运放放大器PCB布局

MicroShip Op Amp Precision Design: PCB Layout TechniquesTEMPERATURE CHANGE IN THE AXIALDIRECTIONIn this illustration, temperature changes horizontally inFigure 2(along the resistor's axial direction), but doesThis section includes several general techniques thatnot change in the normal direction (vertically Let'sprevent the appearance of large temperature gradientsassume ov on the left copper trace, +125c atIt critical componentsJunction #1, a temperature gradient of 10C/in(0. 394C/mm) from left to right(0 in the verticaldirection) and a 1206 SMd resistor.When a PCBs thermal gradient is mainly caused byThe resistor is 0. 12 inches long (3.05 mm) andcomponents attached to it then find components that0.06 inches wide (1.52 mm). assume the end caps aredissipate less power. This can be easy to do(e.gabout 0.01 inches long(0. 25 mm) and the metal film ischange resistors) or hard(change a PICmicroabout 0. 10 inches long (2. 54 mm). The results aremicrocontrollershown in Figure 4Increasing the load resistance, and other resistorvalues, also reduces the dissipated power. Choose8.999mVlower power supply voltages, where possible, to furtherreduce the dissipated power4.000m∨14.010m0 mv0.038mVChanging the direction that heat flows on a PCB, or inits immediate environment, can significantly reducetemperature gradients. The goal is to create nearlyconstant temperatures in critical areasALTERNATE HEAT PATHS+1250°C+126.1C+125.1°C126.2°CAdding heat sinks to parts that dissipate a lot of powerwill redirect the heat to the surrounding air. One form ofheat sink that is often overlooked is either groundplanes or power planes in the PCB; they have theJunction#110400014000advantage of making temperature gradients on a PCBlower because of their large(horizontal) thermalJunction #24|-1001-5.001conductivityJunction#3410115011Adding a fan to a design will also redirect heat to theJunction#4-10-4.048-14.048surrounding air, which reduces the temperature dropon the PCB. This approach, however, is usuallyavoided to minimize other design issues (randomThus, the temperature gradient of10°cn(1.2°ctemperature fluctuations, acoustic noise, power, costincrease from left to right) caused a total of -38 pV toetc.). It is important to minimize air (convection)currents near critical components. Enclose either theappear across this resistor. Notice that adding thearts with significant temperature rise, or the criticalsame temperature change to all junction temperatureswill not change this resultparts. Conformal coating may also helpShifting all of the junction temperatures byISOLATION FROM HEAT GENERATORSthe same amount does not change theIt is possible to thermally isolate critical areas on thetemperature gradient. This means that thePCB. Regions with little or no metal act like a goodvoltage drop between any two points in thethermal insulator. Signals that need to cross thesecircuit using the same conductive materialregions can be sent through series resistors, which willis the same (assuming we're within thealso act as poorly conducting thermal elementslinear region of responsePlace heat sources as far away from critical points aspossible. Since many heat sources are in the externalenvironment, it can be important to place these criticalpoints far away from the edges of the PCBComponents that dissipate a lot of power should bekept far away from critical areas of the PCBO 2009-2012 Microchip Technology IncDS01258B-page 3Low profile components will have reduced exposure tothe external environment. They may have theadditional advantage of reduced electrical crosstalkThermal barriers, such and conformal coating and PCBThis section focuses on methods that minimize theenclosures, can be helpful too. They usually do noteffects of a given temperature gradient. They can behave to be added unless there are other compellingpowerful aids in improving a design because they tendreasons to do soto be low costIn some applications, sudden changes inCritical points, that need to have the same totalthermoelectric voltages can also be a concernthermoelectric voltage, should use the sameconductive material. For example, the inputs to an opAvoid power-up and power-down thermal transientamp should connect to the same materials. The