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Switched-Capacitor Filters: An Introduction

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AnandY
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Silicon area
AnandY   6/12/2013 5:22:50 AM
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In a SCF, capacitors and switches take the place of resistors...

High-value resistors can be implemented in very little silicon area. For example, a 1-Mohm resistor can be realized with a 1-pF capacitor switched at a clock rate of 1-MHz.

AnandY
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Capacitor ratios
AnandY   6/12/2013 5:16:08 AM
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So, for an SCF, the frequency response depends solely on the ratio of two capacitors, C and C1, and the switching-clock frequency, fclk.

@Jon, thanks for the very informative post. Capacitor ratios can take advantage of the superior matching of capacitances fabricated on silicon. Especially in monolithic form capacitor ratios are very robust against changes in temperature.

Jon Titus
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Switched-Cap Filter Applications?
Jon Titus   6/11/2013 5:41:48 PM
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You will find switched-capacitor filters (SCFs) in audio circuits, in anti-aliasing filters, and in tracking filters. The latter type of filter can track a signal by adjusting the filter's clock frequency to "tune" filter characteristics as a sampled signal changes frequency. You also can use SCFs in tone-detection circuits. Unlike a PLL circuit that requires external passive components (and trimming) to detect a tone, an SCF requires only a clock signal. SCFs also serve as notch filters.

Because the filtering occurs based on a "master" clock signal's frequency, you might consider an SCF is you want to scan a given bandwidth to obtain information within it at certain frequencies.  The SCFs would act like the front end of a spectrum analyzer.

Cabe Atwell
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app
Cabe Atwell   6/11/2013 5:20:43 PM
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Jon,

Can you suggest some applications for the filter you describe?

C

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