AN-202 APPLICATION NOTE Amplifier User's Guide Decoupling, Ground
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Paul Brokaw
AN-202 APPLICATION NOTE
Amplifier User's Guide Decoupling, Grounding, Making Things Right Change
"There once breathy baboon always breathed down bassoon, said appears that billions years shall certainly tune" (Sir Arthur Eddington) This quotation seemed proper note with which begin subject that made monkeys most time another. struggle find suitable configuration system power, ground, signal returns frequently degenerates into frustrating glitch hunt. While strictly experimental approach used solve simple problems, little forethought often prevent serious problems provide plan attack some judicious tinkering later required. subject fragmented that completely general treatment difficult tackle. Therefore, like state general principle then look more narrowly subject decoupling grounding relates integrated circuit amplifiers. Principle: Think-where currents will flow. suppose this seems pretty obvious, tend think currents we're interested flowing "out" some place "through" some other place often neglect worry current will find back source. tends "ground" "supply voltage" points equivalent neglect (for long possible) fact that they parts network conductors through which currents flow develop finite voltages. order some advance planning important consider where currents originate where they will return determine effects resulting voltage drops. This, turn, requires some minimum amount understanding what goes inside circuits being decoupled grounded. This information lacking difficult interpret when integrated circuits part design. Operational amplifiers most widely used linear lCs, fortunately most them fall into
classes, problems power grounding concerned. Although configuration system pose formidable problems decoupling signal returns, some basic methods handle many these problems developed from look amps. AMPS HAVE FOUR TERMINALS casual look through almost operational amplifier text might leave reader with impression that ideal three terminals: pair differential inputs output shown Figure quick review fundamentals, however, shows that this cannot case. amplifier output voltage must measured with respect some point point which amplifier reference. Since ideal infinite common-mode rejection, inputs ruled that reference that there must fourth amplifier terminal. Another looking that amplifier supply output current load, that current must into amplifier somewhere. Ideally, input current flows, again conclusion that fourth terminal required.
Figure Conventional "Three Terminal"
common practice say, indicate diagram, that this fourth terminal "ground." Well, without getting into discussion what "ground" observe that most integrated circuit amps (and modular ones well) have "ground" terminal. With these circuits fourth terminal both power supply terminals. There temptation here lump together both supply voltages with ubiquitous ground. And, extent that supply lines really present impedance frequencies within amplifier bandwidth, this probably reasonable. When impedance requirement satisfied, however, door left open variety problems including noise, poor transient response, oscillation.
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DIFFERENTIAL-TO-SINGLE-ENDED CONVERSION fundamental requirement simple that applied signal that fully differential input must converted single-ended output. Single-ended, that with respect often neglected fourth terminal. this lead difficulties, take look Figure
OUTPUT CURRENT MIRROR
Figure Simplified "Real"
signal flow illustrated Figure used several popular integrated circuit families. Details vary, basic signal path same 101, 741, 748, 777, 4136, 503, 515, other integrated circuit amplifiers. circuit first transforms differential input voltage into differential current. This input stage function represented transistors Figure current then converted from differential single-ended form current mirror that connected negative supply rail. output from current mirror drives voltage amplifier power output stage that connected integrator. integrator controls open-loop frequency response, capacitor added externally, 101, self-contained, 741. Most descriptions this simplified model emphasize that integrator has, course, differential input. biased positive couple base emitter voltages, noninverting integrator input referred negative supply. should apparent that most voltage difference between amplifier output negative supply appears across compensation