Switched-capacitor filters

Electrical filters can be highly complex circuits. Built using analog technologies, they generally require components with very precise values, and very stable values as a function of temperature and time. Accuracy better than 1% and stability better than 50 · 10 ^-6 /K are generally required. Even with such high performance, a final adjustment is often necessary to meet the requirements of the template. A priori, such constraints appear incompatible with integrated circuit production, depriving this type of circuit of the cost savings available in mass production. The digitization of telephone networks, which took place in most countries of the world towards the end of the 1970s, made it urgently necessary to overcome this difficulty, or risk making the cost of a digital subscriber telephone set prohibitive. Intense competition ensued to solve this problem. Three solutions were explored:

• digital filters (see ) ;

• integrated charge transfer filters (see ) ;

• switched-capacitor filters.

• High-performance digital filters require complex, and therefore relatively expensive, integrated circuits. What's more, they are very limited in frequency, have high power consumption and need to be combined with an auxiliary anti-aliasing filter. Finally, analog-to-digital and digital-to-analog converters are not easily integrated on the same chip as the processor, and in turn require a smoothing filter.

• Charge transfer filters, originally very promising, ultimately proved unsuitable for providing highly selective filters.

• Switched-capacitor filters, described below, seemed at first sight to be the least well placed, so numerous and insurmountable were the practical difficulties to be solved. Thanks to a fortunate combination of circumstances (or the genius of their creators?), these circuits immediately [8][9] met the requirements of the telephone channel filter: very low cost, low power consumption, fully integrated circuit. First used in mass production as early as 1980 [1][2][3] , this technique was only fully mastered on a theoretical level in 1983 [4][5] . Switched-capacitor...