CircuitNote
CN-0145
CircuitDesignsUsingAnalogDevicesProducts
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DevicesConnected/Referenced
ADuC7060/
ADuC7061
ARM7-BasedMicrocontrollerwithDual
24-BitΣ-ΔADCs,VDAC,andPWM
ADP17202.5VOutputLinearRegulator
OP193/OP293LowPowerOpAmp
ADR280LowPower1.2VReferenceSource
4mA-to-20mALoop-PoweredTemperatureMonitorUsingthe
ADuC7060/ADuC7061PrecisionAnalogMicrocontroller
Rev.0
“CircuitsfromtheLab”fromAnalogDeviceshavebeendesignedandbuiltbyAnalogDevices
engineers.Standardengineeringpracticeshavebeenemployedinthedesignandconstructionof
eachcircuit,andtheirfunctionandperformancehavebeentestedandverifiedinalabenvironment
atroomtemperature.However,youaresolelyresponsiblefortestingthecircuitanddeterminingits
suitabilityandapplicabilityforyouruseandapplication.Accordingly,innoeventshallAnalog
Devicesbeliablefordirect,indirect,special,incidental,consequentialorpunitivedamagesdueto
anycausewhatsoeverconnectedtotheuseofany“CircuitfromtheLab”.(Continuedonlastpage)
OneTechnologyWay,P.O.Box9106,Norwood,MA02062-9106,U.S.A.
Tel:781.329.4700www.analog.com
Fax:781.461.3113?2010AnalogDevices,Inc.Allrightsreserved.
CIRCUITFUNCTIONANDBENEFITS
Thiscircuitprovidesasimplehighlyintegratedtemperature
monitorsolutionthatcaninterfacetoa4mA-to-20mAhost
controller.Thiscircuitprovidesaverylowcostsolutionto
temperaturemonitoringbecausesomuchofthecircuit
functionalityisintegratedontheADuC7060/ADuC7061
precisionanalogmicrocontroller,includingdual24-bitΣ-Δ
ADCs,theARM7processorcore,andtheDAC/PWMfeatures
forcontrollingthe4mA-to-20mAfeedbackcircuit.
TheperformanceoftheADCsandotheranalogcircuits
withintheADuC7060/ADuC7061issuperiortocompetitive
microcontrollersthathaveintegratedanalogcircuits.The
circuitpresentedhererepresentsthemostcost-effective,lowest
power,lowestboardareasolutioncomparedtousingdiscrete
ADCsandaseparatemicrocontroller.Becauseofitshighlevel
ofintegrationandlowpower,theADuC7060/ADuC7061can
operatedirectlyfromthelooppowerinthe4mA-to-20mA
application.WiththeARM7corerunningat640kHz,the
primaryADCactiveandmeasuringtheexternalRTDtempera-
turesensor,andthePWMcontrollingthe4mA-to-20mA
feedbackcircuit,theentirecircuittypicallyconsumes3.15mA.
Furtherdetailsonthepowerconsumptionareprovidedinthe
CircuitDescriptionsection.
DVDD
IEXC0
ADC0
ADC1
V
REF
+
V
REF
–
ADC2
ADC3
DAC
PWM
DGND
0.01μF
0.1μF
0.01μF
AVDDDVDD
AVDDDVDD
0.1μF0.1μF
AGNDDGND
ADuC7061
R
REF
5.62k?
(0.1%)
RTD
10?
10?
PTRTD:ENERCORP
PCS-11503.1,100?
PWR
AVDD
ADR280
GNDREFOUT
LOWPOWEROPTION:
USEADR280
INSTEADOFDAC
ADJ
ADP1720
OUTIN
47k?
0.1μF
47k?
R2
100k?
V
IN
V
RLOOP
V
REF
=1.2V
OP193
MEASUREV
R12
VIAADC2.
OPTIONAL,USEDFOR
FEEDBACKCONTROLOF
LOOPCURRENT
R1
100k?
R
LOOP
47.5?
10k?
BC548
10?
AVDDDVDD
BEAD
4.7μF4.7μF
R3
57.6k?
R4
60.4k?
LOOP+
LOOP–
V
R12
4VTO28V
BEAD:TAYIOYUDEN
BK2125HS102-T
1k?@100MHz
2.5V
08836-
001
Figure1.ADuC7061Controlling4mA-to-20mALoop-BasedTemperatureMonitorCircuit.(SimplifiedSchematic:AllConnectionsandDecouplingNotShown)
CN-0145CircuitNote
Rev.0|Page2of4
Duringtheintervalsbetweentemperaturemeasurements,the
ADCandtheRTDexcitationcurrentsourcescanbeturnedoff
tofurtherreducepower.
The100?PtRTDisanEnercorp#PCS11503.1.Thecomplete
circuitmeetsabetterthan±1°Cspecification.Thetemperature
rangeofthisRTDis?50°Cto+130°C.TheRTDisavailableina
1206sizeSMDpackageandhasatemperaturecoefficientof
0.385?/°C.
