DAC&ADC [EngineeringDuniya.com]

7/28/2019 DAC&ADC [EngineeringDuniya.com]

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DAC/ADC

by

D.V.Kamat , Faculty

Department of E&C Engg., MIT


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Digital versus Analog

Most values are analog in nature

-temperature , speed ,pressure , position etc

Digital systems are used to process information

-CDs,MP3,DVDs

Need to convert an analog signal in to digital value Perform digital operations on data

Need to convert the digital result back into an

analog quantity


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DAC & ADC

DAC Digital-to-Analog Converter

Converts n-bit digital value to an analog voltage

ADC

Analog-to-Digital ConverterConverts analog voltage to a digital value

ADC and DAC operations are required at the

input and output from a digital processingsystem that deals with analog quantities.


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4

Digital to Analog Converter

What is a digital to analog converter (DAC)?

Converts digital input signal to an analog output

signal

0

10

1

0

01

1

0

11

1

1

00

1

1

00

1

1

01

0

1

01

1 DAC
http://www.ti.com/ww/de/graphics/shared/icn-dac-symbol.gif


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Types of DACs

Binary Weighted Resistor DAC

R/2R Ladder DAC


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Analysis

Recall inverting summer Op-Amp:

Set the input resistor values at

multiple powers of two.

Using KCL and Op-Amp properties

V(-) = V(+) = 0 V

Inverting summer Op-Amp

Starting from V1 and going through V3, this would give each input

voltage exactly half the effect on the output as the voltage before it.

R

V

R

V

R

VRVout

42

321


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8


The output will be:

)2

...2

( 1021

Rb

Rb

RbRVrefVs n

nn

)2...22(2

_012

2

1

1

1bbbb

VrefVs n

n

n

n

n


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Example:

)248(8

_ 0123 bbbbVrefVout

4n

VrefVref

Vout8

3))1(1)1(2)0(4)0(8(

8

_

)2...22(2

_012

2

1

1

1bbbb

VrefVsV n

n

n

n

nout

refreffs VVV 9375.02

12

4

4

Full scale voltage:


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Advantage

Easy design principle

Disadvantages Requirement of several

different precise inputresistor values: one

unique value per binaryinput bit. (High bit DACs)


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11

Resolution

The variation of the output voltage corresponding to

the variation of the least significant binary bit (LSB)

Inversely proportional to the number of bits

NLSB

VV

2Resolution Ref
http://www.google.co.in/imgres?imgurl=http://www.ti.com/ww/de/graphics/shared/icn-adc-symbol.gif&imgrefurl=http://www.ti.com/ww/de/prod_dc.html&h=34&w=41&sz=1&tbnid=6LPvJnW_OPRL3M:&tbnh=34&tbnw=41&prev=/images?q=ADC+symbol&zoom=1&q=ADC+symbol&hl=en&usg=__ZquR_5jyZZLvqda6-OeefYmauMc=&sa=X&ei=5sXCTOOSM4quvgOTuOzSCA&ved=0CCYQ9QEwBA


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An analog-to-digital converter (abbreviated ADC, A/D

or A to D) is a device that converts a continuousquantity to a discretedigital number.

ADC output may be fed to Digital displays in

consumer electronic applications and accuracy ofthese consumer electronic applications depend on

the accuracy of the A/D conversion.

Resolution is a function of the number of data bits inthe ADC output. Resolution has a direct impact on

the accuracy of the ADC system

ADC Analog-to-Digital Converter
http://en.wikipedia.org/wiki/Discrete_signalhttp://en.wikipedia.org/wiki/Digitalhttp://www.google.co.in/imgres?imgurl=http://www.ti.com/ww/de/graphics/shared/icn-adc-symbol.gif&imgrefurl=http://www.ti.com/ww/de/prod_dc.html&h=34&w=41&sz=1&tbnid=6LPvJnW_OPRL3M:&tbnh=34&tbnw=41&prev=/images?q=ADC+symbol&zoom=1&q=ADC+symbol&hl=en&usg=__ZquR_5jyZZLvqda6-OeefYmauMc=&sa=X&ei=5sXCTOOSM4quvgOTuOzSCA&ved=0CCYQ9QEwBAhttp://en.wikipedia.org/wiki/Digitalhttp://en.wikipedia.org/wiki/Discrete_signal


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A digital signal is an approximation of an analog one

ADC Analog-to-Digital Converter
http://en.wikipedia.org/wiki/File:ADC_voltage_resolution.svg


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Types of ADC are as follows:

1. Counter type ADC

2. Successive approximation type ADC

3. Flash type ADC or Direct conversion ADC

Types Of ADC
http://en.wikipedia.org/wiki/Flash_ADC


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A direct conversion ADC or flash ADC (also called

parallel A/D converter) consists of a bank of

comparators, each one comparing the input signal to

a unique reference voltage. The comparator outputs

are given to the inputs of a priority encoder circuit,

which then produces a binary output for each voltagerange.

