High-speed low-voltage and ultra low-voltage SAR ADC

In this thesis work several techniques to optimize SAR (Successive Approximation Register) ADC (Analog-to-Digital Converter) performances have been investigated. The thesis work has been focused on two vertical topics: high speed SAR ADC and LV (Low-Voltage) and ULV (Ultra-Low-Voltage) ADC. The structure of the work is based on the research activity carried out in the three years of the PhD. The first activity that was carried out is the design of an HS (High-Speed) SAR ADC that has been realized in foundry. Subsequently, much of the activity has been focused on the development of innovative techniques and topologies aimed at the optimization of LV and ULV dynamic comparators, then a complete ULV ADC has been realized with one of this comparator, finally a wide band LPF (Low-Pass Filter) for analog front-end of HS ADC has been realized. The document is structured as follows: The first chapter introduces the work describing the context and the state of the art of the ADC in general and of the SAR ADC in particular. The chapter reports the aims of this thesis and describes the content of all the chapters. The second chapter contains a theoretical introduction to the ADC in general, reporting the working principles, applications, main characteristics and definitions. The chapter deepens the analysis of the SAR ADC, describing the building blocks and in particular the dynamic comparators. In the third chapter of the thesis, a 166 MS/s 10 bit VDD = 1 V fully differential SAR ADC has been designed and tested. The design has been sent to the foundry to be realized within an integrated circuit. The ADC shows good performance in terms of accuracy despite its FOM (Figure Of Merit) is not competitive. The design of this SAR ADC allowed to validate an innovative switching algorithm used to optimize conventional Strongarm comparator performances. The fourth chapter presents a 6.67 MS/s 8 bit VDD = 0.4 V fully differential ULV SAR ADC. The chapter describes the ADC architectures and every building block, with particular emphasis on the dynamic comparator that is the most innovative element of the ADC. Finally reports the post-layout simulation results both for the comparator and the ADC, highlighting the competitive performances in terms of speed at a supply voltage as low as 0.4 V. The fifth chapter is dedicated to a survey of LV and ULV comparators. A series of innovative topologies and techniques to optimize the speed-consumption trade-off at Low and Ultra low supply voltage are presented in this chapter. Detailed theoretical models on the delay behavior of the Strongarm based ULV comparator are presented in this chapter. Particular attention was dedicated to the study of the comparator transistors operating in sub-threshold region and to the study of the CPB (Charge Pump-Based) dynamic biasing. All the presented comparators have been simulated (almost all with post-layout simulations) and the performances that have been evaluated are competitive with the state of the art. In the sixth chapter of the thesis a 17 GHz inductor-less LPF has been designed and tested, the LPF is an innovative topology based on a modification of conventional Sallen-key topology. This topology involves the internal poles of the amplifier in order to enlarge the bandwidth of the filter. The filter shows very competitive performance in terms of band, noise and linearity.