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A-D A-D CONVERTERSCONVERTERS

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WHAT IS IT FOR?WHAT IS IT FOR?

► It accepts analog i/p It accepts analog i/p and produces o/p and produces o/p binary word(dbinary word(d11dd22……ddnn) of functional ) of functional value D.value D.

► D=dD=d1122-1-1 + d + d2222-2-2 + … + …+d+dnn22-n-n

► dd11=MSB , d=MSB , dnn =LSB =LSB

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CONVERSION PRINCIPLESCONVERSION PRINCIPLES

► Direct type ADCsDirect type ADCs compares a given compares a given analog signal with analog signal with internally generated internally generated equivalent signal.equivalent signal.

► Integrating type Integrating type ADCsADCs perform perform conversion in conversion in indirect manner by indirect manner by first changing the first changing the analog i/p signal to analog i/p signal to a linear function of a linear function of time or freq and time or freq and then to digital code.then to digital code.

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BROAD CLASSIFICATION OF BROAD CLASSIFICATION OF ADCs(based on conversion ADCs(based on conversion

techniquetechnique))

►DIRECT TYPE DIRECT TYPE ADCADC

► Flash(comparator)Flash(comparator)► Counter (ramp) Counter (ramp)

type type ► Tracking (servo) Tracking (servo)

typetype► Successive Successive

approximation typeapproximation type

INTEGRATING INTEGRATING TYPETYPE

►Charge balancing Charge balancing ►Dual slopeDual slope

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THE PARALLEL THE PARALLEL COMPARATOR(FLASH) A/D COMPARATOR(FLASH) A/D

CONVERTERCONVERTER► Simplest,fastest and most Simplest,fastest and most

expensive.expensive.► It compares the analog i/p It compares the analog i/p

voltage with each node voltage with each node voltage.voltage.

► Conversion takes place Conversion takes place simultaneously rather than simultaneously rather than sequentially.sequentially.

► Typical conversion time is Typical conversion time is 100 ns or less.100 ns or less.

► No of comparators required No of comparators required are 2are 2nn – 1. – 1.

► Conversion time is limited Conversion time is limited by speed of comparators.by speed of comparators.

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+

-

Voltage input Logic output Y

Va >Vd X=1

Va <Vd X=0

Va =Vd Previous value

Input voltage x7 x6 x5 x4 x3 x2 x1 x0 y2 y1 Y0

0-VR/8 0 0 0 0 0 0 0 1 0 0 0VR/8-VR/4 0 0 0 0 0 0 1 1 0 0 1VR/4-3VR/8 0 0 0 0 0 1 1 1 0 1 03VR/8-VR/2 0 0 0 0 1 1 1 1 0 1 1VR/2-5VR/8 0 0 0 1 1 1 1 1 1 0 05VR/8-6VR/8 0 0 1 1 1 1 1 1 1 0 16VR/8-7VR/8 0 1 1 1 1 1 1 1 1 1 07VR/8-VR 1 1 1 1 1 1 1 1 1 1 1

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COUNTER (RAMP) TYPE COUNTER (RAMP) TYPE COMPARATORCOMPARATOR

► Low speedLow speed► Conversion time may Conversion time may

be as long as (2be as long as (2n n -1) -1) clock periods clock periods depending upon depending upon magnitude of i/p magnitude of i/p voltage.voltage.

► Counter freq must be Counter freq must be low enough to give low enough to give sufficient time for DAC sufficient time for DAC to settle and for to settle and for comparator to respond. comparator to respond.

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SERVO TRACKING A/D SERVO TRACKING A/D CONVERTERCONVERTER

► It is an improved It is an improved version of ADC.version of ADC.

► It is simple.It is simple.► The time needed to The time needed to

stabilize is more as stabilize is more as new conversion is new conversion is directly prop to the directly prop to the rate of change of rate of change of analog signal.analog signal.

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SUCCESSIVE SUCCESSIVE APPROXIMATION COUNTERAPPROXIMATION COUNTER

► Completes n-Completes n-bit bit conversion in conversion in just n clock just n clock periods.periods.

► It uses SAR to It uses SAR to find req value find req value of each bit by of each bit by trial & error.trial & error.

► This ckt is This ckt is more more versatile and versatile and superior to all superior to all ckts ckts discussed.discussed.

SAR

DAC

d1 d2 d8

+

-

start EOC

MSBd1 d2

d8

CLK

Va

Functional block diagram of the successive approximation ADC

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DUAL SLOPE(DUAL RAMP) DUAL SLOPE(DUAL RAMP) ADCADC

► It provides excellent It provides excellent noise reduction due the noise reduction due the integrator.integrator.

