolithicintegrated circuits. In order to find the optimal usage of CMOStechnology and robustness against undesired mixed-signalcouplings, new approaches to CMOS front-end integration areneeded. The key aspect for such new architectures is theidentification of the optimal sampling devices and theirarchitecture. In this work the aim has been to utilizesigma-delta noise shapers for wideband applications, e.g. radiosystem applications having a digital baseband of 1 - 3 MHz. Thesigma-delta noise shaper uses oversampling to trade resolutionin time for resolution in amplitude. Until recently, technologyconstraints have limited the use tothe lower frequency ranges,e.g. audio applications. The sigma-delta noise shaper utilizesdigital signal processing extensively, which means thattolerances on the analog circuit blocks can be significantlyrelaxed and at the same time it is possible to benefit fromfast signal processing provided by digital deep submicron CMOStechnologies. The sigma-delta noise shaper is therefore anattractive candidate for wideband applications.In this work, appropriate sigma-delta modulator structureshave been modeled and analyzed for wideband applications.Simultaneous demands for high sample rate and low oversamplingratio within the technology constraints have been addressed.Furthermore, a low-distortion wideband sigma-delta noise shaperstructure is presented. A critical block of the sigma-deltamodulator is the sampling switch, as its nonlinearities aredirectly distorting the input signal. A set of samplingswitches have been analyzed in detail regarding dynamic andfrequency behavior. Process scaling towards smaller geometriesin deep submicron CMOS has also resulted in negative effects onthe performance. Substrate noise coupling has become a veryimportant limitation on the design of mixed-mode circuits indeep submicron CMOS today. A brief overview and some results onthis problem are also presented.