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Unformatted text preview: 1122 IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. SC-20, NO. 6, DECEMBER 1985 A High-Performance Micropower Switched-Capacitor Filter RINALDO CASTELLO AND PAUL R. GRAY, FELLOW, IEEE ,4mtract MOS technology scaling requires the use of lower supply voltages. Analog circuits operating from a low supply and achieving a sufficiently farge dynamic range must be designed if analogidigital inter- faces are to be implemented in scaled technologies. This paper describes a high-performance fifth-order low-pass switched-capacitor filter operating from a single 5-V supply. The filter uses a fully differential topology combined with input-to-output class A B amplifier design, dynamic biasing, and switched-capacitor common-mode feedback (CMFB). An experimental prototype fabricated in a 5- p m CMOS technology requires orlly 350 pW of power to meet the PCM channel filter requirements. Typical measured results are a dynamic range of 92 dB, a supply rejection (PSRR) of 40 dB over the entire Nyquist range, and a total harmonic distortion (THD) of 73 dB for a 2-V rms dlfferentiaf output signal. The chip active area is about 3900 milz. I. INTRODUCTION T HE PERFORMANCE of switched-capacitor filters has steadily improved during the last several years, primarily as a result of improvements in the performance of CMOS operational amplifiers. This improvement has been particularly evident in the PCM channel filter appli- cation . However, the most recent commercial PCM filter implementations still require a power-per-pole of about 1 mW and operate from a ~ 5 V supply. It has recently been shown ,  that, from a fundamental standpoint, the absolute minimum achievable power dis- sipation in a voice-band filter with a dynamic range of 90 dB in a 3-pm technology operated from a +-5 V supply is less than 1 pW per pole. A large margin for improvement in power consumption over existing filter designs is there- fore possible in principle. The realization of such a reduc- tion, while maintaining high-performance levels, would have important implications in the realization of battery- operated analog/digital interfaces. A second important consideration in the realization of switched-capacitor filters is the fact that the technological scaling of the mainstream MOS technologies dictates the use of lower power-supply voltages . This fact, and the need for an analog/digital compatible technology, create a strong motivation for developing new analog circuit tech- niques suitable for low-voltage operation. Recently several circuit approaches to the implementa- tion of low-power MOS switched-capacitor filters have Manuscript received October 25, 1985; revised August 1, 1985. This work was supported by NSG Grants ECS-8023872 and ECS-81OOO12, The authors are with the Department of Electrical Engineering and Computer Science, University of California, Berkeley, Berkeley, CA 94720....
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