capacitor. negative supply voltage changed abruptly, integrator amplifier will force output follow change. When entire amplifier closed-loop configuration resulting error signal input will tend restore output, recovery will limited slew rate amplifier. result, amplifier this type have outstanding frequency power supply rejection, negative supply rejection fundamentally limited high frequencies. Since feedback signal input that causes output restored, negative supply rejection will approach zero signals frequencies above closed-loop bandwidth. This means that high-speed, high-level circuits "talk low-level circuits through common impedance negative supply line. Note that problem with these amplifiers associated with negative supply terminal. Positive supply rejection also deteriorate with increasing frequency, effect less severe. Typically, small transients
positive supply have only minor effect signal output. difference between these sensitivities result apparent asymmetry amplifier transient response. amplifier driven produce positive voltage swing across rated load, will draw current pulse from positive supply. pulse result supply voltage transient, positive supply rejection will minimize effect amplifier output signal. opposite case, negative output signal will extract current from negative supply. this pulse results "glitch" bus, poor negative supply rejection will result similar "glitch" amplifier output. While positive pulse test give amplifier transient response, negative pulse test actually give pretty good look your negative supply line transient response, instead amplifier response! Remember that impulse response power supply itself what likely appear amplifier. Thirty forty centimeters wire like high inductor high-frequency component normally overdamped supply response. decoupling capacitor near amplifier won't always cure problem either, since supply must decoupled somewhere. decoupled current flows through long path, still produce undesirable glitch. Figure illustrates three possible configurations negative supply decoupling. dotted line shows negative signal current path through decoupling along ground line. load "ground" decoupled "ground" actually join power supply, "glitch" ground lines similar "glitch" negative supply bus. Depending upon feedback signal sources "grounded," effective disturbance caused decoupling capacitor larger than disturbance intended prevent. Figure shows decoupling capacitor used minimize disturbance ground buses. high-frequency component load current confined loop that does include part ground path. capacitor sufficient size quality, will minimize glitch negative supply without disturbing input output signal paths. When load situation more complex, little more thought required. amplifier driving load that goes virtual ground, actual load current does return ground. Rather, must supplied amplifier creating virtual ground shown figure. this case, decoupling negative supply first amplifier positive supply second amplifier closes fast signal current loop without disturbing ground signal paths. course, still important provide impedance path from "ground" second amplifier avoid disturbing input reference. understanding decoupling circuits note where actual load signal currents will flow. optimizing circuit bypass these currents
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around ground other signal paths. Note, that Figure "single point grounding" oversimplified solution complex problem. jingle small damped signal supply terminal. residual larger low-frequency components, these handled supply rejection.
OUTPUT TRANSITOR
LOAD LOAD LOAD GROUND SIGNAL CURRENT LOOP POWER SUPPLY TERMINAL
LOAD
LOAD
Figure Damping Parallel Decoupling Resonances
FREQUENCY STABILITY There temptation forget about decoupling negative supply when system intended handle only low-frequency signals. Granted that decoupling required handle low-frequency signals, still required frequency stability amps. Figure more detailed version Figure showing output stage separated from integrator (since this usual arrangement) showing negative power supply wiring impedance lumped together single constant. amplifier connected unity gain follower. This makes closed-loop path from amplifier output through differential input integrator input. There second feedback path from collector output transistor back other integrator input. input integrator difference signals through these paths. frequencies this net, negative feedback. high-frequency feedback depends upon both load reactance reactance supply.
