Analog to Digital Converter

  
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Before examining the various analog to d igital (A-D) and digital to analog (D-A) conversion processes it is useful to review the properties of each type of representation; in particular this may help select the representation most suited to the problem at hand. An analog signal is a signal whose value varies continuously with time, its instantaneous amplitude itself varying continuously within a limited range.
Analog to Digital Converter - schematic

The simplest example is that of a sinusoidal signal Asin(w t + f ), whose instantaneous value covers all the values within the range (-A, +A). An analog signal may very often be expressed as a weighted sum of sinusoidal signals. The analog signal is a simple type of signal which can quite conveniently be transmitted, but its simplicity results in several drawbacks. It is sensitive to parasitic signals and its amplitude or phase can be distorted by the transmitting system. When it undergoes such operations as analog multiplication the accuracy with which the signal is known is often reduced. Moreover it is difficult to store an analog signal. On the other hand a digital signal usually appears as a series of symbols. Thus, in a binary system a signal consists of a series of numbers, each of which is 0 or 1, that may be given physical form by the absence or presence of pulses. It can be said that the signal is represented by a word of a given format that is of a given structure. The digital signal represents the value of a quantity at a specific instant. It is not a continuous signal and since the symbols making up this signal (usually numbers) can only vary by `steps` the value represented by the signal must, perforce, be discrete. These difference from the analog signal, which may appear to be obstacles, are largely compensated by the advantages gained by the digital representation. First of all a digital signal is far less...



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