TheprimaryADCwithintheADuC7060/ADuC7061hasa
peak-to-peaknoise-freecoderesolutionofgreaterthan18bits.
ThePWMbasedDACoutputprovides12effectivebitsof
resolution.Theperformanceoftheoverallcircuitisdiscussed
intheCircuitDescriptionsection.
DuetothesmallformfactoroftheADuC7061,a5mm×5mm
32-LFCSPpackage,theentirecircuitwillfitonanextremely
smallPCBthusfurtherreducingcost.
Thefocushereisprimarilythe4mA-to-20mAinterface
aspectofthecircuit.FordetailsontheRTDsensorinterface
totheADCandonlinearizationtechniquesoftheRTD
measurements,seeApplicationNoteAN-0970andCircuit
NoteCN-0075.
CIRCUITDESCRIPTION
ThecircuitispoweredviatheADP1720linearregulator
(adjustableversion),whichregulatestheLoop+supplyto2.5V
fortheADuC7060/ADuC706161,theOP193opamp,andthe
optionalADR280reference.
The4mA-20mAfeedbackcircuitisprimarilycontrolledbythe
ADuC7060’son-chip16-bitPWM(pulsewidthmodulator).
ThedutycycleofthePWMisconfiguredinsoftwaretocontrol
thevoltageacrossthe47.5?RLOOPresistor,whichinturnsets
theloopcurrent.NotethetopofRLOOPconnectstothe
ADuC7060ground.ThebottomofRLOOPconnectstotheloop
ground.Becauseofthis,thecurrentduetotheADuC7060/
ADuC7061,ADP1720,ADR280,andOP193,plusthecurrent
setbythefilteredPWMoutput,flowsacrossRLOOP.
VREFissuppliedbytheADR280,a1.2Vprecisionreference.
Alternatively,theon-chipDACoftheADuC7060/ADuC7061
couldbeconfiguredtoprovidethe1.2Vreference,butthis
wouldconsumeextrapowerbyenablingtheinternalDAC.
ThevoltageatthejunctionofR1andR2canbeexpressed:
VR12=(VRLOOP+VREF)×R2/(R1+R2)–VRLOOP
Aftertheloopsettles:
VIN=VR12
SinceR1=R2:
VIN=(VRLOOP+VREF)/2–VRLOOP=VREF/2–VRLOOP/2
VRLOOP=VREF–2VIN
Full-scalecurrentwillflowwhenVIN=0,atwhichpoint
VRLOOP=VREF.Hence,full-scalecurrentisVREF/RLOOP,or≈24mA.
WhenVIN=VREF/2,nocurrentwillflow.
TheOP193amplifierimpedanceatVINishighandwillnotload
thePWMfilteredoutput.Theamplifieroutputvariesonlya
smallamountaround0.7V.
Performanceattherangeextremes,0mAto4mA,and20mA
to24mA,isnotcritical;therefore,theopampdoesnotrequire
goodperformanceatthesupplyrails.
AbsolutevaluesofR1andR2arenotcritical.However,note
thatmatchingofR1andR2isimportant.
Alsonotethepossibilityofmeasuringthevoltageatthe
pointmarkedVR12byusinganinputchannelofADC0on
theADuC7060/ADuC7061.ThisADCmeasurementcanbe
usedasfeedbackforthePWMcontrolsoftwaretoadjustthe
4mA-to-20mAcurrentsetting.
TheprimaryADCoftheADuC7060/ADuC7061measuresthe
voltageacrosstheRTD.TheRTDisexcitedbytheon-chip
excitationcurrentsource,IEXC0.Itisrecommendedthatthe
excitationcurrentisconfiguredfor200μAtoconservepower,
anditshouldbeswitchedoffbetweenmeasurements.The
internalPGAonthefrontendoftheprimaryADCisconfigured
foreitheragainof16or32.ThereferencesourcefortheRTD
measurementmaybeeithertheinternalreferenceortheexter-
nal5.62k?referenceresistor.Selectingtheexternalresistorwill
conservefurtherpower.ForfurtherdetailsontheRTDtoADC
interfaceandonlinearizationtechniquesfortheADCresults
seeApplicationNoteAN-0970andCircuitNoteCN-0075.
Powerrequirementsforthecircuitdependonwhetherthe
moduleispowereddirectlyfromthe4mA-to-20mAloop
supplyoractivelyfroma4-wireloopwherethemoduleis
poweredseparately.Forthisdocument,wewillassumethe
temperaturemonitormoduleislooppowered,thereforethe
totalcurrentconsumptionofthemoduleshouldnotexceed
approximately3.6mA.
Tofacilitatelowpoweroperation,theADuC7060/ADuC7061
coreoperatingspeedcanbereducedbyprogrammingthe
internalPOWCON0register.Thisallowsthetopfrequencyof
10.28MHztobedividedinbinarymultiplesof2to128.During
thetests,aclockdividevalueof16wasused,givingacorespeed
of640kHz.TheprimaryADCwasenabledwithagainof32.
