REV B AN APPLICATION NOTE One Technology Way P - PDF document

REV. BAPPLICATION NOTE One Technology Way ¥ P.O. Box 9106 ¥ Norwood, MA 02062-9106 ¥ 781/329-4700 ¥ World Wide Web Site: http://www.analog.comAn IC Amplifier UserÕs Guide to Decoupling, Grounding,This quotation seemed a proper note with which to beginon a subject that has made monkeys of most of us at onelike to state one general principle and then conductors through which currents flow and developfinite voltages.inputs and an output as shown in Figure 1. A quick which the amplifier has a reference. Since the ideal op A Figure 1.Conventional "Three Terminal" Op Amplines really do present a low impedance at all frequenciesincluding noise, poor transient response, and oscillation. nded, t Figure 2.Simplified ÒRealÓ Op Amppopular integrated circuit families. Details vary, but thebasic 4136, 503, 515, and other integrated circuit amplifiers. Theby PNP transistors in Figure 2. The current is then con-verted from differential to single-ended form by a currentmirror that is connected to the negative supply rail. Theand power output stage that is connected as an The integrator controls the open-loop frequency response,and its capacitor may be added externally, as in the by a couple of base emitter supply voltage is changed abruptly, the integrator ampli- the output to follow the change. When theentire amplifier is in a closed-loop configuration theresulting error signal at its input will tend to restore theof the amplifier. As a result, an amplifier of this type maybut the negative supply rejection is fundamentally limitedinput tsupply rejection will approach zero for signals at cies above the high-speed, high-level circuits can Òtalk toÓ low-levelcircuits through the common impedance of the negativewith the negative stive supply routput. The result in an apparent asymmetry in the amplifier transientpositive pulse test may give the amplifier transient a negative pulse test may actually give look at your negative supply line transient response, insteadinductor to add a high-frequency component to the normallyproduce an undesirable glitch.Figure 3 illustrates three possible configurations for nega-negative signal current path through the decoupling andon the ground lines is similar to the ÒglitchÓ on the nega-tive supply bus. Depending upon how the feedback andcaused by the decoupling capacitor may be larger than thedisturbance it turbance of VÐ and ground buses. The high-frequencycomponent of glitch on the negative supply without disturbing input oroutput signal paths. When the load situation is more com-plex, as in 3c, a little more thought is required. If the ampli-fier is driving a load that goes to a virtual ground, the actualload current does not return to ground. Rather, it must besupplied by supply of the REV. B CURRENT GROUND Figure 3a.Decoupling for Negative Supply Ineffective CURRENT COMMON CAPACITOR Figure 3b.Decoupling Negative Supply Optimized for NPN OUTPUT FREQUENCYSIGNALCURRENTPATH Figure 3c.Decoupling Negative Supply Optimized forhow small resistors can be added to lower the Q of thejingle to a small damped signal at the op amp supply termi- Figure 4.Damping Parallel Decoupling Resonancestive supply when the system is intended to handle onlyFigure 5 is a more detailed version of Figure 2, showing thepower supply and wiring impedance lumped together as asingle constant. The amplifier is connected as a unity gainfollower. This makes a closed-loop path from the amplifier VÐÐ+ Figure 5.Instability Can Result from Neglectingtive supply if there is any substantial lead inductance in therather than to some vague ÒgroundÓ connection.Up to this point we have not considered decoupling the posi- requires 2 apply equally to this second class, but should be appliedis used to bypass an level has poor high-frequency response. Figure 6 illustrates howthis may be done. Each of the amplifiers shown is really ainput to a single-ended signal. The signal drives an inter-bandwidth. The output of this stage drives an integrator-the available integrator response. A feed-forward capacitorpermits high-frequency signals to bypass this stage. As aquency rverting input of the intermediate stage. Note that the sec-ond stage is not an integrator, as it may appear at firstglance, but actually INTEGRATOR AND AMPLIFIERFEED-FORWARDCAPACITOR CAPACITOR REF 1REF 2 Figure 6.Fast Fed-Forward Amplifierence 2 is the negative supply. Signals appearing betweenspeed amplifier designer, it poses special prorequired to maximize bandwidth and minimize noise,amplifiers. Almost without exception, however, signalsance at high frequencies. Many high-speed modularamplifiers include appropriate capacitive decouplingdecoupled supply other such as the 118. capacitor is used to dance path between the ies. The resistor in the V+ lead ensures that the supply lines will be rejected, and prevents the estab- SIGNAL COMMON Figure 7.Decoupling for a Fed-Forward Amplifier REV. BDevices AD507 and AD509. In these circuits, a singlefrequency response of the amplifier will appear withrespect to tor. In these amplifiers a small internal capacitance isthe compensation capacitor can also be connected eitherthe signal common and the compensation connect directly SIGNAL COMMON Figure 8.Decoupling a Wideband Amplifiertion to earth ground, but a common connection to whichsignals and power are referred. It is frequently immaterialemphasize that they must be common. The termsort of irrational reverence that you hold for your mother,remember that while you can always trust y trust your Òground.