► Conversion time is Conversion time is longer.(2longer.(2nn-T)-T)

► These are suitable for These are suitable for accurate accurate measurements of slow measurements of slow varying signals(thermo varying signals(thermo couples,weighing couples,weighing scales,digital panel scales,digital panel meters & multimeters).meters & multimeters).

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POINTS TO REMEMBERPOINTS TO REMEMBER

► ADCs are either direct type or indirect type.ADCs are either direct type or indirect type.► Most direct type ADCs require DAC.Most direct type ADCs require DAC.► Flash converter is fastest but uses max Flash converter is fastest but uses max

hardware.hardware.► Counting type ADC has low speed but simple.Counting type ADC has low speed but simple.► Tracking ADC is simple but gives error when Tracking ADC is simple but gives error when

analog signal varies rapidly.analog signal varies rapidly.► Successive approximation ADCs are most Successive approximation ADCs are most

versatile.Most monolithic ADCs are of this type.versatile.Most monolithic ADCs are of this type.► Dual slope converters are suitable for precise Dual slope converters are suitable for precise

measurement of slowly varying signals.but measurement of slowly varying signals.but takes long conversion time.takes long conversion time.

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DAC/ADC SPECIFICATIONSDAC/ADC SPECIFICATIONS

►RESOLUTIONRESOLUTION►LINEARITYLINEARITY►ACCURACYACCURACY►MONOTONOCITYMONOTONOCITY►SETTLING TIMESETTLING TIME►STABILITYSTABILITY

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DAC/ADC SPECIFICATIONSDAC/ADC SPECIFICATIONS

►RESOLUTIONRESOLUTION: of a converter is the : of a converter is the smallest change in voltage which may smallest change in voltage which may be produced at the o/p (or i/p) of the be produced at the o/p (or i/p) of the converter.converter.

►Resolution of DAC =[VResolution of DAC =[VFSFS/(2/(2nn-1)] =1 LSB-1)] =1 LSB

►Resolution of ADC (for M volts i/p Resolution of ADC (for M volts i/p range) is [M/(2range) is [M/(2nn -1)]. -1)].

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DAC/ADC SPECIFICATIONSDAC/ADC SPECIFICATIONS

►LINEARITYLINEARITY:: it’s a measure of accuracy it’s a measure of accuracy and tells us how close the converter o/p and tells us how close the converter o/p is to it’s ideal transfer characteristics.is to it’s ideal transfer characteristics.

►Linearity error is usually expressed as a Linearity error is usually expressed as a fraction of LSB increment or percentage fraction of LSB increment or percentage of full-scale voltage.of full-scale voltage.

►A good converter exhibits an error of A good converter exhibits an error of less than +(1/2)LSB or –(1/2)LSB.less than +(1/2)LSB or –(1/2)LSB.

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DAC/ADC SPECIFICATIONSDAC/ADC SPECIFICATIONS

►ACCURACYACCURACY: absolute accuracy is the : absolute accuracy is the max deviation b/w actual converter o/p max deviation b/w actual converter o/p and ideal converter o/p.and ideal converter o/p.

►RELATIVE ACCURACYRELATIVE ACCURACY is the max is the max deviation after gain and offset errors deviation after gain and offset errors have been removed.have been removed.

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DAC/ADC SPECIFICATIONSDAC/ADC SPECIFICATIONS

►MONOTONOCITYMONOTONOCITY : a monotonic DAC is : a monotonic DAC is one whose analog o/p increases for an one whose analog o/p increases for an increase in digital i/p.increase in digital i/p.

► If a DAC has to be monotonic,error If a DAC has to be monotonic,error should be less than + or -0.5 LSB.should be less than + or -0.5 LSB.

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DAC/ADC SPECIFICATIONSDAC/ADC SPECIFICATIONS

►SETTLING TIMESETTLING TIME : represents the time : represents the time taken for o/p to settle within specified taken for o/p to settle within specified band +or -0.5 LSB. band +or -0.5 LSB.

► It depends on switching time of logic It depends on switching time of logic circuitary.circuitary.

►Settling time ranges from 100ns to 10 Settling time ranges from 100ns to 10 micro secs depending on wordlength micro secs depending on wordlength and type of ckt used.and type of ckt used.

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DAC/ADC SPECIFICATIONSDAC/ADC SPECIFICATIONS

►STABILITYSTABILITY : performance of converter : performance of converter changes with temp,age and power changes with temp,age and power supply variations.supply variations.

►so all relevant parameters as so all relevant parameters as offset,linearity and monotonocity must offset,linearity and monotonocity must be specified over the full temp and be specified over the full temp and power supply ranges.power supply ranges.

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SAR

DAC

d1 d2 d8

+

-

start EOC

MSBd1 d2

d8

CLK

Va

Functional block diagram of the successive approximation ADC

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21

Num

ber of clock cycles

required

Tracking devices faster

Successive approximation device faster

Number of increments

Input change