ThePWMwasalsoenabled.Allotherperipheralswere
disabled.
Forourcircuitandtestsetup,Table1givesabreakdownofthe
IDDcurrentconsumption,andTable2givesthecurrentsforthe
variousperipherals.
CircuitNoteCN-0145
Rev.0|Page3of4
Table1.TypicalIDDValuesforComponentsofTemperature
MonitorCircuit
Component
IDDValue
at25°C
IDDValue
at85°C
ADuC7060/ADuC7061
ADC0On,Gain=32,FADC=100Hz
CPUspeed=640kHz
(POWCON0=0x7C)
PWMOn.PWMCON1=0x100
ExternalreferenceselectedbyADC0.
Allotherperipheralsoff.
Note:Addexcitationcurrentvalueto
thisfigure.Typicalvalueis200μA.
2.45mA
0.2mA
2.74mA
0.2mA
ADR280,1.2VReference12μA15μA
ADP1720,2.5VOutputLinearRegulator200μA300μA
OP193,LowPowerOpAmp15μA25μA
RemainingCircuitry50μA50μA
TotalCurrentLessExcitationCurrent2.73mA3.13mA
TheDNLgraphinFigure2showsabetterthan0.6LSB
typicalperformanceinthecritical4mA-to-20mArange.
Thesetestswereconductedwithasecondorderfilteron
thePWMoutput.Two47k?resistorsandtwo100nF
capacitorswereusedasshowninFigure1.
LOOPCURRENT(μA)
DNL(LSBs)
08836-
002
Figure2.TypicalDNLperformanceofcircuit
TheperformanceofthePWMbasedoutputcouldbe
enhancedbyusingtheADCtomeasurethevoltagepoint
V
R12
andotherpointsonthecircuit.Thismethodof
feedbackcouldbeusedtocalibratethePWMoutputfor
greaterprecision.
NotethatthePWMcircuitwasonlyusedtosettheoutput
voltagebetween0Vand600mV,hencethereducednumberof
codes.Code0aboveisnotimportantasitrepresentsavalue
greaterthan24mA.
ForADCmeasurementperformance,seeAN-970andCN-0075
andtheADuC7060/ADuC7061datasheet.
Table2.TypicalIDDValuesforDifferentPeripheralsonthe
ADuC7060/ADuC7061
PeripheralofADuC7060/61IDDValue,Typical,25°C
ARM7Core@10.24MHz
5.12MHz
2.56MHz
1.28MHz
640kHz
320kHz
160kHz
80kHz
5.22mA
4.04mA
2.7mA
2mA
1.674mA
1.5mA
1.42mA
1.38mA
PrimaryADC,G=1
G=4
G≥128
30μA
440μA
630μA
AuxiliaryADC350μA
DAC330μA
PWM340μA
SPI40μA
UART200μA
COMMONVARIATIONS
InsteadofusingthePWM,theon-chipDACcouldbeusedfor
thesamefunction.Theadvantagewouldbequickerresponse
timeandacheapersolution.TheADR280referencewouldnot
berequiredinthiscase.
TheOP90canbeusedinsteadoftheOP193.
LEARNMORE
ADIsimPowerDesignTool,AnalogDevices.
CN-0075CircuitNote,USBBasedTemperatureMonitorUsing
theADuC7061PrecisionAnalogMicrocontrollerandan
ExternalRTD.AnalogDevices.
Kester,Walt.SensorSignalConditioning,AnalogDevices,1999,
Chapter7,TemperatureSensors.
Kester,Walt.SensorSignalConditioning,AnalogDevices,1999,
Chapter8,ADCsforSignalConditioning.
Looney,Mike.AN-0970ApplicationNote,RTDInterfacingand
LinearizationUsinganADuC706xMicrocontroller,Analog
Devices.
MT-022Tutorial,ADCArchitecturesIII:Sigma-DeltaADC
Basics,AnalogDevices.
MT-023Tutorial,ADCArchitecturesIV:Sigma-DeltaADC
AdvancedConceptsandApplications,AnalogDevices.
MT-031Tutorial,GroundingDataConvertersandSolvingthe
Mysteryof"AGND"and"DGND",AnalogDevices.
MT-101Tutorial,DecouplingTechniques,AnalogDevices.
CN-0145CircuitNote
Rev.0|Page4of4
DataSheetsandEvaluationBoards
ADP1720DataSheet
ADR280DataSheet
ADuC7060DataSheet
ADuC7060EvaluationBoardandSoftware
ADuC7061DataSheet
ADuC7061EvaluationBoardandSoftware
OP193DataSheet
OP293DataSheet
OP90DataSheet
REVISIONHISTORY
2/10—Revision0:InitialVersion
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CN08836-0-2/10(0)
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