Switched-Capacitor Circuits: From Maxwell to the Internet of Things
Maxwell introduced the concept of the equivalent switched-capacitor resistance in Vol. 2 of his Treatise on Electricity and Magnetism in 1873. The concept laid dormant for almost a century until it became commercially viable by exploiting the switches, native capacitors, and operational amplifiers of MOS IC technology. CMOS switched-capacitor circuits have been used in high-volume data converters and signal processing ICs for nearly four decades, and are ubiquitous in modern RF transceiver circuits and emerging as a dominant design approach in CMOS bio-medical and internet of things circuits and systems, etc. This talk will begin with a brief history of SC circuits as applied to data converters, precision high-order filters, operational amplifiers, etc. Next, SC circuits are described for body-area-networks (BAN) that integrate multiple sensor nodes in the portable and wearable bio-medical systems that are revolutionizing healthcare. A typical BAN comprises several bio-signal and motion sensors and uses ultra-low-power short-haul radios in conjunction with nearby smart-phones or handheld devices (with GPS capabilities) to communicate via the internet with a doctor or other healthcare professional. Higher energy efficiency is critical to the development of feature-rich, wearable and reliable personal health monitoring systems. The amount of data transmitted to the smart-phone increases as more sensors are added to the BAN. Because the energy consumed for RF transmission is proportional to the data rate, it is advantageous to compress the bio-signal at the sensor prior to digitization and transmission. This energy-efficient paradigm is possible using compressed sensing—a sampling theory wherein a compressible signal can be acquired using only a few incoherent measurements. For ECG signals, for example, large compression factors are achievable which means similar reductions in energy consumption. SC circuits are having a huge impact on wireless communications. A major challenge is the RF power amplifier dissipates a large fraction of the total power of a transceiver because of its low efficiency. Despite more than two decades of extensive research, the challenge of on-chip RF Pas with high efficiency in digitalfriendly CMOS technologies has not been met. Switching PA topologies with relatively high efficiency have gained momentum, and relatively high output power is being delivered using power combining techniques. Supply regulation techniques have enabled higher efficiency when amplifying non-constant envelope modulated signals. The switched-capacitor RF power amplifier technique which meets many of the remaining challenges is described and some future directions are presented.