VLSI - 2 ( Analog)

For those of you who wanted to make a career in analog, be a devotee of IIT Madras faculty irrespective of which IIT or which college you end up joining for your Mtech.

1. Shanti Pavan - IIT Madras videos
2. Nagendra Krishnapura - IIT Madras videos
3. Razavi - Videos and Text book
4. Anirudhan  - IIT Madras videos

I would like to discuss few common blunders, that we end up doing, which henceforth make us loose confidence in analog

Mistake 1 :  Two current sources in series

In simulation if you put two current sources in series and simulate this, you observe some crazy things happen, kindly do this and learn on this.

Put up a current source with NMOS sinking the current with very low Vgs and observe

Repeat the experiment with PMOS too. Try increasing the sizing for the same Vgs and observe what happens.

This experiments are really the most powerful in analog. The deeper one digs into this, the faster he understands the analog concepts well.

Spend great time dealing with this circuit.

Feel free to discuss if any doubts.......

Will come back with the next post. and discuss the solution and concepts here.

Analog to Digital Converter

In real world, all signals like light, sound etc, are analog signals. These signals have to be converted into digital form so that they can be manipulated by digital equipment. Device used to convert analog signal into digital signal is called Analog to Digital Converter (ADC). An example of an analog to digital converter is a Scanner – It takes a picture (analog) as input and convert into digital picture. ADC is an electrical circuit that converts continuous time and continuous amplitude signal into discrete time and discrete amplitude signal.

Let us first discuss basic concept of analog to digital conversion. The process of digitizing the domain(time) is called sampling and the process of digitizing range(voltage/current) is called quantization.

Sampling : An ADC circuit samples analog signal from time to time. Then, each sample is converted into a number based on its voltage level. The frequency at which sampling occurs is called sampling rate or sampling frequency. e.g if sampling frequency is 22000 Hz, it means, in one second 22000 input points will be sampled and distance between two adjacent time points is 1/22000 seconds. Higher the sampling frequency, more perfect will be the analog signal produced by DAC (when it is required to reconstruct the analog signal from digital samples). But more memory will be needed to store these samples. So there is always a trade off between memory required to store samples and accuracy of signal. But to reproduce analog signal from digital samples, there should be some minimum number of samples. And

According to Nyquist sampling theorem, sampling rate must be at least twice the highest frequency component to avoid aliasing.

                                      Fs = 2Fmax

Quantization: Quantization is the process of converting continuous value signal into discrete value signal so that signal takes only finite set of values. Unlike sampling (where we saw that under some conditions, it is possible to reconstruct the signal), quantization results in some loss of information called quantization error. One of basic choice in quantization is the number of discrete quantization levels to use. Fundamental tradeoff in this choice is the resulting signal quality vs data(bits) needed to represent each sample. With L levels, number of bits required to represent each level,

                     
              N = logL/log2.

Analog to Digital Converter with 32 levels(5 bits)

­Our world – Digital or analog

There are two kinds of electronic systems that we encounter in our daily life – digital and analog. Digital systems are the ones in which the variables to be dealt with can presume only some specified values whereas in analog systems, these variables can assume any of the infinite values. The superiority of digital devices over analog devices has ever been a topic of discussion. This is the reason why digital devices have taken over analog in almost all the areas that we encounter today. Digital computers, digital watches, digital thermometers etc. have replaced analog computers, analog watches and analog thermometers, and so on. Digital devices have replaced the analog ones due to their superior performance, better ability to handle noise and reliability in spite of being more costly than analog ones. Although most of the devices used today are digital, but in general, the world around us seems to be analog. All the physical quantities around us; i.e. Light, heat, current are analog. The so called digital devices have to interface with this analog real world only. For instance, a digital camera interfaces with analog signal (light) and converts it into information in the form of pixels that collectively form a digital image. Similarly, a music system converts the digital information stored in a music CD into pleasant music which is nothing but analog sound waves. All the digital devices that we know have this characteristic in common. Simply speaking, there are devices known as Analog to Digital converters (ADC) and Digital to Analog converters (DAC) that acts as an interface between the real analog world and the digital devices and converts the data sensed by analog sensor into the digital information understood by the digital system and vice-versa. They all interface with the so called analog world. But is the analog world really analog? Is it true that analog variables can take any number of values? Or is there some limit of granularity for them too. Is this world inherently digital or analog in nature? Is digital more fundamental than analog?

 As we all know, there are many fundamental quantities in this universe viz. Mass, length, time, charge, light etc. We have been encountering these ever since the world has begun. Now the question arises – whether all these quantities are inherently analog or digital? Finding the answer to this question will automatically bring us to the answer of our main question; i.e. whether the basics of this world lie in analog or digital. It is often said that “Heart of digital devices is analog.” (See figure below). This is because, as visible on a macroscopic scale, the current and voltage waveforms produced by a digital circuit/system are not digital in fact. This can be observed from the fact that the transition from one logic state to another cannot be abrupt.  Also, there are small spikes in the voltage levels even if the system is stable in one state. But, seen at microscopic level in terms of transfer of current by transfer of electrons, since, there can only be

transfer of an integral number of electrons, current can only take one of numerous values, and not just any value. Let us take an illustration. The charge on an electron is 1.6E19 (or 0.00000000000000000016) represented as ‘e’. It is the smallest charge ever discovered. It is well known that charge can exist only in the multiples of ‘e’. Thus, electric charge is a digital quantity with the smallest unit ‘e’. When we say that the value of charge at a point is +1C, we actually mean that the charge is caused by transfer of 6250000000000000000 electrons. Since, the smallest unit of charge is 0.00000000000000000016 C, hence, there cannot exist any charge of value 1.00000000000000000015 C, since that will make the number of electrons to be a fraction. Since, the magnitude of 1C is very large as compared to charge on 1e, it appears to us as continuous and not discrete. For us, there is no difference between 1. 00000000000000000015 and 1 as the devices we use don’t measure with that much precision. Hence, we infer these quantities as analog. Similar is the case with other physical quantities.

Many laws have been formed by our great scientists postulating about the quantization of some basic physical quantities. Viz. Planck’s quantum theory states that angular momentum of an electron in the orbit of an atom is quantized. Simply stated, it states that the angular momentum can take only specified values given as multiples of h/2Π. Thus, the smallest angular momentum an electron can have is h/2Π and the angular momentum can increment only in steps of h/2Π. If we take h/2Π as one unit, then we can say that angular momentum of an electron is a digital quantity. Similarly speaking, Light is also known to consist of photons. According to Planck’s quantum theory, the light intensity is also an integral multiple of the intensity of a single photon. Thus, light is also inherently a digital quantity. Also, as stated above, the charge is also quantized.

But there are some physical quantities of which quantization is yet to be established. Mass is one of those quantities. But, it is believed that the quantization of mass will be established soon.

Thus, we have seen that most of the physical quantities known are known to be digital at microscopic level. Since, we encounter these at macroscopic level having billions and billions of basic units, the increments in these seem to be continuous to us as the smallest incremental unit is negligible in comparison to actual measure of the quantity and we perceive them as analog in nature.

Thus, we can come to the conclusion that most of the quantities in this world are digital by their blood. Once the quantization of mass will be established, we can conclude with surety that digital lies in the soul of this world. This digital is similar to our definition of digital systems; just the difference is that it occurs at a very minute scale which we cannot perceive at our own.

Read also:

• Enhancement and depletion mode MOSFETs
• Analog to digital converter