PCBproblems by minimizing the currents drawn duringtraces will match well, but components with differentthese times. Also, reducing the times can helpconstructions may be a source of troubleQuick changes in voltages at heavy loads can beanother source of concern if the load cannot be madeIt is possible to find combinations of metals and soldersthat have low seebeck coefficients While thislighter, then isolation is usually the best approach tosolving this problemobviously reduces voltage errors, this can becomplicated and expensive to implement inmanufacturingPlace critical components so that their current flowfollows constant temperature contour lines; thisminimizes their thermoelectric voltages. Figure 5shows an inverting amplifier that will be used toillustrate this concept; RN: RG and R are the criticalcomponents in this circuitRRVIN OOUTDDCFigure 6 shows one implementation of this conceptConstant temperature contour lines becomereasonably straight when they are far from the heatsource. Placing the resistors in parallel with these linesminimizes the temperature drop across themHeat source1>○TemperatureContoLinesVIN-RNETA-U1 VDDRGLIRNELVOuTDS01258B-page 4C 2009-2012 Microchip Technology IncThe main drawback to this technique is that the contourThe output has a simple relationship to the inputS (VINlines change when the external thermal environmentand the three VTHx and VTHy sources)changes. For instance, picking up a PCB with yourhands adds heat to the PCB, usually at locations notaccounted for in the designIt is possible to cancel thermoelectric voltages whenthe temperature gradient is constant. Severalexamples will be given to make this technique easy toWhen the gain(Gn)is high, the thermoelectric voltageunderstandcontributes little to the output error. This layout may begood enough in that case. Notice that the cancellationTRADITIONAL OP AMP LAYOUT APPROACHbetween RN and Re is critical to good performanceFigure 7 shows a non-inverting amplifier that needs toWhen the gain is low, or the very best performance ishave the resistors' thermoelectric voltage effectdesired, this layout needs improvement. the followingminimized. The traditional approach is to lay out thesections give guidance that helps achieve this goalinput resistors(RN and RG) close together, at equaldistances from the op amp input pins and in parallelSINGLE RESISTOR SUBSTITUTIONSA single resistor on a PcB will produce a thermoelectricvoltage, as discussed before. Replacing that resistor- VDDwith two resistors that are properly aligned will cancelthe two resulting thermoelectric voltagesFigure 9 shows the original resistor and its model on⊥MMAOUTthe top, and a two series resistor substitution and itsRdel on the bottomThe original resistor has a thermally induced voltageTHx that is based on the temperature gradient in theFigure 8 shows one layout that follows the traditionalX-direction(horizontal)approach, together with a circuit diagram that includesthe resulting thermoelectric voltages(VTHx and VTHvThe two resistors on the bottom have thermally inducedoltages Vth that are based on the temperatureVTHX is positive on the right side of a horizontallyoriented component (e.g, RN). VTHy is positive on thegradient in the y-direction(vertical); they are equalbecause the temperature gradient is constant and thetop side of a vertically oriented component (e.g, REresistor lengths are equal. Due to their parallel alignment, these voltages cancel; the net thermally inducedRFVDDvoltage for this combination (as laid out)is zeroRGRN口VINR1RTHxVOUTCURIA RIBTHTTHxW<+W+OVOUTR11BRG VTHXRWhereRA=R1A1BrecommendedThe orientation of these two resistors(R1Aand Rib)is critical to canceling thethermoelectric voltagesO 2009-2012 Microchip Technology IncDS01258B-page 5Figure 10 shows the original resistor and its model onthe top, and a two parallel resistor substitution and itsVDDmodel on the bottomRERGo口The original resistor has a thermally induced voltageRNLIVTHx that is based on the temperature gradient in theViNVOUTX-direction(horizontal)The two resistors on the bottom have thermally inducedvoltages VTHy that are based on the temperature gradient in the y-direction(vertical); they are equal becausethe temperature gradient is