A bank ofcomparators compare the input signal in

parallel with different reference voltages.

Flash ADC
http://en.wikipedia.org/wiki/Flash_ADChttp://en.wikipedia.org/wiki/Comparatorhttp://en.wikipedia.org/wiki/Flash_ADChttp://en.wikipedia.org/wiki/Flash_ADChttp://en.wikipedia.org/wiki/Comparatorhttp://en.wikipedia.org/wiki/Flash_ADChttp://en.wikipedia.org/wiki/Flash_ADChttp://en.wikipedia.org/wiki/Flash_ADC


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3-bit flash ADC circuit


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Vrefis a stable reference voltage provided by a precision voltage

regulator.As the analog input voltage exceeds the reference voltage at each

comparator, the comparator outputs will change or saturate to a

high state.

The priority encoder generates a binary number based on the

highest-order active input, ignoring all other active inputs.

Fl h C t l


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Flash Converter example

Fl h C t l


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Flash Converter example

Number of comparators = number of quantization levels

Converterinput

Comparator Outputs Encoder Output

range (V) A B C D E F G

< 1 0 0 0 0 0 0 0 000>1-2 1 0 0 0 0 0 0 001>2-3 1 1 0 0 0 0 0 010>3-4 1 1 1 0 0 0 0 011>4-5 1 1 1 1 0 0 0 100>5-6 1 1 1 1 1 0 0 101>6-7 1 1 1 1 1 1 0 110>7 1 1 1 1 1 1 1 111

M it d d it f Fl h ADC


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Benefits :

i. Flash converters are extremely fast compared to many other

types of ADCs. Direct conversion is very fast, capable of

gigahertz sampling rates. Hence , flash ADCs are employed in

Gigahertz applications like radar detection, wide band radioreceivers and optical communication links.

ii. Construction and design procedure of Flash converter is

simple and straight forward.

iii. They are accurate, and easy to operate.iv. They are often used forvideo, wideband communications, or

other fast signals in optical storage

Merits and demerits of Flash ADC

M i d d i f Fl h A
http://en.wikipedia.org/wiki/Gigahertzhttp://en.wikipedia.org/wiki/Videohttp://en.wikipedia.org/wiki/Optical_storagehttp://en.wikipedia.org/wiki/Optical_storagehttp://en.wikipedia.org/wiki/Optical_storagehttp://en.wikipedia.org/wiki/Optical_storagehttp://en.wikipedia.org/wiki/Videohttp://en.wikipedia.org/wiki/Gigahertz


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Drawbacks :

i. A Flash converter requires a huge number of comparators

compared to other ADCs, especially as the precision

increases.

ii. The 3-bit flash ADC requires 7 comparators. A 4-bit versionwould require 15 comparators. A Flash converter requires

comparators for an n-bit conversion. The number of

comparators needed, doubles with each additional bit,

requiring a large expensive circuit. ADCs of this type have alarge die size, a high input capacitance, high power

dissipation.

iii. The size and cost of all those comparators makes Flash

converters generally impractical for precisions much greater

than 8 bits (255 comparators).

Merits and demerits of Flash ADC
http://en.wikipedia.org/wiki/Comparatorhttp://en.wikipedia.org/wiki/Die_(integrated_circuit)http://en.wikipedia.org/wiki/Capacitancehttp://en.wikipedia.org/wiki/Capacitancehttp://en.wikipedia.org/wiki/Die_(integrated_circuit)http://en.wikipedia.org/wiki/Comparator


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-

+

D-AConverter

Counterclock and

control logic

comparitor

analogue input

Counter-ramp ADC


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Comprises a D-A converter, a single comparator, a counter, a clock

and control logicWhen a conversion is required

A signal (conversion request) is sent to the converter and thecounter is reset to zero

a clock signal increments the counter until the reference voltagegenerated by the D-A converter is greater than the analogueinput

At this point in time the output of the comparator goes to alogic 1, which notifies the control logic the conversion has

finished

The value of the counter is output as the digital value

Counter-ramp ADC


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END

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