Optimizing Capacitance Ratio Assignment for Low-Sensitivity SC Filter Implementation

Accurate capacitance matching is essential for switched-capacitor filters implementation because the filter coefficients depend upon the capacitance ratios. The use of identical unit capacitors in parallel to form larger capacitances and careful layout design can provide, in many cases, an accuracy of 0.1%. Unfortunately, this technique can be directly applied only if the filter coefficients are rational numbers. In general, coefficient approximations are required, leading to frequency response errors. In this paper, a new design method, using a genetic algorithm, is proposed to find the optimum capacitance ratio approximations by rational numbers which minimize the total number of unit capacitors for a given acceptable frequency response error, in order to save die area.

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