C.F.T. Soares and A. Petraglia (Brazil)
Switched-capacitor filters, capacitance matching.
A critical issue in the design of SC filters is the capacitance matching, because the filter coefficients depend on capac itance ratios. In order to achieve a better matching, each capacitor is implemented as a parallel arrangement of unit capacitors laid out in a common centroid geometry. This procedure can be directly applied if the filter coefficients can be written as rational numbers. This paper presents a systematic procedure to approximate the filter coefficients by integer ratios without causing significant errors in the filter frequency response, while keeping the total number of unit capacitors small in order to save die area. This pro cedure is applied in a design example of a SC band-pass filter, using a 0.35 µm CMOS technology.
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