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Two key concepts of pipelining and background offset trimming are applied to demonstrate a 13-b 40-MSamples/s CMOS analog-to-digital converter (ADC) based on the basic folding and interpolation architecture. Folding amplifier stages made of simple differential pairs are pipelined using distributed interstage track-and-holders. Background offset trimming implemented with a highly oversampling delta-sigma modulator enhances the resolution of the CMOS folders beyond 12 bits. The background offset trimming circuit continuously measures and adjusts the offsets of the folding amplifiers without interfering with the normal operation. The prototype system is further refined using subranging and digital correction, and exhibits a spurious-free dynamic range (SFDR) of 82 dB at 40 MSamples/s. The measured differential nonlinearity (DNL) and integral nonlinearity (INL) are about /spl plusmn/0.5 and /spl plusmn/2.0 LSB, respectively. The chip fabricated in 0.5-/spl mu/m CMOS occupies 8.7 mm/sup 2/ and consumes 800 mW at 5 V.