Micro-electronics and so integrated circuit design are heavily driven by technology scaling. The main engine of scaling is an increased system performance at reduced manufacturing cost (per system). In most systems digital circuits dominate with respect to die area and functional complexity. Digital building blocks take full advantage of reduced device geometries in terms of area, power per functionality, and switching speed. On the other hand, analog circuits rely not on the fast transition speed between a few discrete states but fairly on the actual shape of the transistor characteristic. Technology scaling continuously degrades these characteristics with respect to analog performance parameters like output resistance or intrinsic gain. Below the 100 nm technology node the design of analog and mixed-signal circuits becomes perceptibly more difficult. This is particularly true for low supply voltages near to 1V or below. The result is not only an increased design effort but also a growing power consumption. The area shrinks considerably less than predicted by the digital scaling factor. Obviously, both effects are contradictory to the original goal of scaling. However, digital circuits become faster, smaller, and less power hungry. The fast switching transitions reduce the susceptibility to noise, e.g. flicker noise in the transistors. There are also a few drawbacks like the generation of power supply noise or the lack of power supply rejection. Still, the advantages are overwhelming and suggest to implement as much system components as possible in the digital domain. As digital functionality nearly comes for free it usually makes no sense to optimize single gates or to elimination a few logic functions. In most cases the effort for these optimizations does not pay off. If however analog or mixed-signal performance can be increased by digital assist or enhancement techniques the effort should be spent. If digital circuit add-ons help to reduce analog power or improve robustness and reliability, the investment in these add-ons should be done. In the deep sub-micron regime a few thousand gates are not worth to be discussed. Very often the introduction of digital assist techniques or even all digital implementations cause a one time area and power overhead. The increased performance and new functionality should justify this expense. Moreover, the overhead will shrink in future technology generations so the full advantage of the increased performance and the robustness will pay off one or two generations later.
KeywordsAnalog Circuit High Dynamic Range Digital Circuit Sigma Delta Modulator Integrate Circuit Design
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