AD540

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HighAccuracy,low Cost, FET-InputOperationalAmplifier

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Low Cost: $4.45 (100's,J) Low Ib:25pA max (K) Low Vas: 20m V max (K) Low Vas Drift:25/lVtC max (K) High DifferentialInput Voltage Capability::t20V PRODUCT DESCRIPTION The AD540 isthe lowest cost,high accuracy FET-input op amp availablewhich providesthe user with low biascurrents, high overallperformance, and accuratelyspecifiedpredictable operation.The deviceoffersmaximum biascurrentsas low as 25pA, offsetvoltagesbelow 20mV, maximum offsetvoltage driftbelow 2511V/C and a minimum gain of 50,000. All devicesarefreefrom latchup and are short-circuit protected. No externalcompensation isrequiredas the internal6dB/octave roll-offprovidesstabilityin closedloop applications, The AD540 issuggestedfor allFET~input amplifierrequire- ments where low costisof prime importance. Itsperformance iscomparable to modular FET op amps, but itsIC constructionreduces the pricesignificantlybelow that of modules. Allversionsof the AD540 aresuppliedin the hermetically- sealed,8-pin,TO-99 package. The AD540J and AD540K are specifiedfor 0 to +70°C applications,while the AD540S isofferedfor operation over the fullmilitarytemperature range of -55 to +125°C. PRODUCT HIGHLIGHTS 1. The AD540 op amp meets specifiedinput biascurrentand offsetvoltagevaluesafterfullwarm-up. Conventional high speed IC testingdoes not allow for self-heatingof the ch~p due to internalpower dissipationunder operating conditions. Information furnished by Analog Devices is believed to be accurate and reliable.However, no responsibility is assumed by Analog Devices for itsuse; nor for any infringements of patents or other rightsof third parties which may result from itsuse. No license isgranted by implica- tion or otherwise under any patent or patent rightsof Analog Devices. 2. The biascurrentsof the AD540 are specifiedas a maximum for eitherinput. Conventional IC FET op amps generally specifybiascurrentsasthe averageof the two input currents. 3. Unlike many FET-input op amps, the AD540 allows a maximum differentialinput voltageof :t20VDC. Standard "bootstrapped" FET-input op amps permit maximum differentialinput voltagesof only about :t3V. 4. Offsetnullingof the AD540 isaccomplished without affectingthe operatingcurrentof the FET's and results in relativelysmallchanges in temperature drift characteristics.The additionaldriftinduced by nullingis only :t2.0/lV1°C per millivoltof nulledoffset,compared to severaltimes thisfor other IC FET opamps. 5. The gain of the AD540 ismeasured with the offset voltagenulled. Nullinga FET-input op amp can cause the gain to decreasebelow itsspecifiedlimit.The gain of the AD540 isfullyguaranteed with the offsetvoltageboth nulledand unnulled. 6. To maximize the reliabilityinherentin IC construction, every AD540 isstoredfor 40 hours at+150°C, temperature cycled from -65 to +125°C and receivesa high impact shock test. Route 1 IndustrialPark;P.O. Box 280; Norwood, Mass. 02062 Tel: 617/329-4700 TWX: 710/394-6577 West Coast Tel: 213/595-1783 Mid-West Tel:312/297-8710 ~-- OBSOLETE

SPECIFICATIONS(typical@ +25°C and Vs = :!::15VDCunlessotherwisespecified) MODEL OPEN LOOP GAIN (Note 1) Vout = :!::10V,RL > 2kil TA = min tomax OUTPUT CHARACTERISTICS Voltage@ RL = 2kil,TA = min tomax Voltage@ RL = 10kil,TA = min tomax ShortCircuitCurrent FREQUENCY RESPONSE Unity Gain, Small Signal FullPower Response Slew Rate, Unity Gain INPUT OFFSET VOLTAGE (Note 2) vs.Temperature vs.Supply, T A = min to max INPUT BIAS CURRENT EitherInput (Note 3) INPUT IMPEDANCE Differential Common Mode INPUT VOLTAGE RANGE Differential(Note 4) Common Mode Common Mode Rejection,Vin = :!::10V POWER SUPPLY Rated Performance Operating Quiescent Current TEMPERATURE RANGE Operating,Rated Performance Storage PRICE (1-24) (25-99) (100-999) NOTE: 1. Open Loop Gain isspecifiedwith Vos both nulledandunnulled. 2. Input OffsetVoltage specificationsareguaranteed after5 minutes of operationatT A =+2SoC. 3. BiasCurrent specificationsareguaranteed after 5 minutes of operationatT A =+ 25°C. For higher temperatures, the current doubles every lOoC. 4. Defined as voltage between inputs, such that neither exceeds :!::lOVfrom ground. * Specificationssame as AD540]. Specificationssame as AD540K. Specifications and prices subject to change without notice. PIN CONFIGURATION TAB OUTUNE DIMENSIONS Dimensions shown in inchesand (mm). INiNEp~T~NG ( 2 REF PLANE 0.335 (8.501 0370] 0.02910.741 0.04511.141 0.028{0.711 0.034 10.861 OFFSET v- OFFSET TOP VIEW ~ v- TO.' TO-99 -7- AD540j AD540K AD540S 20,000 min 50,000 min 15,000 min 25,000 min ** :!::10Vmin (:!::13Vtyp) * * :!::12Vmin (:!::14Vtyp) * * 25mA * * 1.0MHz * " 100kHz * * 6.0V //lsec * * 50mV max 20m V max 75/lV/oC max 25/lV/oC max 50/lVtC max 400/lV/V max 300/lVN max 50pA max 25pA max ** 1010illl2pF * * 101 1illl2pF * * :!::20V * * :!::10Vmin (:!::12Vtyp) * * 70dB min * * :!::15V * * :!::(5to 18)V * * 7mA max (3mA typ) * * 0 to +70°C * -55 to +125°C -65 to +150°C * * $6.45 $8.95 $14.95 $5.70 $7.20 $11.95 $4.45 $5.95 $ 9.95 OBSOLETE