POWER GROUND
Figure Decoupling Negative Supply Ineffective
OUTPUT TRANSITOR
LOAD
SIGNAL CURRENT LOOP CIRCUIT COMMON DECOUPLING CAPACITOR
Figure Decoupling Negative Supply Optimized "Grounded" Load
OUTPUT TRANSITOR
HIGH FREQUENCY SIGNAL CURRENT PATH
OUTPUT TRANSITOR
CURRENT MIRROR
Figure Decoupling Negative Supply Optimized Virtual Ground" Load
Figures have been simplified illustrative purposes. When entire circuit considered, conflicts frequently arise. example, several amplifiers powered from same supply, individual decoupling capacitor required each. gross sense decoupling capacitors paralleled. fact, however, inductance interconnecting power ground lines convert this harmless-looking arrangement into complex network that often rings like "Avon Lady." circuits handling fast signal wavefronts, decoupling networks paralleled more than centimeters wire generally mean trouble. Figure shows small resistors added lower undesired resonant circuits. resistors generally tolerated since they convert high-frequency
IMPEDANCE
Figure Instability Result from Neglecting Decoupling
When supply lead reactance inductive, tends destabilize integrator. This situation aggravated capacitive load amplifier. Although difficult predict under exactly what circumstances circuit will become unstable, generally wise decouple negative supply there substantial lead inductance lead common return load amplifier input signal source. decoupling effective, course, must with respect actual signal returns, rather than some vague "ground" connection. POSITIVE SUPPLY DECOUPLING this point have considered decoupling positive supply line, with amplifiers typified Figures
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there need other hand, there number integrated circuit amplifiers that refer compensating integrator positive supply. Among these 108, 504, families. When these circuits used, positive supply that requires most attention. considerations techniques described class circuits shown Figure apply equally this second class, should applied positive supply rather than negative. FEED-FORWARD technique that most frequently used improve bandwidth called feed-forward. Generally, feed-forward used bypass amplifier level translator stage that poor high-frequency response. Figure illustrates this done. Each amplifiers shown really subcircuit, usually single stage, overall amplifier. illustration, input stage converts differential input single-ended signal. signal drives intermediate stage (which, practice, often includes level translator circuitry) that low-frequency gain, limited bandwidth. output this stage drives integratoramplifier output stage. overall compensation capacitor feeds back input second stage includes integrator loop. compromises necessary obtain gain level translation intermediate stage often limit bandwidth slow down available integrator response. feed-forward capacitor permits high-frequency signals bypass this stage. result, overall amplifier combines low-frequency gain available from three stages with improved frequency response available from 2-stage amplifier. feed-forward capacitor also feeds back noninverting input intermediate stage. Note that second stage integrator, appear first glance, actually positive feedback connection. Fed-forward amplifiers must carefully designed avoid internal oscillations resulting from this connection. Improper decoupling upset this plan permit this loop oscillate.
COMPENSATING CAPACITOR FEED-FORWARD CAPACITOR INPUT SECTION INTERMEDIATE AMPLIFIER
Reference positive supply, while signal Reference negative supply. Signals appearing between positive negative supply terminals effectively inserted inside integrator loop! Obviously, while feed-forward valuable tool high-speed amplifier designer, poses special problems application. thoughtful approach decoupling required maximize bandwidth minimize noise, error, likelihood oscillation. Some fed-forward amplifiers have other arrangements, which include "ground" terminal inverting only amplifiers. Almost without exception, however, signals between some combination supply terminals "inside" amplifier. vital proper operation that involved supply terminals present common impedance high frequencies. Many high-speed modular amplifiers include appropriate capacitive decoupling within amplifier, with amps this impossible. user must take care provide cleanly decoupled supply fed-forward amplifiers. Figure shows decoupling method that applied AD518 well other fast fed-forward amplifiers such 118. capacitor used provide lowimpedance path between supply terminals high frequencies. resistor lead ensures that noise supply lines will rejected, prevents establishment resonances with other decoupling circuits. second capacitor decouples side integrator load.
SUPPLY
AD518 LOAD SIGNAL COMMON SUPPLY
Figure Decoupling Fed-Forward Amplifier
Alternatives include resistor both supply leads and/ decoupling from load. principle, positive negative supply should tied "tight knot" with signal return. extent that this cannot done, there slight advantage favoring negative supply high-frequency limitations transistors used junction-isolated lCs. OTHER COMPENSATION While most integrated circuit amplifiers three compensation schemes already described, significant fraction some other plan. 725-type amplifiers combine referred integrator with network manufacturers recommend connected from signal ground integrator input. This makes circuit extremely liable pick noise between ground. many circumstances wiser connect external compensation negative supply, rather than signal ground.
