A highly digital, reconfigurable and voltage scalable SAR ADC

Micropower sensor networks have a broad range of applications which include military surveillance, environmental monitoring, chemical detection and more recently, medical monitoring systems. Each node of the sensor network requires energy efficient circuits powered off small batteries or harvested energy. In such systems, a single reconfigurable analog-to-digital converter (ADC) is needed to digitize a wide range of signals with varying bandwidth and resolution requirements. This thesis describes the design of an ADC whose power scales exponentially with resolution and linearly with frequency to maximize the system lifetime. The proposed ADC has reconfigurable resolution from 5 to 10-bits and a scalable sample rate from 0 to 1-MS/s. The successive approximation register (SAR) architecture was chosen for its highly digital nature which enables low voltage operation. The supply voltage can be scaled from 1V down to 0.4V such that the ADC maintains a constant energy efficiency across all modes of operation when normalized with respect to sample rate and resolution. A capacitive digital-to-analog converter (DAC) in a split capacitor topology with a sub-DAC is used to minimize the DAC power and area. Top plate switches are used to decouple the MSB capacitors as resolution is scaled to avoid parasitic loading of the DAC. The DAC capacitors are laid out in a common-centroid configuration with edge effects minimized at each resolution mode to improve matching. A fully dynamic latched comparator is used to avoid static bias currents. Power gating of the digital logic is used to reduce leakage power at low sample rates. Reconfigurability between single-ended or differential modes enables a power versus performance trade-off. Lastly, programmable sampling duration and internal bootstrapping is used to maintain sampling linearity at low voltages. The ADC has been submitted for fabrication in a low power 65nm digital CMOS process and simulation results are presented. Thesis Supervisor: Anantha P. Chandrakasan Title: Professor of Electrical Engineering Acknowledgments This thesis would not have been possible without the help and support of an endless list of colleagues, friends and family. Let me attempt to express my sincere gratitude here. First, I'd like to thank my supervisor, Professor Anantha Chandrakasan, for inspiring me to get excited about research. I am grateful for his guidance, support and insight. I would also like to thank Naveen Verma and Brian Ginsburg for answering all of my SAR-related questions, no matter how trivial. Thanks also goes to Denis, Joyce, Yogesh, Ersin, Masood, Payam, Fred, Courtney, Patrick, Vivienne and the rest of ananthagroup for helping me with technical things somewhere along the way, and also for making the lab a fun place to be. Also, I'd like to thank Margaret for keeping everything running smoothly. I'd like to acknowledge MOSIS for chip fabrication, and Cambridge Analog Technologies for graciously providing me with pads at the last minute, without which tape-out would not have happened. To my friends, who all remind me that work is just that, work. There is life outside school and I am grateful to be surrounded by such wonderful friends. Edward, Alan, Anita, Cindy, Hannah and Polly, thanks for making Toronto such a great place to come home to. Bryan, Andy and George, thanks for keeping me sane with all the ridiculousness that always seems to surround us. Oh, and how can I forget to thank Young Shin, Chester and Dave Campbell for many fun-filled late night discussions. To everyone I've met at MIT, thanks for making Boston a fun place to live. Finally, I would like to thank my family for their continued love and support.