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Analog to DigitalConverters
• Representing a continuouslyvarying physical quantity by
asequence of discrete numericalvalues.
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03 07 10 14 09 02 00 04
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Conversion Methods
• Ramp ADC
• Successive Approximation
• Flash Comparison
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The Comparator
– Most A-D converters use a comparatoras part of the
conversion process
– A comparator compares 2 signals A andB
• if A B the comparator output is in onelogic state !"# sa$%
• if B A then it is in the opposite state !sa$%
– A comparator can 'e 'uilt using an opamp (ith no
feed'ac)
-
+analogueinput
referencevoltage
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Digital-Ramp ADC
• Conversion from analog to digitalform inherentl$ involves
comparator
action (here the value of the analogvoltage at some point in
time iscompared (ith some standard*
•A common (a$ to do that is to appl$the analog voltage to one
terminal ofa comparator and trigger a 'inar$counter (hich drives a
DAC*
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Ramp ADC
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Successive Approximation
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Successive approximation
ADC• Much faster than
the digital rampADC 'ecause ituses digital logicto converge on
thevalue closest tothe input voltage*
• A comparator anda DAC are used inthe process*
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Successive-Approximaion A/!
Successive
Approximaion
"e#iser
!/A $onverer %re&
clock
analo#
inpu
!i#ial
'upu
!aa
A iniiali(aion) all *is &rom +e SA" are se o (ero) an,
conversion *e#ins * akin# ST"T line lo.
comparaor
ST"T
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Successive
Approximaion
"e#iser
!/A $onverer %re&
clock
analo#
inpu
!i#ial
'upu
!aa
comparaor
ST"T
Successive-Approximaion A/!
irs +e lo#ic in +e SA" ses +e S *i e3ual o 1
5 %6 "emem*er +a a 1 in *i 7 .ill *e +al& o&
&ull
scale
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Successive
Approximaion
"e#iser
!/A $onverer %re&
clock
analo#
inpu
!i#ial
'upu
!aacomparaor
ST"T
Successive-Approximaion A/!
T+e oupu o& +e SA" &ee,s +e !/A converer pro,ucin#
an
oupu compare, o +e analo# inpu vola#e & +e !/A
oupu is %in +en +e S is le& a 1 an, +e nex *i is
+en ese,
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Successive
Approximaion
"e#iser
!/A $onverer %re&
clock
analo#
inpu
!i#ial
'upu
!aacomparaor
ST"T
Successive-Approximaion A/!
& +e !/A oupu is 9 %in +en +e S is se o 0 an, +e
nex *i is se e3ual o 1
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Successive *is are se an, ese, * comparin# +e !A$ oupu o
+e inpu %in in an 8 sep process &or an 8-*i converer6 +a
resuls
in a vali, 8-*i *inar oupu +a represens +e inpu vola#e
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$:'$; P
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Successive approximaion searc+ ree
&or a 4-*i A/!
1111
1110
1101
1100
1011
1010
1001
1000
0111
0110
0101
0100
0011
0010
0001
!/A oupu
compare, .i+ %in
o see i& lar#er orsmaller
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Noe +a +e successive approximaion process akes a
&ixe, ime - 8 clock ccles &or +e 8-*i example
or #reaer accurac) one mus use a +i#+er *i converer)
ie 10-*i) 12-*i) ec Bo.ever) +e ,ep+ o& +e searc+ an,+e ime
re3uire, increases .i+ +e *i coun
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Flash Comparison
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& N is +e num*er o& *is in +e
oupu .or,C
T+en 2 N comparaors .ill *e
re3uire,
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Flash ADC
• +t is the fastest t$pe of ADCavaila'le# 'ut re,uires
acomparator for each value of
output*!. for -'it# 2// for 0-'it# etc*%
• Such ADCs are availa'le in +C formup to 0-'it and &"-'it
1ash ADCs!&"2. comparators% are planned*
• The encoder logic executes a truthta'le to convert the
ladder of inputsto the 'inar$ num'er output*
Illustrated is a 3-bit flash ADC with resolution 1
voltIllustrated is a 3-bit flash ADC with resolution 1 volt
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Flash ADC
• The resistor net and comparatorsprovide an input to the
com'inationallogic circuit# so the conversion time is
ust the propagation dela$ throughthe net(or) - it is not
limited '$ thecloc) rate or some convergence
se,uence*
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Resolution
• Misal)an 'ilangan 'iner dengan n 'itdiguna)an untu) me(a)ili
nilaianalog mulai dari " )e A
• There are 2n possi'le num'ers
• Resolution 3 A 4 2n
•FS 3 !2
n
-&% step si5e
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Resolution 6xample
• Temperature range of " 7 to ."" 7 to'e linearl$
converted to a voltagesignal of " to 2*/ 8# then digiti5ed
(ith an 8-bit A4D converter
• 2*/ 4 20 3 "*""90 8# or a'out &" m8per step
• ."" 7 4 20 3 &*2 7 per step
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Resolution 6xample
• Temperature range of " 7 to ."" 7 to'e linearl$
converted to a voltagesignal of " to 2*/ 8# then digiti5ed
(ith a 10-bit A4D converter
• 2*/ 4 2&" 3 "*""2::8# or a'out 2*:
m8 per step• ."" 7 4 2&" 3 "*29 7 per step
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;uanti5ation
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Conversion Time
• >a)tu $ang di'utuh)an untu)memperoleh sampel dari sin$al
analog danmenentu)an representasi numeri)*
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