The Vchip Blog
Which circuits are easier to design among Analog and Digital circuit.
July 26, 2017 by Vartul Sharma
Which circuits are easier to design among Analog and Digital circuit?
This is a bit easier question as at this level we all have an intuition to say that analog circuits are way more difficult to design than digital circuits. But why? And the answer is simply due the following reasons:
- Less Robust: The robustness of digital signal comes with the concept of noise margin. This makes a signal with 0 volts will detected as zero as well as a signal with an added noise 0.2 volts also to be detected as a logic zero. But in analog, there is no concept of noise margin. Any signal with 0 volts is treated as 0 volts and if any noise changes the signal to 0.2 volts, it reflects at the output. To further realize the impact of such a small noise, let us assume a case when a signal is fed to an amplifier of gain 100. For a signal of 1 volt, the output will be 100 volts. In case of any noise addition of 0.1 volts, the signal becomes 1.1 volts and the output of amplifier is now 110 volts. See the impact. HUGE.
- Re-designing: In digital, most of the previously designed blocks and symbols can be further used directly in other circuit designs like AND gate designed once can be used in any other circuits without much change. But, in analog designs, each block has to be designed again for different specifications. An op-amp designed for one circuit can’t be used in another circuit. This makes the analog design more cumbersome.
- Lack of HDL: Use of Verilog and VHDL has made even very complex designs very simple by just writing codes. The analog designs on the other hand are still designed by placing each transistor manually and then varying their parameters to get desired output. The bunch of permutation and combinations make analog design very tedious (but very interesting) job. This requires a clear understanding of circuit and components and a lot of intuition which comes with experience.
- More Number of Players: In digital designs, the optimization is done by keeping an eye on performance (delay) and power. With only these two parameters, the digital circuits easily reach to an optimal design. Whereas in analog world, apart from these two parameters, some more parameters namely gain, bandwidth, noise, port impedances etc are to be taken care of. With such large numbers of players on the table, analog designers have to be one of the smartest guys in the world to manage all of them at the same time.
- Digital is BOSS: Today, there is hardly any independent analog chip. This is the era of mixed signals and both digital and analog blocks share the same die. Here, digital is the BOSS because almost 80% of the area is covered by digital. So, all the specifications are decided in accordance with the digital blocks only. Analog blocks are given very less room and very tight environment.
For example: shrinking the channel length has increases the performance of digital blocks (less distance, less time to travel), but for analog larger channel lengths are required for large unity gain bandwidth product. So, analog wants to work on 0.35 micron and digital compels it to work on 22nm with the desire of very good results. This is the most challenging task to be done by analog engineers. You are given a bad tool and restricted area to achieve the wonders
. But, that is the most interesting thing as an engineer you can do. Remember BAHUBALI: covering a large army with a large cloth sheet (a bad tool) and burning them under fire (achieving success).
So, we have asked some repeated questions again and we got some amazing and unexpected answers. In this era of engineering, the technology is changing very swiftly and so the world. I am sure, the next time someone asks the same question, the answers will be equally unexpected and astonishing. So, KEEP ASKING..KEEP LEARNING..
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.