Tham khảo tài liệu 'biosignal and biomedical image processing phần 2', kỹ thuật - công nghệ, kĩ thuật viễn thông phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 22 Chapter 1 Figure Block diagram of an analog-to-digital converter. The input analog voltage is compared with the output of a digital-to-analog converter. When the two voltages match the number held in the binary buffer is equivalent to the input voltage with the resolution of the converter. Different strategies can be used to adjust the contents of the binary buffer to attain a match. proportional voltage VDAC. This DAC voltage VDAC is then compared to the input voltage and the binary number in the buffer is adjusted until the desired level of match between VDAC and vin is obtained. This approach begs the question How are DAC s constructed In fact DAC s are relatively easy to construct using a simple ladder network and the principal of current superposition. The controller adjusts the binary number based on whether or not the comparator finds the voltage out of the DAC VDAC to be greater or less than the input voltage vin. One simple adjustment strategy is to increase the binary number by one each cycle if VDAC vin or decrease it otherwise. This so-called tracking ADC is very fast when vin changes slowly but can take many cycles when vin changes abruptly Figure . Not only can the conversion time be quite long but it is variable since it depends on the dynamics of the input signal. This strategy would not easily allow for sampling an analog signal at a fixed rate due to the variability in conversion time. An alternative strategy termed successive approximation allows the conversion to be done at a fixed rate and is well-suited to digital technology. The successive approximation strategy always takes the same number of cycles irrespective of the input voltage. In the first cycle the controller sets the most significant bit MSB of the buffer to 1 all others are cleared. This binary number is half the maximum possible value which occurs when all the bits are Copyright Marcel Dekker Inc. All rights reserved. Marcel Dekker Inc. 270 Madison Avenue New York New