The AD540 isespeciallydesigned for low costapplications involvingthe measurement of low levelcurrentsor small voltagesfrom high impedance sourcesinwhich biascurrent can be a primary source of error.Input biascurrent contributesto errorin two ways: (1)in currentmeasuring configurationsthe biascurrentlimitsthe resolutionof a currentsignal;(2)the biascurrentproduces a voltageoffset which isproportionalto the value of input resistance(inthe caseof an invertingconfiguration)or source impedance (when the non-inverting"buffer" connection isused). The AD540 FET-input operationalamplifieris,therefore,of use where smallcurrentsare to be measured or where relativelylow voltagedriftisnecessarydespitelargevaluesof source resistance. APPLICATIONS CONSIDERATIONS BIAS CURRENTS. Most IC FET op amp manufacturers specifymaximum biascurrentsas the valueimmediately after turn-on. Since FET biascurrentsdouble every 10°C and since most FET op amps have casetemperature increasesof 15°C to 20°C above ambient, initial"maximum" readingsmay be only ~ of the true warmed-up value. Furthermore, most IC FET op amp manufacturers specifyIb as the averageof both input currents,sometimes resultingin twice the maximum bias currentappearing at the input being used. The totalresultis that 8X the expected biascurrentmay appear at eitherinput terminalin a warmed-up operatingunit. The AD540 specifiesmaximum biascurrentat eitherinput afterwarm-up, thus givingthe user the valueshe expected. IMPROVING BIAS CURRENT BEYOND GUARANTEED VALUES. Biascurrentscan be substantiallyreduced inthe AD540 by decreasingthe junctiontemperature of the devices. One technique to accomplish thisisto reduce the operating supply voltage.This procedure willdecreasethe power dissipationof the device,which willin turn resultina lower junctiontemperature and lower biascurrents.The supply voltageeffecton biascurrentisshown in Figure 1. ,.0 ..s>!;0.9 1 ,!z 0.8w a:- ~ i;;0.70 - ~ :'0.6 '". c> 0.5 w 0N '" ~ '" 0.4 :;; ~ 0.3z 0.2L0 5 10 SUPPLY VOLTAGE - ! Vs-Volts Figure1. NormalizedBiasCurrentvs.Supply Voltage Operation of the AD540K at:t5V reduces the warmed-up biascurrentby 70% to a typicalvalueof 8pA. A second technique isthe use of a suitableheat sink. Wakefield EngineeringSeries200 heat sinkswere selectedto demonstrate thiseffect.The characteristicbiascurrentvs. case temperature above ambient isshown inFigure 2. Bias currenthas been normalized with unity representingthe 25°C freeairreading.Note thatthe use of the Model 209 heat sink reduces warmed-up biascurrentby 60% to 10pA in the AD540K. 1.0 0 0.9 ..s'1~ !z- 0.8w" ~~ 0.7 0 !;( ~ ~ 0.6 '"0 c z 0.5w:>:N I- ~ § 0.4 :;; 0 ~,'f0.3 WAKEFIELD MODEL NUMBERS- 0.2 0 5 10 15 20 25 30 CASE TEMPERATURE ABOVE 25°C-oC Vs ! 15V Figure2. NormalizedBiasCurrentvs.CaseTemperature Both of thesetechniquesmay be used togetherfor obtaining lower biascurrents.Remember that loadingthe output can alsoaffectthe power dissipation. ~ 100I '"H I-Zwa: a:::>0 U)" jjj 10I-::>a.~ 25 50 AMBIENT TEMPERATURE TA-oC VS= ! 15V Figure3. InputBiasCurrentvs.Temperature n~1 -5 0 10 Figure4. BiasCurrentvs.Common Mode Voltage. VCM-Volts INPUT CONSIDERATIONS. Unlike some FET-input operationalamplifiers,the AD540 accommodates differential input voltagesof up to :t20V without any degradationin biascurrent.In certaintime-dependent applications,such as charge amplifiersand integrators,largedifferentialinput voltagestemporarilyoccur which may exceed the ratedvalue of a typicalFET op amp (approximately :t3V differential). -':/.. I / V OBSOLETE