I. Introduction
For four decades the evolution of integrated circuits has followed Moore's law, according to which the number of transistors per square micron millimeter doubles every 18 months. At the same time, this scaling towards deep submicron CMOS technologies provides faster transistors. Therefore, the processing power of digital circuits increases every generation. For analog circuits, this evolution of technology scaling is not as beneficial. That's why there is a trend to move signal processing from the analog into the digital domain. This leads to a higher level of accuracy, savings in power consumption and area, increases robustness and brings flexibility and programmability.