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This chapter is conceived as an introduction to CMOS Sigma-Delta () analog-to-digital converters (ADCs). Their operation principle consists in combining oversampling, quantization error processing, and negative feedback for improving the effective resolution of a coarse quantizer. Before presenting practical topologies for the implementation of modulation, the large variety of the existing realizations is briefly classified according to the type of modulator architecture (single loops or cascades), the circuit techniques employed (discrete time (DT) or continuous-time (CT)), and the nature of the signals being converted (low pass (LP) or band pass (BP)). Starting from the case of DT, LP, single-bit modulators, the implications of these different alternatives are then presented in an incremental way.