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An Asynchronous Sigma-Delta Modulator (ASDM) that achieves a SFDR of 72 dB with a bandwidth of 8 MHz is presented. The ASDM operates as a closed-loop, nonlinear system. Spectral analysis of the ASDM shows that a center frequency of at least 100 MHz is required for these specifications. Circuit implementations are presented and a prototype ASDM is realized in a digital 0.18 micrometer, 1.8V CMOS technology. The ASDM dissipates 1.5mW and has an area of 0.026mm2.