Ó E of resis-V.Ó This signal acts in series with thenoninverting input and can result in significant errors. Forexample, the typical gain of an AD510 amplifier is 8 V of input signal is required to up or oscillate for large clos R DVOUTPUT SIGNALIaRf b ARi Figure 9.Proper Choice of Power Connectionsfiers as previously illustrated in Figure 3c. The only curflowing in signal common is the amplifierÕs input cur REV. BZi1 L f i TO POWER Zf1 L1 Figure 10.Minimizing Common Impedance Couplingand its assumption that a ground is a ground is a ground. Someof a system. Quantitative approaches are quite uthe characteristic impedance of interconnections is con-RF circuitry, the unavoidable impedances are taken themselves to transmission line theory, and thepower and ground impedances are difficult to control orand restrictive quantitative analysis, seems to be to Rf Ri fRi Figure 11.Subtractor Amplifier Rejects Common-ModeIn Figure 11 a subtractor circuit is used to amplify a normalpowered with respect to the output signal common. If itssubtractor. It is just this kind of effect that makes it impor-tant to use care in grounding and decoupling. A subtractoror using its output signal. In Òsingle-endedÓ systems itshould also be decoupled to the input signal return asreferenceÓ to another. The normal mode input signal isto be used by a system that has an interfering tween its own common and the signal sin Figure 12, by the ratio of a pair of resistors that are MODERSRg SIGNALCOMMONMODE SIGNAL REFERENCE Figure 12.Applying an In-Amp REV. BSince instrumentation amplifiers have a reference orÒgroundÓ terminal, they have the potential to be free of thesupply. In the case of the AD521, the ctor is nal is particularly important, and it shouldnal, or actually to the point to which this termsmall voltage on them and by loading the terminals with apotentiometer the amplifier offset voltage can be adjusted.generally looked at as inputs, most amplifiers are quitefamily amplifiers the output voltage gain from An illustration of the type of problems that can arise withthe ÒotherÓ input is shown in Figure 13. The figure is an V ADJ V3kV DV SIGNAL COMMONTO POWER SUPPLY Figure 13.Details of V null pot wiper connects to a pointsmall voltage, amplifier VÐ terminal and the null pot wiper. If the null terminals and will have little effect on the ampli- terminals. For instance, suppose that a 10k null pot terminals will be about 1/8 discussion of Figure 9, the current i10 microvolt input error signal. In this case, however,rent comes from the negative supply. When the load isdriven positive the error will disappear. As a rsignal, and the voltage developed by it will manifest itself Figure 14.Connecting the Null Pot for Trouble-Freerants lists Devices, including some popular second-source families, Referred to: CommentAD OP 07/V+, VÐInternal Feedforward Cap V+ to VÐ27/37and integrator VÐ to OutputAD380V+AD390VÐOutput and Reference AmplifierAD394/AD395VÐOutput AmplifiersAD396VÐOutput AmplifiersAD507ÐExternal Cap to Signal Common or V+AD508ÐExternal Cap to Signal Common or V+AD510V+AD517V+AD518V+, VÐInternal Feedforward Cap V+ to VÐ|AD521VÐOutput Amplifier IntegratorAD524VÐOutput Amplifier IntegratorAD526VÐOutput Amplifier IntegratorAD532/AD533V+MAD534/AD535VÐOutput AmplifierAD536AVÐ, V+External Integrator to V+, InternalCommonFeedforward VÐ to CommonAD538VÐInternal AmplifiersAD542/AD642VÐAD544/AD644VÐAD545AVÐAD546VÐAD547/AD647VÐAD548/AD648VÐAD549VÐAD557/AD558CommonOutput Amplifier and DAC ControlLoop Integrator Referred to CommonAD561VÐ,DAC Control Loop Integrator andCommonRef. Amp Referred to Common andAD565A/VÐDAC AD566Ato ÐV. Reference Input CommonAD568V+Reference AmplifierAD580VÐOutput AmplifierAD581VÐOutput AmplifierAD582VÐOutput AmplifierAD584VÐOutput AmplifierAD586/AD587VÐOutput AmplifierAD588VÐOutput AmplifierAD624/AD625VÐOutput Amplifier IntegratorAD636VÐ, V+,External Integrator to V+, InternalCommonFeedforward VÐ to CommonAD637VÐ,Internal Feedforward VÐ to CommonAD645VÐAD650/AD652V+Internal AmplifierAD662CommonDAC Control Loop Integrator andAD664VÐOutput AmplifiersAD667VÐ,Output Amplifier Referred to VÐCommonand Reference Amplifier ReferredAD668V+Reference Amplifier Referred to: CommentAD688VÐOutput AmplifierAD689VÐOutput AmplifierAD704/AD705/V+AD707/AD708V+, VÐInternal Feedforward Cap V+ to VÐAD711/AD712/VÐAD736/VÐ,External Integrator to VÐAD737CommonInternal Feedforward VÐ to CommonAD741VÐAD744/AD746VÐAD766VÐOutput and Reference AmplifierAD767VÐ,Output Amplifier Referred to VÐ andCommonReference Amp Referred to CommonAD840/AD841/V+, VÐAD843V+, VÐAD844/AD846V+, VÐAD845V+AD847/AD848/V+, VÐAD1856/AD1860VÐOutput and Reference AmplifierAD1864VÐOutput and Reference AmplifierAD2700/AD2710CommonOutput AmplifierAD2701VÐOutput AmplifierAD2702/VÐ,Output AmplifiersAD2712CommonAD7224/AD7225VÐOutput AmplifiersAD7226/AD7228VÐOutput AmplifiersAD7237/V+,Reference Amplifier to CommonAD7247CommonOutput Amplifier to Both V+AD7245/V+,Reference Amplifier to V+AD7248CommonOutput Amplifier to Both V+AD7569/AD7669VÐAll AmplifiersAD7769CommonAll AmplifiersAD7770CommonAll AmplifiersAD7837/AD7847V+All AmplifiersAD7840V+,Output Amplifiers to V+CommonReference Amplifier to CommonAD7845V+All AmplifiersAD7846V+All AmplifiersAD7848V+,Output Amplifier to V+CommonReference Amplifier to CommonThis collection of examples will not solve all your also give you some of the Òinside storyÓ on the ICs, general grounding method that will mother, but....