Analog and Digital Instruments
About the analog and digital instruments
An analog instrument gives an output
which varies continuously as the quantity being measured changes. The
output can have an infinite number of values within the range that the
instrument is designed to measure. The deflection type of pressure
gauge a good example of an analog instrument. As the input value
changes, the pointer moves with a smooth continuous motion. Whilst the
pointer can therefore be in an infinite number of positions within its
range of movement, the number of different positions which the eye can
discriminate between is strictly limited, this discrimination being
dependent upon how large the scale is and how finely it is divided.
A digital instrument has an output which varies in discrete steps and
so can only have a finite number of values. A cam is attached to the
revolving body whose motion is being measured, and on each revolution
the cam opens and closes a switch. The switching operations are
counted by an electronic counter. This system can only count whole
revolutions and cannot discriminate any motion which is less than a
full revolution.
The distinction between analog and digital instruments has become
particularly important with the rapid growth in the application of
microcomputers to automatic control systems. Any digital computer
system, of which the microcomputer is but one example, performs its
computations in digital form. An instrument whose output is in digital
form is therefore particularly advantageous in such applications, as
it can be interfaced directly to the control computer. Analog
instruments must be interfaced to the microcomputer by an
analog-to-digital (AID) converter, which converts the analog output
signal from the instrument into an equivalent digital quantity which
can be read into the computer. This conversion has several
disadvantages. First, the A/D converter adds a significant cost to the
system. Secondly, a finite time is involved in the process of
converting an analog signal to a digital quantity, and this time can
be critical in the control of fast processes where the accuracy of
control depends on the speed of the controlling computer. Degrading
the speed of operation of the control computer by imposing a
requirement for A/D conversion thus impairs the accuracy by which the
process is controlled.
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