EE Fundamentals – Circuit Development

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:

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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