UNIVERSITY OF GONDAR 1
INSTITUTE OF TECHNOLOGY
Department of Electrical and Computer Engineering
Course name: Digital Logic Design
Course code: ECEg3141
Course instructor: Amare Worku
Contact information:
Email: amareworku2154@gmail.com
VHJ
Consultation hours: friday: from 8:00-11:00
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1. Introduction to Digital and analogue Systems
Continuous Vs Discrete (Analog Vs Digital)
There are basically two way of representing the numerical value of quantities i.e Analogue
& digital.
Analogue
An analogue system is one having continuous value.
In the world most quantities are analogue-in digital, we must convert analog to digital.
In analog representation quantities are continuous range of value.
Analog
Signal
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Cont.…
Analog means Continuous (both in time and amplitude)
Analog information exhibit smooth, gradual changes over
time and assume a continuous (infinite) range of amplitudes
Examples:
Body temperature
Our speech
Room temperature
Electric current,volt… etc
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Cont…
Analogue is used to express the numerical value of the quantity as a continuous range of values
between the two expected extreme values.
For example, the temperature of an oven settable anywhere from 0 to 100 °C may be measured to be
65 °C or 64.96 °C or 64.958 °C or even 64.9579 °C and so on, depending upon the accuracy of the
measuring instrument.
Most physical quantities – position, velocity, acceleration, force,pressure, temperature and flowrate
are analogue in nature.
Digital 5
Digital Discrete, Not continuous
Digital information assume a limited (finite) set of “Discrete”
values, not a continuous range of values
Values change suddenly (not smoothly) by “Jumping” between
values
Examples:
Energy levels of electrons in atoms.
Digital clock
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cont…
If we have only two signal levels Binary signal
… So binary is a special case of digital representation .
For example,
the temperature of the oven may be represented in steps of 1 °C as 50°C and above and
0°C as 50 °C and below.
Only 4 allowed
Signal levels
Digital
Signal
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Cont.….
The major difference between analog and digital quantities then,
can be simply stated as follows:
Analog = continuous
Digital = discrete (step by step)
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Advantage and limitation of digital system
Advantage:-
Digital system are easier to design.
Information storage is easy.
Accuracy and precision are greater.
Digital system are less affected by noise.
Limitation:-
There is really only one major drawback when using digital techniques-
The real world is analogue.
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ADC and DAC
Analog-to-digital-converters (ADC) are used to digitize raw analog inputs.
Digital-to-analog-converters (DAC) are used to regenerate analog signals from their digitized form.
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Example 1.
Which of the following involve analog quantities and which involve digital
quantities?
(a) Ten-position switch
(b) Current flowing from an electrical outlet
(c) Temperature of a room
(d) Automobile fuel gauge
Exercise 1.
1.What are the advantages of digital techniques over analog?
2. What is the chief limitation to the use of digital techniques?
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solution
(a) Digital
(b) Analog
(c) Analog
values and not every possible value over a continuous range
(d) Analog, if needle type; digital, if numerical readout.
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Comparison chart
Analog Digital
Signal :Analog signal is a continuous Signal: Digital signals are discrete time signals
signal which represents physical generated by digital modulation.
measurements. Waves: Denoted by square waves.
Waves: Denoted by sine waves
Representation: Uses discrete or discontinuous
Representation: Uses continuous range values to represent information
of values to represent information. Example: Computers, CDs, DVDs, and other
Example: Human voice in air, analog digital electronic devices.
electronic devices. Technology: Samples analog waveforms into a
Technology: Analog technology records limited set of numbers and records them.
waveforms as they are.
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Cont….
Data transmissions: Subjected to deterioration Data transmissions: Can be noise-immune without
by noise during transmission and write/read deterioration during transmission and write/read cycle.
cycle
Response to Noise: Less affected since noise
Response to Noise: More likely to get response are analog in nature.
affected reducing accuracy. Flexibility: Digital hardware is flexible in
Flexibility: Analog hardware is not flexible implementation
Memory: Stored in the form of wave signal. Memory: Stored in the form of binary bit
Power: Analog instrument draws large power. Power: Digital instrument draws only negligible
power.
Cost: Low cost and portable. Cost : Cost is high and not easily portable
Errors: Analog instruments usually have a scale Errors: Digital instruments are free from observational
which is cramped at lower end and give errors like parallax and approximation errors.
considerable observational errors
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Binary digit
Each of the two digit in binary system,1 and 0 is called a bit,which is the contraction of the word binary digit.
In digital circuit ,two different voltage level are used to represent the two bits
Generally one is represented by the higher voltage, which we will refers as to HIGH and zero is represented
by the lower voltage level ,which we will refers to as a LOW.
This is called positive logic.
HIGH = 1 LOW = 0
In another system one is represented by LOW and one is represented by HIGH,this system is called negative
logic.
A group of bits(combination of 1’s and 0’s) are called code.
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Logic level
The voltage used to represent 1 and 0 are called logic level, ideally one voltage level represents a high and
another voltage level represents low.
In practical HIGH can be any voltage between the specified maximum value and the specified minimum value.
likewise LOW can be any voltage between the specified maximum value and the specified minimum value
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Digital wave form
Digital wave form is made up of series of pulses.
Digital wave form consists of voltage level that are changing back and so forth between the high and low level or
state.
Positive going pules is generated, the voltage or current goes from its LOW level to HIGH level.
negative going pules is generated, the voltage or current goes from its HIGH level to low level.
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Waveform characteristic
Digital wave form are can be classified as periodic or non periodic.
A periodic pulse wave form is one that repeat itself at fixed interval of timecalled period(T).
frequancy)(f) is the rate at which it repeat itself and is measured in hertz(Hz).
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Cont.…
The frequency(f) of the pulse(digital) waveform is the reciprocal of the period.
T=1/f and f =1/T
Another important characteristics of periodic digital wave form is its duty cycle, which is the ratio of the
pulse width to(tw) to the period(T).
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Example 2.
From the digital waveform figure below calculate period, frequency and duty cycle ?
period =10
frequancy,f = 1/T= 1/10ms = 100 hz
Duty cycle = (tw/T) =(1ms/10 ms)100%=10%
