In order to make analog to digital conversion we should convert the analog signal into digital by sampling and quantization processes ( Pulse Code Modulation) .
Sampling of Analog Signals
The principle underlying digital signal processing is that of sampling the analog signal Fig 4.2 illustrates in a conceptual form the process of obtaining samples of an analog signal. The switch shown closes periodically under the control of a periodic pulse signal (clock). The closure time of the switch, τ, is relatively short, and the samples obtained are stored (held) on the capacitor. The circuit of the following figure is known as a sample-and-hold (S/H) Circuit. As indicated, the S/H circuit consists of an analog switch that can be implemented by a MOSFET transmission gate. A storage capacitor, and (not shown) a buffer amplifier.
Between the sampling intervals-that is, during the hold intervals-the voltage level on the capacitor represents the signal sample we are after. Each of these voltage levels is then fed to the input of an A/D converter, which provides an N-bit binary number proportional to the value of signal sample. The fact that we can do our processing on a limited number of samples of an analog signal while ignoring the analog-signal details between samples is based on the shannon’s sampling theorem
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Fig 4.2 The
process of periodically sampling an analog signal (a) Sample-and-hold
(S/H) circuit The switch closes for a small part of time of every clock
period (T). (b) Input signal waveform. (c) Sampling signal (control
signal for the switch). (d) Out put signal (to be fed to A/D converter).
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