Why do we need Analog ?

July 26, 2017 by Vartul Sharma

                                                                                        Why do we need Analog?

This is not a new question that has been asked since the evolution of digital circuits. So, why this question again? The evolution of digital circuits has proven to be a “MAGIC STICK” for electronics. High noise margin and high immunity provides great robustness to the digital circuits. Also, by using more advanced techniques like Dynamic circuits and domino logics, the power (rather energy) dissipated by the digital circuits can be assured to be minimal. Also, ability to do fast computations has made the digital circuits favorites for almost all kind of processors.

Why do we need Analog?
From all the points discussed above, it can easily be concluded that we don't need any analog system. And yes, the Scientists, way back in 1980s, had predicted that after few years (from then) the analog will DIE. So, now we are in 2017, so has analog already died? Are all the devices we are seeing today made of only digital blocks? The answer is a big NO. A number of devices are still using analog blocks. Analog has not died, but it has flourished more after 2000s like never before. What happened after 1980s? Of course, digital signal processing and digital ICs have also advanced tremendously since then. Why the earlier predictions went false? Why the analog was not fully replaced?

Apart from digital blocks used in circuit design, analog blocks are also necessary in these circuits. There are many applications where analog designs are still preferable and in some cases there is no other alternative to an analog circuit. Few of them are:

  • Processing Natural Signals: Nature is full of analog signals. Temperature, pressure, light etc. all are given by nature continuously. Before the processing of these quantities, they must be converted into digital signals because in Digital processing is much . This conversion has done by ADC. The design of ADC for a particular application is one of the most difficult challenges in Analog design.
  1. Design of Amplifier : For some applications like detection of seismic signals, the strength natural signals produced by earthquake is so small that it can't directly given to ADC, otherwise it may result in false digital sequence. So, the signal must be amplified before giving to the ADC. The design of such low noise amplifiers are one of the major problems in Analog world.
  2. Design of Filter: The natural signal is not an ideal signal. It also has some out of the band noise. Before applying the signal to the ADC this noise should filter. The design of such filters has done in the analog domain.
  • Microcontroller design: Not all the things in a microcontroller is digital. Multi clock environment generated by PLLs and power supply are designs in analog domain.
  • Memory Design: Yes, you heard it correct. Reliable data storage can be best done in digital domain. So, really we are using something analog in memories too? The answer is YES. With decrement in semiconductor size and power supply, the identification of stored data (0 or 1) in the memory is becoming difficult day by day. For this purpose, circuits known as “sense amplifiers” are employed to sense the data correctly. This sense amplifiers are nothing but a kind of differential amplifiers, which a very common circuit in analog world.
  • Digital Communication: Even the digital communication itself uses analog designs. Most of the received data are attenuate and distorted, which barely resembles any digital form. This distorted data is first processed in analog domain and then converted back into the digital signal.
  • Optical Communication: It is used for very high-speed data transfer (10 to 40 GBPS) due to bandwidth limitations of cables. The data is converted into light by using laser diodes and then transmitted over the optical fiber cable. This data (in the form of light) is sense by a photodiode which converts it back into the electrical signal. This signal is fed to the receiver. The receiver then must be able to process a very low-level and high-speed signal. This type of low noise and high bandwidth receivers are designs in analog domain.
  • Sensors: Most of the sensors interact with the natural environment, so they are analog designs. Various sensors in automobiles and spacecrafts are needed to be of very high precision which can sense even 1% change in the signal. Any digital processing here can lead to quantization noise which can corrupt the data. For example: A capacitance based sensor can detect a change in humidity or temperature if the resistivity of that dielectric material changes with the change in humidity (or temperature). This change in capacitance can be sensed by an interface circuit which is designed in analog domain. A wheatstone bridge is simplest type of interface circuit with the sensor capacitor as one of its arm.
With the demand of high speed data transfer, the need of analog designs has emerged drastically. The design of USB, HDMI, SerDes etc highly use analog techniques. It is often stated that high-speed digital designs are in fact analog designs.
So, this was about the applications of analog designs (hoping you all know the wide application of digital designs) and we do find a lot of unexpected area were analog is being used these days.
Please go through Which circuits are easier (or difficult) to design among Analog or Digital circuit? for further reading.....

i) “Design of Analog CMOS Integrated Circuits” by Behzad Razavi, 3rd Edision.
ii) NPTEL video lectures by Prof. A. N. Chandorkar
iii) Some discussion with my colleagues and my seniors.


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