TOPIC 6 - (Converter)

8/13/2019 TOPIC 6 - (Converter)

1/45


2/45

COURSE LEARNING OUTCOMES

(CLO)

CLO1. explain correctly the principles of

electronic circuits by using block diagram orcircuit diagram (C4)

CLO2. conduct the construction of electronic

circuits application during practical works

based on the theory and principle operation of

the circuits. (P4)CLO3. deliver an oral presentation to display

good communication skills. (A2)

2


3/45

LEARNING OUTCOMES

Upon completion of this topic students should be able to:

Explain the methods of converting digital to analogue signals6.1

Explain the application of D/A converter6.2

6.3

6.4

6.5

Draw the input and output waveform

Explain the methods of converting analogue to digital signals

Explain the application of A/D converter

3


4/45

INTRODUCTION

Most real-world information is analog. Forinstance, time, speed, weight, pressure, lightintensity, and position measurements are allanalog in nature. The digital system in Figurebelow has an analog input. The voltage variescontinuously from 0 to 3 V. The encoder is aspecial device that converts the analog signal todigital information. The encoder is called an

analog-to-digital converter or, for short, an A/Dconverter. The A/D converter, then, convertsanalog information to digital data.


5/45

INTRODUCTION

The digital system diagrammed in Figure below

also has a decoder. This decoder is a special

type; it converts the digital information from the

digital processing unit to an analog output. Forinstance, the analog output may be a continuous

voltage change from 0 to 3 V. We call this

decoder a digital-to-analog converter or, for short,

a D/A converter. The D/A converter, then, decode

digital information to analog form.


6/45

Digital System


7/45

Digital to Analog Converter (DAC)

A block diagram of a D/A converter is

shown below. The digital inputs (D, C, B, A)

are at the left. The decoder consists of twosections: the resistor network and the

summing amplifier. The output is shown as

a voltage reading on the voltmeter at the

right.


8/45

Digital to Analog Converter (DAC)

Basically, D/A conversion is the

process of taking a value represented

in digital code (such as straightbinary or BCD) and converting it to a

voltage or current which is

proportional to the digital value


9/45

A block diagram of a D/A converter


10/45

Symbol of a D/A converter


11/45


12/45

Resistive Divider

One method of the D/A conversion uses a resistornetwork with resistance values that represent thebinary weights of the input bits of the digital code.Figure below shows a 4-bit DAC of this type. Eachof the input resistors will either have current orhave no current, depending on the input voltagelevel. If the input voltage is zero (binary 0), thecurrent is also zero. If the input voltage is HIGH(binary 1), the amount of current depends on theinput resistor value and is different for each inputresistor, as indicated in the figure.


13/45

Resistive Divider


14/45

Resistive Divider

From the above figure

Since there is practically no current into the op-

amp inverting input, all of the input currents sum

together and go through Rf. Since the inverting

input is at 0 volt (virtual ground), the drop acrossRf is equal to the output voltage, so Vout= If Rf.


15/45

Resistive Divider

The values of the input resistors are chosen to beinversely proportional to the binary weights of thecorresponding input bits. The lowest value resistor(R) corresponds to the highest binary-weightedinput (23). The other resistors are multiples of R(2R, 4R, 8R) and correspond to the binaryweights 22, 21, and 20, respectively. The inputcurrents are also proportional to the binaryweights. Thus, the output voltage is proportionalto the sum of the binary weights because the sumof the input currents is through Rf.


16/45

Example 1

Determine the output of the DAC in figure

below if the waveforms representing a

sequence of 4-bit numbers given below areapplied to the inputs. Input D0 is the least

significant bit (LSB).


17/45


18/45


19/45

Example 2

Determine the output of the DAC in the

figure (a) below if the sequence of 4 bit

numbers in part (b) is applied to theinputs. The data inputs have a low value

of 0V and a high value of +5V.


20/45


21/45


22/45

Binary Ladder

1) Difference with resistor network because it

only uses two values of resistors R and 2R

2)Binary weighted DAC circuit have too manyresistors

3)R-2R circuit is used more often to get high

accuracy and precision


23/45

Binary Ladder


24/45

Binary Ladder


25/45

Binary Ladder


26/45

Binary Ladder


27/45

Digital analog converter spesification


28/45

Analog to Digital Converter

An analog-to-digital converter is a specialtype of encoder. A basic block diagram ofan A/D converter is shown below. The inputis a single variable voltage. The voltage inthis case varies from 0 to 3V. The output of

the A/D converter is in binary. The A/Dconverter translates the analog voltage atthe input into a 4-bit binary word.


