This paper gives an overview of Cognitive-Radio (CR) circuits and systems, that will enable the implementation of new technology paradigms such as software-defined electronics and Artificial Intelligence (AI) managed Internet-of-Things (IoT). A survey of the state of the art, trends and design challenges is presented from a top-down perspective – from system-level to circuit and chip implementation. As an application, special emphasis is put on analog/digital interfaces as one of the key building blocks in CR-based devices. Cutting-edge architectures – mostly based on Sigma-Delta Modulators (SDMs) – are discussed, as well as the best candidate circuit strategies to implement CR-based digitizers in deep nanometer CMOS.