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EE Fundamentals -
Analog vs. Digital
Design Considerations
Since the rate
and flow of power and information in circuits or PCBA circuit assemblies
are
encoded differently in analogue and digital electronics, the way they
process a signal is developed and implemented differently.
The choice of the most appropriate Analogue or digital or a combination
of the two for circuit design and component
selection is really a matter of considerations of the advantages and
disadvantages of each approach. Operations that can be
performed on an analogue signal such as amplification, filtering,
limiting, and others, can also be duplicated in the digital domain. Following
is an overview and some design considerations of the two domains:
(See also EE Development Process)
Analogue
Electronics
Analogue electronics
are those electronic systems with a continuously variable signal. In
contrast, in digital electronics signals usually take only two different
levels. The term "analogue" describes the proportional relationship
between a signal and a voltage or current that represented the signal.
An analogue signal uses some property of the medium to convey the
signal's information. Electrical signals may represent information by
changing their voltage, current, frequency, or total charge. Information
is converted from some other physical form (such as sound, light,
temperature, pressure, position) to an electrical signal by a
transducer.
The signals take any
value from a given range and each unique signal value represents
different information. Any change in the signal is meaningful and each
level of the signal represents a different level of the phenomenon that
it represents. Another method of conveying an analogue signal is to use
modulation in which some base carrier signal has one of its properties
altered: amplitude modulation (AM) involves altering the amplitude of a
sinusoidal voltage waveform by the source information, frequency
modulation (FM) changes the frequency. Other techniques, such as
changing the phase of the carrier signal are also used.
In an analogue sound
recording, the variation in pressure of a sound striking a microphone
creates a corresponding variation in the current passing through it or
voltage across it. An increase in the volume of the sound causes the
fluctuation of the current or voltage to increase proportionally while
keeping the same waveform or shape. Mechanical, pneumatic, hydraulic
and other systems may also use analogue signals.
Digital
Electronics
Digital electronics are electronics systems that use digital signals. Digital electronics
are representations of Boolean algebra and are used in computers,
cellular phones, and other consumer products. Digital electronics or
digital circuits are usually made from large assemblies of logic gates,
with simple electronic representations of Boolean logic functions. To
most electronic engineers, the terms "digital circuit", "digital system"
and "logic" are interchangeable in the context of digital circuits.
The main differences
between analogue and digital electronics are listed below:
Inherent Noise
Analogue systems exhibit
noise; that is, random disturbances or variations. Since all variations
of an analogue signal are significant, any disturbance is equivalent to
a change in the original signal and so appears as noise. As the signal
is copied and re-copied, or transmitted over long distances, these
random variations become dominant and lead to signal degradation.
Electrically these disturbances are reduced by shielding, and using low
noise amplifiers. The effects of random noise can make signal loss and
distortion impossible to recover, since amplifying the signal to recover
attenuated parts of the signal often generates more noise and amplifies
the noise as well.
Noise
Analogue circuits
are more susceptible to noise than digital circuits, since a small
change in the signal can represent a significant change in the
information present in the signal and can cause information distortion
or lost. Since digital signals take on one of only two different
values, a disturbance would have to be about one-half the magnitude of
the digital signal to cause an error; this property of digital circuits
can be exploited to make signal processing noise-resistant. In digital
electronics, because the information is quantized, as long as the signal
stays inside a range of values, it represents the same information.
Digital circuits use this principle to regenerate the signal at each
logic gate, lessening or removing noise.
Precision
A number of
factors affect how precise a signal is, mainly the noise present in the
original signal and the noise added by processing. See signal-to-noise
ratio. Fundamental physical limits such as the shot noise in components
limits the resolution of analogue signals. In digital electronics
additional precision is obtained by using additional digits to represent
the signal; the practical limit in the number of digits is determined by
the performance of the analogue to digital converters, since digital
operations can usually be performed without loss of precision.
Analogue vs. digital electronics
The advantages of digital circuits when compared to
analog circuits are:
-
Digital systems interface well with computers and are easy to control
with software. New features can often be added to a digital system
without changing hardware. Often this can be done outside of the
factory by updating the product's software. Product errors or updates
can be corrected after the product is in a customer's hands.
-
Information storage can be easier in digital systems than in analog
ones. The noise-immunity of digital systems permits data to be stored
and retrieved without degradation. In an analog system, noise from
aging and wear degrade the information stored. In a digital system, as
long as the total noise is below a certain level, the information can
be recovered perfectly.
Power
In some cases, digital
circuits use more energy than analog circuits to accomplish the same
tasks, thus producing more heat as well. Digital circuits are sometimes
more expensive, especially in small quantities. The sensed world is
analog, and signals from this world are analog quantities. For example,
light, temperature, sound, electrical conductivity, electric and
magnetic fields are analog. Most useful digital systems must translate
from continuous analog signals to discrete digital signals. This causes
quantization errors. Quantization error can be reduced if the system to
stores enough digital data to represent the signal to the desired degree
of fidelity. Digital systems can be fragile, in that if a single piece
of digital data is lost or misinterpreted, the meaning of large blocks
of related data can completely change.
Cost versus
Performance
One of the primary
advantages of digital electronics is its robustness. Digital electronics
is robust because if the noise is less than the noise margin then the
system performs as if there were no noise at all. Therefore, digital
signals can be regenerated to achieve lossless data transmission, within
certain limits. Analog signal transmission and processing, by contrast,
always introduces noise. Digital systems are much
smaller
and somewhat
easier to design
than comparable analogue circuits. This is one of the main reasons
why digital systems are more common than analog. Development of an
analogue circuit is typically created manually and much less automated
than for digital systems. Additionally, because the smaller scale,
digital circuits
and integrated
circuit (chips) are cheaper to manufacture
than analog
circuits.
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