Continuous-Time Circuit Design
Even if they do not directly map on the concept of “sampled system” normally used to describe a ΣΔ ADC, continuous-time (CT) circuits are often employed to implement the loop filters of such ADCs. This is done by taking advantage of the periodic operation of the quantizer, which makes possible the mapping of discrete-time loop filters into CT equivalents . The main returns of this mapping effort are a lower power consumption of CT circuits  (especially at high sampling frequencies) and intrinsic anti-alias filtering .
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