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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