INTEGRATOR OUTPUT SECTION
Figure Fast Fed-Forward Amplifier
Note that internal input stages shown being referred separated reference points. Ideally, these will same reference signals concerned, although they differ bias level. practice, this case. Examples fed-forward amplifiers AD518 AD707. these amplifiers, signal
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more class amplifiers typified Analog Devices AD507 AD509. these circuits, single capacitor used induce dominant pole response without resorting integrator connection. highfrequency response amplifier will appear with respect "ground" compensation capacitor. these amplifiers small internal capacitance connected between compensation point. Unity gain compensation added parallel pinout arranged make this simple. free compensation capacitor also connected either signal common. extremely important that signal common compensation connect directly through low-impedance decoupling. Although main signal path these amplifiers compensated variety ways, some care required ensure stability internal structures. always wise extra care decoupling wideband amplifiers avoid problems with output stage other subcircuits that similar main integrator problem illustrated Figure effective compensation decoupling circuit AD509 shown Figure This arrangement similar Figure these circuits likely suitable many types wideband amplifier. Depending upon power distribution, small (1052 5052) resistor appropriate both supply leads reduce power lead resonance interference both from circuits sharing power supply.
COMPENSATION AD510 INPUT SIGNAL AD509 OUTPUT OUTPUT SIGNAL RLOAD
important have look currents that flow ground circuit. Allowing these currents share path with low-level signal result trouble. Figure illustrates careless grounding degrade performance simple amplifier. amplifier drives load that represented load resistor. load current comes from power supply controlled amplifier amplifies input signal. This current must return supply some path; suppose that points alternative power supply "ground" connections. Assuming that figure represents proper topology, ordering connections along "ground" bus, connecting supply will cause load current share segment wire with input signal connection. Fifteen centimeters number wire this path will present about resistance load current. With load, 10-volt output signal will result about microvolts between points marked "V." This signal acts series with noninverting input result significant errors. example, typical gain AD510 amplifier million that only input signal required produce volt output. ground error signal will result 32-times increase circuit gain error! This degradation could easily most serious error high-gain precision application. Moreover, error represents positive feedback that circuit will latch oscillate large closed-loop gains with Rf/R greater than about 250k.
SIGNAL COMMON
Figure Decoupling Wideband Amplifier
GROUNDING ERRORS Ground most electronic equipment actual connection earth ground, common connection which signals power referred. frequently immaterial function equipment whether point actually connects earth ground. prefer some distinguishing name names these common points emphasize that they must made common. term "ground" often seems associated with sort cure-all concept, like snake oil, money, motherhood. those regards ground with same sort irrational reverence that hold your mother, remember that while always trust your mother, should never trust your "ground." Examine think about
Figure Proper Choice Power Connections Minimizes Problems
Reconnecting power supply point will correct problem eliminating common impedance feedback connection. real system, problem more complex. input signal source, which represented floating Figure also produce current that must return power supply. With supply point current that flows additional loads (other than interfere with operation amplifier shown. Figure illustrates amplifiers cascaded still drive auxiliary loads without common impedance coupling. output currents flow through auxiliary loads back power supply through power common. currents input feedback resistors supplied from power supply amplifiers previously illustrated Figure only current flowing signal common amplifier's input current, effect generally negligibly small.
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INPUT SIGNAL POWER SUPPLY SIGNAL COMMON POWER COMMON OUTPUT
Figure Minimizing Common Impedance Coupling
Having given example simple "grounding error" solution, will back soap that grounding errors result from neglect, based assumption that ground ground ground. Some impedance will present interconnection path, effect should considered overall design system. Quantitative approaches quite useful specialized applications. fast logic circuitry, characteristic impedance interconnections controlled that proper terminations reduce problems. circuitry, unavoidable impedances taken into account incorporated into design circuit. With circuitry, however, impedance levels lend themselves transmission line theory, power ground impedances difficult control analyze. most expedient procedure, short difficult restrictive quantitative analysis, seems arrange unavoidable impedances minimize their effects arrange circuitry overcome effects. Figures illustrate sort simple considerations that substantially reduce practical ground problems. Figure illustrates circuitry used reduce effect ground problems that cannot corrected topological tricks.