29/45



30/45



31/45


The A/D conversion process is generally

more complex and time-consuming than

the D/A process, and many differentmethods have been developed and used.

We shall examine several of these methods

in detail. Among them are a digital-ramp

method and a successive approximationmethod.


32/45

Digital-ramp A/D Converter

The digital-ramp method of A/D conversionis also known as the stairstep-ramp or the

counter method. It employs a DAC and abinary counter to generate the digital valueof an analog input. Figure below illustratesthe block diagram of this type of converter.

It contains a counter, a DAC, an analogcomparator, and a control AND gate.


33/45

Digital-ramp A/D Converter


34/45

Example 1

Assume the following values for the digital-

ramp ADC: clock frequency= 1MHz; DAC

full-scale output= 1.5V and a 4-bit input.Determine the following values.

The digital equivalent obtained for VA= 0.78V

The conversion time

The resolution of this converter


35/45

Example 2

Digital Ramp ADC has frequency clock of1.25 MHz, threshold sensivity voltage of0.1 mV and full scale voltage DAC 10 bit

of 10.23 V.i) when Va=3.57 V, calculate equivalent

output value

ii) Determine the convertor timeiii) Calculate the resolution in volt and

percentage.


36/45

The stairsteps


37/45

Successive Approximation A/DConverter

The successive-approximation converter is

one of the most widely used types of A/D

converter. It has more complex circuitrythan the digital ramp A/D converter but a

much shorter conversion time. In addition,

successive approximation converters

(SAC) have a fixed value of conversiontime that is not dependent on the value of

the analog input.


38/45



39/45


The input bits of the DAC are enabled (made equal to 1)one at a time, starting with the MSB. As each bit isenabled, the comparator produces an output thatindicates whether the analog input voltage is greater or

less than the output of the DAC. If the DAC is greaterthan the analog input, the comparators output is LOW,causing the bit of the register to RESET. If the output isless than the analog input, the 1 bit is retained in theregister. The system does this with the MSB first, thenthe next most significant bit, then the next, and so on.

After all the bits of the DAC have been tried, theconversion cycle is complete.


40/45

Example

Describe the operation of the 4-bit SAC.

Assume that the constant analog input

voltage is +5V. Lets assume the output

characteristics of DAC are: Vout= 8V for

the 23 (MSB), Vout= 4V for the 22 bit, and

Vout= 1V for the 20 bit (LSB).


41/45

Solution


42/45

Solution

Figure (a) shows the first step in the conversioncycle with the MSB = 1. The output of the DACis 8V. Since this is greater than the analog inputof 5 V, the output of the comparator is LOW,causing the MSB in the SAR to be RESET to a0.

Figure (b) shows the second step in theconversion cycle with the 22 bit equal to a 1.

The output of the DAC is 4V. Since this is lessthan the analog input of 5 V, the output of thecomparator switches to a HIGH, causing this bitto be retained in the SAR.


43/45

Solution

Figure (c) shows the third step in the conversioncycle with the 21 bit equal to a 1. The output ofthe DAC is 6V because there is a 1 on the 22 bitinput and on the 21 bit input; 4V + 2V = 6V.Since this is greater than the analog input of 5V, the output of the comparator switches to aLOW, causing this bit to be RESET to a 0.

Figure (d) shows the fourth and final step in the

conversion cycle with the 20 bit equal to a 1.The output of the DAC is 5 V because there is aIon the 22 bit input and on the 20 bit input; 4V +1V = 5V.


44/45

Solution

The four bits have all been tried, thus

completing the conversion cycle. At this

point the binary code in the register is

0101, which is the binary value of the

analog input of 5V. Another conversion

cycle now begins, and the basic process is

repeated. The SAR is cleared at thebeginning of each cycle.


45/45

THANK YOU

GOOD LUCK FOR YOURFINAL EXAM and

TRY THE BEST

TOPIC 6 - (Converter)

Documents

Transcript of TOPIC 6 - (Converter)