TABLE OF CONTENTS This book is primarily about analog components and issues, but because ADCs and DACs are only half-digital devices we ought to say a few words about them.
Any time-dependent signal, such as a varying voltage or current, that is band limited (i.e., composed of frequencies within a restricted bandwidth) can be completely reconstructed from its value at a tiny number of discrete points, that is, from samples of the waveform. According to another theorem of the inevitable Dr. Nyquist, the frequency with which the samples must be acquired is twice the bandwidth of the signal. An example of a sampled signal is depicted in Figure 3.6. The rather complex analog signal is the sum of a signal at normalized frequency 1, another component at f = 2, a third at f = 5, and a final component at f = 7. We show the sampling operation taking place on each component separately, which is correct because sampling is a linear operation (a sample of ( A + B) = the sum of a sample of A and a sample of B). Because the highest frequency is 7, 14 samples are needed to define the signal. It is obvious that this is more than we need to determine the amplitude of the f =1 and f = 2 components but just enough to find the f = 7 component. Note that the theorem...
