A JSSC Classic Paper:

Switched Capacitor Filters Grew the
Communications Industry

Filtering circuits, the most ubiquitous of electronic signal-processing functions, made for spirited discussions as analog-to-digital technology solutions unfolded. Bedrich Hosticka's 1978 paper on a MOS switched-capacitor filter marked the beginning of the dominance of switched-capacitor techniques for integrated audio-band filtering. In the ISSCC 40th anniversary Commemorative Supplement in 1993, Robert W. Brodersen remembered that in the year following Hosticka's paper two papers at ISSCC 1979 demonstrated the manufacturability of the approach in high-volume applications. The circuit served as part of an icon of the era of IC telecom, when a chip plot of 30 poles of switched-capacitor filtering by B.J. White, G. M. Jacobs, and G.F.Landsburg hung in the New York Museum of Modern Art for a number of years.

Published in the December 1977 JSSC, "MOS-sampled data recursive filters using switched-capacitor integrators" by Bedrich Hosticka, Robert Brodersen, and Paul Gray, University of California, Berkeley, California, is the fourth most frequently cited article ever published in the JSSC. It has been cited more than 150 times according the Journal Citation Report 2000 Science Edition. The concepts were presented first in February of the same year at ISSCC.

Unknown to Hosticka and his coauthors in 1978, James Clerk Maxwell had explained the equivalence of a periodically switched capacitor to a resistor in A Treatise on Electricity and Magnetism, Oxford, The Clarendon Press, pp. 420-421, 1892. The opportunity for practical realization of filters came only with the development a zero-offset electronic switch and an infinite input impedance electronic amplifier. The MOS transistor was the first device to meet both requirements, 80 years later. Professor Yannis Tsividis at Columbia found this early prior art.

Prof. David Hodges of UC Berkeley, a co-recipient of the 1983 IEEE Morris Liebmann Technical Field Award for switched capacitor filters, notes: "SC filters found effective use only below 1 MHz analog. In that range, due to the progress in digital technology, digital filters used with delta-sigma A/D converters have almost entirely superseded SC filters for high-order functions.

Switched-C filters still are used for simple filter functions that can't justify the overhead associated with DSP filters, e.g. some toys. L-C filters still are used for very simple filters, such as the signal splitter that separates the DSL channel from the voice/fax channel on a home telephone line - excellent surge immunity, zero power consumption!"

As so frequently happens in the industry, a new technology emerged, had major impact, and then was superseded by subsequent advances and requirements.

The original paper is available on the Solid-State Circuits Digital Archive 2002 DVD. See page 13. (IEEE product # JD3755B)