A/D Converters

An analog-to-digital (A/D) converter is defined as a device that receives an analog input and supplies a digital output (usually binary or decimal number). In designing A/D conversion devices, several basic requirements must be taken into consideration:

Sampling Rate. This is the rate at which the readings of the input signal value are taken. Clearly, by increasing the sampling rate, one can obtain a better approximation to the waveform of the original signal which is being digitized. The sampling rate is thus a conversion characteristic of primary importance.

Resolution. Resolution determines the precision of conversion at each interval. The precision of conversion is, in turn, determined by the accuracy or fineness of quantization in the A/D conversion process. If a signal value is quantized by 7 bits (binary digits) the precision of conversion would be 1/2⁷=1/128; and if a signal value is quantized by 10 bits, the precision of conversion would be 1/2¹⁰=1/1024. Thus, resolution is a function of the number of bits in the binary code which is used in the conversion device. For instance, a resolution of one part in 1000 would require 10 bits in the binary code.

Conversion Time. The conversion time is defined as the time interval between the instant at which an input signal to be converted is applied to the conversion device and the instant at which the output signal has been established within desired accuracy. In other words, it is the time required to complete one converting process. The conversion time is related to the sampling rate and must be much smaller than the sampling period. The frequency of conversion, defined as the reciprocal of conversion time, may be likened to the bandwidth of an analog system. This is readily seen by noting that as the frequency of the input signal to the conversion device increases, the accuracy of the output decreases, if the conversion frequency is kept constant. The conversion frequency of an A/D converter is related to the time constants of the conversion device in much the same manner as the bandwidth of an analog system is related to the various time constants of the system.

Several commonly used methods of performing analog-to-digital conversion are explained in the next sections.