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ApplicationReport

SLAA172–March2003

BuildingaStableDACExternalReferenceCircuit

JoselitoParguian

ABSTRACT

ThisapplicationreportiswrittentohelpdesignengineersimplementastableDAC

externalreferencecircuitforTI’sdigital-to-analogconverters(DACs)thatdonothavean

portpointsoutthecriticalaspectsandtheimportanceofa

referencebufferdriver.

DataAcquisitionProducts

Introduction

Theperformanceofadigital-to-analogconverterdependsonvariousdesignpractices,mostof

,inreality,therearemore

subtleareasinDACdesigntobeconsideredtogetgoodperformancefromaDAC,depending

portfocusesononekeyspecificarea,whichisthe

ingastablereferencesourceisoneofthebiggest

eofitssimpleconcept,the

DACreferencecircuitcanbemisunderstood,overlooked,ly

thisisnotabigproblemforTIDACswithabuilt-inreferencesource,becauseitscomplexities

arealreadytakenintoconsiderationandthesolutionsarealreadyimplementedinternaltothe

assumingthattheinternalreferencefitstherequirementofthesystemapplication.

Unfortunately,notalldigital-to-analogconvertersincludeabuilt-inreferencecircuit,mainlydue

torestrictionsinlayoutdesignand,insomecases,becausetheinternalreferenceisjustnot

practicalfortheintendedDACapplication,whereanexternalsourceofreferenceisbettersuited

fortheapplication.

ForDACsthatrequireanexternalsourceofvoltagereference,carefulconsiderationsshouldbe

rceshouldatleastbecleanandstable

becauseitservesastheDAC’’sgeneralpurposeDACsaremeticulously

plannedandanyimpendingproblemsattributablebyinternalfactorsarecarefullytakeninto

ceandsenseconceptisimplementedtotakecareofthese

foreseeableerrorspertainingtothereferencecircuitoftheDAC.

TheForceandSenseCircuit

Thebasicconceptoftheforceandsensecircuitistoprovideareferencedriverthatassuresthe

DACastablevoltagebysensingtheIRdropcausedbythefixedinternalresistance(due

primarilytocopper,metalandpin)hereferencecurrent

iscodedependent,itvariesthroughthespanofthecode,therefore,theIRdropbecomeshard

totracksincethevoltageisnowvaryingconstantlyasthecodeischanged.

Simplysaid,thedigitalinputcodeoftheDACdictatestheamountofcurrentthathastobe

suppliedintotheV

REF

HinputpinandoutofV

REF

nvaryfromafewmicroampsto

approximately2mA,whichmakesthereferenceinputappearasavaryingloadtothereference.

1

SLAA172

TheSolution

BecausetheIRdropisnotafixedvoltage,itcannotbeeasilycompensatedbybuildingasimple

ution,thereferencesensepinisusedtoprovideafeedbackpathtothe

outsidesuch

op-ampisprovidedprimarilytomaintainastablevoltageforthereferenceandminimizeerrors

-ampbasicallydetectsthevoltagedropacrossthe

internalw

essence,theop-ampadjustsfortheIRdropinV

REF

ocess

maintainsa

configurationnowyieldsthebestlinearityperformanceoftheDAC.

AnexamplereferencedrivingcircuitisshowninFigure1belowforadual±10-Vreferencecircuit

7744pinoutprovidesreferenceforceandsensepinsso

thattheexternal

betterunderstand,considertheinternalwiringmetalresistanceoftheDACreferenceinputsas

shownbyR

W1

,R

W2

,R

W3

,andR

W4

.SincetheDACreferenceinputcurrentscanvaryfromafew

microampstoapproximately2mAdependingontheDACcode(seeFig.7and8),internalIR

dropsaregeneratedacrossR

W2

andR

W3

.TheseinternalDACIRdrops,ifleftuncompensated,

cancauseundesirablelinearityerrors(seeFig.3and4).Byusingthereferencebuffer

configurationshown,theoutputvoltageofeachop-ampbufferisself-adjustedforeachDAC

codetoensurethattheinternalDACreferencesensepointmaintainstheexternalbuffered

referencevoltage.

Theadditionofthe100-Ωresistorand1000-pFcapacitortoeachop-ampbufferconfiguration

actsasbothadecouplingcircuitandalsoprovidesnoisereductionfortheexternalreference.

The2200-pFcapacitorprovidesalowimpedancepathathighfrequenciestoclosetheop-amp

loopprovidingstablecompensation.

Thisconfigurationcanalsobeappliedfor±5-Vreferenceapplication.

2200pF

R

W1

DAC

REF

L

Sense

V

REF

L

Sense

100Ohms

1000pF

R

W2

R

W3

V

REF

L

-

+

-10V

V

REF

H

DAC

REF

H

Sense

R

W4

100Ohms

V

REF

H

Sense

1000pF

+

-

2200pF

+10V

pply±10-VBufferedConfiguration

2BuildingaStableDACExternalReferenceCircuit