Modeling Device Mismatch: Miles Copeland's Vision

The mid-1970s is widely regarded as the time when analog metal?oxide?semiconductor (MOS) circuits saw fundamental breakthroughs. Until then, almost all analog circuits were being built using bipolar transistors. Researchers at the University of California, Berkeley, showed the realization of data converter circuits using MOS capacitors [1], which was quickly followed by building pulse code modulation codecs that would eventually be incorporated into every telephone [2]. The next functional block that needed integration for voice telephony was filters for anti-aliasing and reconstruction purposes. In addition to Berkeley [3], a lesser-known center of excellence in Canada led the way. The Canadian team consisting of researchers from Carleton University (doctoral student C.F. Rahim) and Northern Telecom, both based in Ottawa, Canada, was headed by Prof. Miles A. Copeland [4]. They showed that precision analog filters could be realized by using a switched capacitor as a resistor equivalent. The accuracy of the filter is then decided by the matching of capacitors and not by the absolute value of any element. Thus the MOS capacitor became the precision element used in the data converters and filters. These inventions helped establish Northern Telecom as the leader in digital switching products for telephony.