constant and the resistorRN VTHXgths are equal. due to theirbecause R1A=R1b, these voltages produce currentsO VOUTThe net thermally ingge for thiRVcombination (as laid out)is zeronot recom-mendedR1M(一The output has a simple relationship to the inputs ( VINand the three VTHx sourcesR1AT乙RBRR18WhereR1A=R18=2RWhen the gain (GN) is high, the thermoelectricvoltage's contribution to the output error is relativelysmall. This layout may be good enough in that caseNotice that the cancellation between RN and RG iscriticalNON-INVERTING AMPLIFIERWe have a better layout shown in Figure 13Figure 11 shows a non-inverting amplifier. We will startRecognizing that subtracting the last term in the Vouwith the layout in Figure 12 (previously shown inequation(middle equation in Equation 3) completelyFigure 6). The resistor R is horizontal so that all of thecancels the thermoelectric voltages, the resistor REthermoelectric voltages may be(hopefully)cancelledwas oriented in the reverse directionThe model shows how the thermoelectric voltagesmodify the circuitREDDRGc口ICOURNLC1ViN.VOUTRNO VOuTRGF-URN VTH-W(+M+oVoUTRTHDS01258B-page 6C 2009-2012 Microchip Technology IncWith the reversed direction for Re, the output voltage isnowRFIVDDVIN RGIRGVREF VOUTHXGR1FO VREFThe cancellation between rn and rg is critical to thislayout; the change to REs position is not as importantDD○INVERTING AMPLIFIERInverting amplifiers use the same components as noninverting amplifiers, so the resistor layout is the same+Msee Figure 14gVTHxRE VTHXREVDDVINThe output has a simple relationship to the inputs ( VINF and the four VTHx sources)VOUTRR1VIN O-I↑VourDDCDIFFERENCE AMPLIFIERINSTRUMENTATION AMPLIFIER INPUTFigure 15 shows a difference amplifier. This topologySTAGEhas an inherent symmetry between the non-invertingFigure 17 shows an instrumentation amplifier inputand inverting signal paths, which lends itself tostage, which is sometimes used to drive the input of acancelling the thermoelectric voltages. Figure 16differential ADC. While this is a symmetrical circuit,shows the layout and its modelachieving good thermoelectric voltage cancellation onthe PCB presents difficulties. It is best to use a dual opamp, so the re resistors have to be on both sides of theRGR1op amp, while RG connects both sides; the distancesWbetween resistors are too large to be practical(thermalgradient is notNDRR1RRNROUTF1Bnot recommendedO 2009-2012 Microchip Technology IncDS01258B-page 7The solution to this problem is very simple, split RG intoThe VTHx sources cancel, for the reasons alreadytwo equal series resistors so that we can use thegiven, so the differential output voltage is simplynon-inverting layout (see Figure 13)on both sides ofthe dual op amp. Each side of this amplifier will cancelts thermoelectric voltages independently; this is shownin Figure 18 and Figure 191AO VDDMODIFICATIONS FOR NON-CONSTANTRCTEMPERATURE GRADIENTSTemperature gradients are never exactly constant. OneRG/2cause is the wide range of thermal conductivities(e. gOUTtraces VS FR4)on a PCB, which causes complex tem-RG/2 Rperature profiles. Another cause is that many heatsources act like point sources, and the heat is mainlyconducted by a two dimensional object( the PCB); theRtemperature changes rapidly near the source andVUBslower far awayNon-constant temperature gradients will cause thetemperature profile to have significant curvature, whichcauses all of the previous techniques to have less thanperfect success. Usually, the curvature is small enoughso that those techniques are still worth using SomeC1times, additional measures are needed to overcome[RFthe problems caused by the curvatureRFRG/2LIRNELIRG/2One method is to minimize the size of critical compo口]RNnents(e.g, resistors). If we assume that temperaturehas a quadratic shape, then using components that are当朝half as long should reduce the non-linearity error toabout one quarter the sizeAnother method is to keep all heat sources and sinksCfar away from the critical components. This makes theDD Ocontour lines straighter.1AThe contour lines can be deliberately changed inshape. Using a ground plane (also power planes)toconduct heat away from the sources helps equalize theRo/2THxRE VTHX NOUTtemperatures, which reduces the