What are Analog and Digital signals & what is the difference?

Analog Signal: 

We hear lot about digital computer, digital watch Vs analog watch etc. around us. So what are these analog and digital stuffs around us? Let us try to understand the basics first so that it will be easy to understand these technologies in further topics. Though there are many more components left to learn, we will cover them slowly as when I will see the need to explain them. First we will see what is Analog voltage or Analog signal, as this came first and then came the Digital technology. 

Analog Signal

Think of anything around yourself in the environment like sound, temperature, pressure, humidity, light intensity, height, length, depth, Force, Weight etc. All these variables around you can have any value at any given point of time and place. Temperature can be anything like -50, -40, -10, 0, 35, 100, 120…Degree Celsius. Pressure can be 2, 5, 100, 1000…Pascal, similarly height, length, depth, weight etc, all can have any different value at any given condition. So, if we measure any of these variables in the universe through any type of sensor it will have infinite number of different values. And in case if we measure and convert these variables to electrical signal (Voltage or Current) obviously the voltage and Current will also have infinite values at different time, location, height etc. And if we plot the graph of this measured electrical voltage we may get any graph as shown in above image. There is no guarantee to have one, two or three fix value at all time intervals. In image Red, Blue and Green plots are imaginary data measured for any of the above mentioned variable and plotted here. If we draw vertical lines at different time intervals and check the corresponding Voltage from Y-axis, they have infinite number of values. This type of signal or voltage having infinite different values at different point of time is called Analog Signal or Analog Voltage.

Digital Signal:

Digital Signal

So what is then a digital signal? Opposite to analog signal, a digital signal will have only TWO values at any given point of time. There cannot be any other voltage level in digital signal. It will be either “High” or “Low”. High level is also referred as digital – ‘1’ and low level is referred as digital – ‘0’. Refer the graph shown for a digital signal; it has only two values i.e. either ‘5V’ or ‘0V’ at any given point of time. So any signal having precisely two voltage levels ONLY at any given point of time can be considered a digital signal. Here we shown two levels as 5V and 0V, there can be other levels also like 12V and 0V, 3.3V and 0V which we will see in different digital components or Integrated Circuits in future topics. But one condition is same for all such different signals, and that is they can have only TWO levels, no other level will be seen in digital signals. ‘0V’ is always considered as “Digital – 0” or “Low” and the other voltage level i.e. ‘3.3V’, ‘5V’ and ‘12V’ is always treated as “Digital-1” or “High”. Any digital computer can understand only these two levels i.e. ‘0’ or ‘1’.

This number system of ‘0’ and ‘1’ is called as “Binary Number System” as it has only two digits in this system and all other numbers can be formed from these two digits also called as “Bits” in digital world.

We have seen in transistor and MOSFET graphs that they have two region of operation. One is Ohmic region the other is saturation region.

In case of analog systems, transistors or MOSFETS are operated in Ohmic region hence based on different level of input signals, there is different but amplified level of output signal.

But in case of Digital systems, transistors and MOSFETS are always used in Saturation and cut-off region, where the transistor or MOSFET is either fully turned ON or is completely turned OFF.

Look at the small MOSFET based circuit and its related graph. In the circuit the position of switch is correlated with digital-0 and 1. When the Switch is OFF, V_G=0V hence this position is take as “digital input-0” and when the switch is ON, V_G=5V which is taken as “digital input-1”.

- When input is ‘1’, the MOSFET is turned ON and act as a Closed switch between Source and Drain, hence the “Vout” is connected virtually to negative of battery and hence Vout = 0V or Digital-0.

- When the input is ‘0’, the MOSFET is turned OFF and act like an Open switch and hence Vout = 5V or Digital-1.

In the graphical form same thing has been shown, while the digital input is LOW (0) from 0 to 2 sec, the digital output is HIGH (1). Similarly from 2 to 6 sec, the digital input is HIGH (1) hence the digital output is LOW (0) and so on.

This is a sample digital circuit, and actually an example of digital gate (NOT Gate) which we will see in more detail when we will learn about other digital gates which are the building blocks of any digital circuit.

So, as we saw digital signals seems easy to understand as they have only two states “HIGH” and “LOW” and Analog signals have infinite number of values at different point of time, hence the graph seems complex. But it is not that, both have their own complications when we start implementing them, but if we are able to understand the basic concepts behind them it will be easier to handle both types of signals. In fact analog signals are more interesting as we can modify the analog signals to get different wave forms, but in case of digital signals, we have only one type of waveform that is “Square Wave” or “Rectangular Wave” as we can see in the graph.

So, you will be surprised that if all computers work on digital platform, then how they are able to interact with the variables around us which is completely analog in nature. The answer is we have what is called “Analog to Digital Converter” and “Digital to Analog Converter” circuits to interface analog world to digital technology and vice-e-versa.

Will explain different types of wave forms in upcoming posts hence keep visiting this space or subscribe over email.

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