Analog to Digital Conversion (A/D)

About the conversion from analog to digital

Many computer inputs, particularly those from transducers within intelligent instruments, consist of analog signals which must be converted to a digital form before they can be accepted by the computer. This conversion is performed by an analog-to-digital conversion circuit within the computer interface.


Important factors in the design of an analog-to-digital converter are the speed of conversion and the number of digital bits used to represent the analog signal level. The minimum number of bits used in analog-to-digital converters is eight, which means that the analog signal can be represented to a resolution of 1 part in 256 if the input signal is carefully scaled to make full use of the converter range. It is more common, however, to use either 10 bit or 12 bit analog-to-digital converters which give resolutions respectively of 1 part in 1024 and 1 part in 4096. Several types of analog-to-digital converter exist; they differ in the technique used to effect signal conversion, in operational speed and in cost.


The simplest type of analog-to-digital converter is the counter analog-to-digital converter. This, like most types of analog-to-digital converter, does not convert continuously, but in a stop-start mode triggered by special signals on the computer's control bus. At the start of each conversion cycle, the counter is set to zero. The digital counter value is converted to an analog signal by a digital-to-analog converter, and a comparator then compares this analog counter value with the unknown analog signal. The output of the comparator forms one of the inputs to an AND logic gate. The other input to the AND gate is a sequence of clock pulses. The comparator acts as a switch which can turn on and off the passage of pulses from the clock through the AND gate. The output of the AND gate is connected to the input of the digital counter. Following reset of the counter at the start of the conversion cycle, clock pulses are applied continuously to the counter through the AND gate, and the analog signal at the output of the digital-to-analog converter gradually increases in magnitude. At some point in time, this analogue signal becomes equal in magnitude to the unknown signal at the input to the comparator. The output of the comparator changes state in consequence, closing the AND gate and stopping further increments of the counter. At this point in time, the value held in the counter is a digital representation of the level of the unknown analog signal.


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