non-linear errorsAdding guard traces or thermal heat sinks thatRG/2THX Rsurround the critical components also help equalize thetemperaturesWe can modify the sizes of the critical components sothat the cance lation becomes closer to exact. In orderWto match resistors for instance we need to make surethat the temperature change across each of thematched resistors is equal; see Figure 20 for an illusDS01258B-page 8C 2009-2012 Microchip Technology Inc(△T=+10°c+25.0°C+26.0°CWhile the techniques previously shown are a great helpin producing an initial PCB layout, it is important toverify that your design functions as specified. Thissection includes methods for measuring the responseof individual components and of a PCB. With thisinformation, it is possible to make intelligent designtweaksTEMPERATUREThere are many ways to measure temperature [4, 5, 6We could use thermocouples, RTDs, thermistorsdiodes, ICs or thermal imagers(infrared cameras)tomeasure the temperatureFigure 21 shows a circuit based on the mcp9700 Ic+25.1°C+26.1C+26.3°Ctemperature sensor because all of the components(AT=+1.0°C)draw very little current, their effect on PCB temperaturewill be minimal. There is enough filtering and gain tomake VOuT easy to interpret. This circuit can be built ona very small board of its own, which can be easilylaced on top of the pcb of interestVon=5.0VDU 100 nFCMCP97001.0μFSensR100k100nFDDR113k9RR124kQ100kQ1.00MQ1.00kQ⊥1.0μF22 nFThe MCP9700 outputs a voltage of about 500 mv plus10.0 mV/C times the board temperature(TPCB, inC)The amplifier provides a gain of 10 V/ centered on500 mV(when VDD =5.0V), givingO 2009-2012 Microchip Technology IncDS01258B-page 9Since the MCP9700 outputs a voltage proportional totemperature, VOuT needs to be sampled by an ADCRFIthat uses an absolute voltage referenceVDD 10 OhmThe absolute accuracy of this circuit does not supportRGVDD/2our application, so it is important to calibrate the errorsLeave the PCB in a powered-off state(except for thetemperature sensor) for several minutes. MeasureRF□VREFVOUT1WouT at each point, with adequate averaging. Thechanges in VouT from the calibration value representsRTHXRTHxthe change in TpcB from the no power conditionW(+,WO VREFTHERMAL GRADIENTSDD9 CTo measure thermal gradients, simply measure theVop/20temperature at several points on the PCB. the gradientis then the change in temperature divided by thedistance between points. More points give betterM-(OUTresolution on the gradient, but reduce the accuracy ofRG VTHXRTHxthe numerical derivativePACKAGE THERMAL RESISTANCEThe way to estimate the temperature (T inC)of a component is to multiply its dissipated power(P in W)by thepackage thermal resistance (OJA inC/W). This helpsWe can also place a short across one component, of aestablish temperature maximum pointsmatched pair, with a copper trace on the PCBFigure 23 shows a non-inverting amplifier layout thatTo measure OJA, when it is not given in a data sheet,shorts RN (with a copper trace)to unbalance theplace the temperature sensor at the IC(usually, athermoelectric voltages. It also connects the two inputsthermocouple between the package and the PCB)together, and uses larger resistors, to simplifyInsert a small resistor in the supply to measure themeasurements (VOUT =VDD/2, ideally ) The short issupply current when on(pp in A) Measure the changein temperature(△Tin°C) between the off and oneasily removed from the PcBconditions, supply voltage (VDp in V)and IDD. ThenRFVDD 10 OhmRGVDDI2(RN)ETC1VOUT1WTHERMOELECTRIC VOL TAGESVDD♀CRTHxThe easiest way to measure thermoelectric voltages isto thermally imbalance a difference amplifier circuitThe thermoelectric voltages have a polarity that adds(instead of cancelling) in Figure 22(compare to∧(+-)-VouTFigure 16). The differential input voltage is zero, andthe resistors are larger to emphasize the thermoelectricRG VTHXRE VTHXvoltagesThe large resistor on the right of the layout can be usedto generate heat, causing a horizontal temperaturegradient at the resistors RG and Re. The gain(G)is setWith the unbalance we now have the thermoelectrichigh to make the measurements more accurate. Thevoltagethermoelectric voltage(VTHx) across one resistor isDS01258B-page 10C 2009-2012 Microchip Technology Inc