Sampling is the basis of A/D conversion. A continuous time signal(analog) is consisted of infinite data points and it is impossible to process infinite data points. So, A/D converter sample enough points to represent the continuous signal and form discrete signal(Digital).
Very simple model for sampling
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References
^ Walt Kester (2003). Mixed-signal and DSP design techniques. Newnes. p. 20. ISBN 9780750676113. ^ Hiroshi Harada, Ramjee Prasad (2002). Simulation and Software Radio for Mobile Communications. Artech House. ISBN 1580530443. ^ Angelo Ricotta. "Undersampling SODAR Signals".
