The sampling rate used to discretize a continuous signal is critical for the generation ofan accurate digital approximation. If the sampling rate is too low, distortions willoccur in the digital signal. Nyquist’s theorem states that the minimum sampling rateused,fs, should be at least twice the maximum frequency of the original signal topreserve all of the information of the analog signal. The Nyquist rate is calculated asfnyquist¼2fmax (10:2)wherefmaxis the highest frequency present in the analog signal. The Nyquist theoremtherefore states thatfs must be greater than or equal to 2fmaxto fully represent the560 CHAPTER 10 BIOSIGNAL PROCESSINGanalog signal by a digital sequence. Practically, sampling is usually done at five to tentimes the highest frequency,fmax.The second step in the A/D conversion process involves signal quantization.Quantizationis the process by which the continuous amplitudes of the discrete signalare digitized by a computer. In theory, the amplitudes of a continuous signal can beany of an infinite number of possibilities. This makes it impossible to store all thevalues, given the limited memory in computer chips. Quantization overcomes this byreducing the number of available amplitudes to a finite number of possibilities that thecomputer can handle.Since digitized samples are usually stored and analyzed as binary numbers oncomputers, every sample generated by the sampling process must be quantized.During quantization, the series of samples from the discretized sequence are transformed into binary numbers. The resolution of the A/D converter determines thenumber of bits that are available for storage. Typically, most A/D converters approximate the discrete samples with 8, 12, or 16 bits. If the number of bits is not sufficientlylarge, significant errors may be incurred in the digital approximation. Quantizationerrors occur when the sampled binary numbers are significantly different from theoriginal sample values.A/D converters are characterized by the number of bits used to generate adigital approximation. A quantizer withNbits is capable of representing a totalof 2Npossible amplitude values. Therefore, the resolution of an A/D converterincreases as the number of bits increases. A 16-bit A/D converter has betterresolution than an 8-bit A/D converter since it is capable of representing a total of65,536 amplitude levels, compared to 256 for the 8-bit converter. The resolutionof an A/D converter is determined by the voltage range of the input analog signaldivided by the numeric range (the possible number of amplitude values) of the A/Dconverter.
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