dynamic bridge, like Figure will ineffective correcting grounding problem amplifier itself carelessly decoupled. general, should decoupled point that reference measuring using output signal. "single-ended" systems should also decoupled input signal return well. When impossible satisfy both these requirements once, there high probability noise oscillation problem both. Frequently difficulty resolved with subtractor, like Figure where network like single-ended feedback network (which need resistive) joins input output signal reference points provides "clean" reference point noninverting input amplifier. problem with subtractor that uses balanced bridge reject common-mode signal between input output reference points. arms network must carefully balanced, since extent they don't match, unwanted signal will amplified. Although even poorly matched network will probably eliminate oscillation problems, noise rejection will suffer direct proportion mismatches. easier reject large "ground noise" signals true instrumentation amplifier. INSTRUMENTATION AMPLIFIERS true instrumentation amplifier very visible "fourth terminal." output signal developed with respect well-defined reference point that usually "free" terminal that tied output signal common. instrumentation amplifier also differs from that gain fixed well defined, there feedback network coupling input output circuits. Figure shows instrumentation amplifier used translate signal from "ground reference" another. normal mode input signal developed with respect reference point which common generating circuits. signal used system that interfering signal between common signal source. instrumentation amplifier high-impedance differential input which desired signal applied. high common-mode rejection eliminates unwanted signal translates desired signal output reference point. Unlike dynamic bridge circuit, gain common-mode rejection depend network connecting input output circuits. gain set, Figure ratio pair resistors that
SENSE NORMAL MODE SIGNAL
INPUT SIGNAL INPUT SIGNAL COMMON
SIGNAL
"GROUND NOISE"
OUTPUT SIGNAL COMMON
Figure Subtractor Amplifier Rejects Common-Mode Noise
GETTING AROUND PROBLEM Figure subtractor circuit used amplify normal mode input signal reject ground noise signal which common both sides input signal. This scheme uses common-mode rejection amplifier reduce noise component while amplifying desired signal. important aspect this arrangement, which often overlooked, that amplifier should powered with respect output signal common. power pins exposed high-frequency noise input common, compensation capacitor will direct noise right output defeat purpose subtractor. just this kind effect that makes important care grounding decoupling. subtractor
AD521
REFERENCE
OUTPUT
INPUT SIGNAL COMMON
OUTPUT COMMON COMMON MODE SIGNAL
Figure Applying In-Amp
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connected inside amplifier. amplifier very high input impedance, that gain common-mode rejection greatly affected variations unbalance source impedance. Since instrumentation amplifiers have reference "ground" terminal, they have potential free power supply sensitivities amps. practice, however, most instrumentation amplifiers have internal frequency compensation which referred power supply. case AD521, compensation integrator referred negative supply terminal. decoupling this terminal particularly important, should decoupled with respect output reference terminal, actually point which this terminal refers. "OTHER" INPUT Most amps amps include offset voltage pulling terminals. These terminals generally have small voltage them loading terminals with potentiometer amplifier offset voltage adjusted. While their impedance level much lower than normal input, null terminals another differential input amplifier. Although null terminals generally looked inputs, most amplifiers quite sensitive signals applied here. example, 741family amplifiers output voltage gain from null terminals greater than gain from normal input! illustration type problems that arise with "other" input shown Figure figure circuit with some offset null detail shown. terminals will have little effect amplifier output. other hand, null unbalanced, correct amplifier offset, bridge will longer balance. this case, voltages developed along "clothesline" will result difference voltage terminals. instance, suppose that null balances offset when with branches shown figure. internal resistors about that difference signal terminals will about gain from these terminals about twice gain from normal input, that disturbance acts were input signal about Using same assumptions discussion Figure current will result microvolt input error signal. this case, however, error will appear only when amplifier load current comes from negative supply. When load driven positive error will disappear. result, input signal will result distortion rather than simple gain error! additional problem created current returning power supply from other circuits. current from other circuits generally related signal, voltage developed will manifest itself noise. This signal null terminals easily dominant noise system. milliamps current through centimeters wire result interference that orders magnitude larger than inherent input noise amplifier. remedy make connection from null wiper direct amplifier, shown Figure Some amplifiers, such AD504 AD510, refer null offset terminals Obviously, wiper should terminal this type amplifier. important connect line directly terminal minimize common impedance shared current null connection.
