4. Imperfections due to active elements
The performance of a switched-capacitor filter depends critically on the quality of its components. Capacitors must be in precise, stable ratios, with only low-value parasitic capacitances. Amplifiers need to transfer charge as quickly as possible, which implies a short settling time and a very fast rise time. They must have a low offset voltage. Switches must have very low forward resistances and low parasitic capacitances. Finally, and most importantly, amplifiers must generate low noise and be able to reject external noise, especially from the power supply.
A detailed study of these various imperfections and the means used by manufacturers to remedy them is beyond the scope of this article. The interested reader may wish to consult the excellent review article by J. Allstot and W. Black
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Imperfections due to active elements
Implementation
Bibliography
Basic works
Analysis software
A number of analysis software packages are available, based on the principles of nodal analysis, to which refinements have been added to reduce calculation times and introduce the imperfections of active elements. Comparative analyses are available at
Examples of catalogued tours
Several manufacturers offer switched-capacitor biquadratic cells in the form of cataloged circuits. The best-known are the MF10 and LMF100 (National Semiconductor), containing two second-order biquadratic cells in a 20-pin package (DIP package)
Manufacturers
(non-exhaustive list)
Linear Technology
Maxim
National Semiconductor
Texas Instruments
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