Fundamentals and Recent Advances on Continuous-Time Sigma-Delta Modulators
DLP Series, sponsored by the Circuits and Systems Society
Jose Silva-Martinez
Texas A&M University

Abstract

Recent developments in mobile computing and wireless internet have led to exponential growth in demand for portable computers and smart phones equipped with WLAN operating at different standards. The digital computing required by these gadgets is facilitated by process scaling that follows Moore’s law and is expected to continue down to 10nm physical gate lengths. Various wireless standards have been developed over the years due to the high demand for faster data rate in portable wireless communications, which has pushed baseband bandwidths up to a few tens of MHz. When high-resolution continuous-time lowpass S? ADC architectures are selected for emerging products because of their efficiency, a wide bandwidth is essential in multi-standard applications to accommodate receiver bandwidth requirements. 
In this lecture, the fundamentals will be revised and limitations due to clock jitter and presence of strong blockers will be quantified; technology trends will be highlighted. A top-down design approach is followed, starting with system specifications down to design issues of main blocks. Properties of main architectures are fully described. Stability, linearity and power consumption issues in ?? Modulators are addressed. Also, out-of-band blocker effects on modulator’s SQNR and loop stability as well as loop saturation effects due to agile blockers, usually not properly discussed neither in books nor in scientific papers, are covered. Even though multi-bit architectures have been successfully utilized in multi-MHz bandwidth designs, significant research efforts are devoted to the find efficient solutions for the remaining issues: better linearity, wider bandwidth, robustness to clock jitter and co-existence with other standards. In particular, the feedback DAC nonlinearity significantly affects the ADC performance because it directly adds error to the filter input signal and it is not noise-shaped. The foundations on ?? modulators will be covered first and then we will elaborate on linearity limitations as well as jitter and blocker tolerance issues. Two case studies experimentally verified are presented to illustrate design issues and to give insights into the possibilities that exist for solving these contemporary challenges with analog hardware and software-based processing techniques.