SIGNAL COMMON POWER SUPPLY
Figure Connecting Null Trouble-Free Operation
considerations null pots also apply similar trimmers almost types integrated circuits. example, AD521 null terminals exhibit gain about output. Although this much less than case most amps, still warrants care controlling null wiper return. Table lists integrated circuits manufactured Analog Devices, including some popular second-source families, indicates internal conversions from differential-to-single-ended referred. That signals made appear with respect terminal(s) listed.
Figure Details Nulling-the "Other" Input
drawn, null wiper connects point along "clothesline" that carries both return current from amplifier currents from other circuits back power supply. These currents will develop small voltage, along conductor between amplifier terminal null wiper. null center, equal halves will form balanced bridge with resistors inside amplifier. effect voltage generated along wire balanced
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Table
Internal Integrator Referred 27/37 AD380 AD390 AD394/AD395 AD396 AD507 AD508 AD510 AD517 AD518 AD521 AD524 AD526 AD532/AD533 AD534/AD535 AD536A AD538 AD542/AD642 AD544/AD644 AD545A AD546 AD547/AD647 AD548/AD648 AD549 AD557/AD558 AD561 Common Common Common Output Amplifier Control Loop Integrator Referred Common Control Loop Integrator Ref. Referred Common Ref. Bias Amplifier Referred Control Loop Integrator Referred Reference Input Common Control Loop Isolated from Output Common Reference Amplifier Output Amplifier Output Amplifier Output Amplifier Output Amplifier Output Amplifier Output Amplifier Output Amplifier Integrator External Integrator Internal Feedforward Common Internal Feedforward Common AD7845 AD7846 AD7848 Internal Feedforward Integrator Output Output Amplifier Integrator Output Amplifier Integrator Output Amplifier Integrator Multiplier Output Amplifier Integrator Output Amplifier External Integrator Internal Feedforward Common Internal Amplifiers Output Reference Amplifier Output Amplifiers Output Amplifiers External Signal Common External Signal Common AD711/AD712/ AD713 AD736/ AD737 AD741 AD744/AD746 AD766 AD767 AD840/AD841/ AD842 AD843 AD844/AD846 AD845 AD847/AD848/ AD849 AD1856/AD1860 AD1864 AD2700/AD2710 AD2701 AD2702/ AD2712 AD7224/AD7225 AD7226/AD7228 AD7237/ AD7247 AD7245/ AD7248 AD7569/AD7669 AD7769 AD7770 AD7837/AD7847 AD7840 Common Common Common Common Common Common Common Common Common Common Output Reference Amplifier Output Reference Amplifier Output Amplifier Output Amplifier Output Amplifiers Output Amplifiers Output Amplifiers Reference Amplifier Common Output Amplifier Both Common Reference Amplifier Output Amplifier Both Common Amplifiers Amplifiers Amplifiers Amplifiers Output Amplifiers Reference Amplifier Common Amplifiers Amplifiers PRINTED U.S.A. Output Amplifier Reference Amplifier Common Output Reference Amplifier Output Amplifier Referred Reference Referred Common External Integrator Internal Feedforward Common AD688 Comment AD689 AD704/AD705/ AD706 AD707/AD708 Internal Integrator Referred Comment Output Amplifier E1393b-1-2/00 (rev. Internal Feedforward Integrator Output Output Amplifier
Internal Feedforward integrator Output
AD565A/ AD566A
AD568 AD580 AD581 AD582 AD584 AD586/AD587 AD588 AD624/AD625 AD636 AD637 AD645 AD650/AD652 AD662
Common Common Common
Internal Amplifier Control Loop Integrator Reference Amplifier Referred Common Output Amplifiers Output Amplifier Referred Reference Amplifier Referred Common Reference Amplifier
AD664 AD667
Common
This collection examples will solve your potential grounding problems. hope that will give some good ideas prevent some them, should also give some "inside story" ICs, which work very practical ways. There general grounding method that will prevent possible problems. only generally applicable rule attention detail, remember that always trust your mother,
AD668
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