Decoupling capacitor: Difference between revisions

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{{short description|Capacitor used to prevent energy transfer between two circuits}}

{{About|the use of capacitors to filter undesired noise from power supplies|the use of capacitors to allow AC signals to pass while blocking DC offsets|Capacitive coupling capacitor}}

{{Redirect|Bypass capacitor|class-Y safety capacitors|Line-bypass capacitor}}

[[File:MLCC-Imp-versus-Freqenz.engl.png|thumb|right|Typical impedance curves of X7R and NP0 [[Ceramic capacitor|MLCC chip capacitors]].]]

[[File:LM7805 with Decoupling Capacitor.svg|thumb|right|LM7805 5V [[Voltage_regulator#Linear_regulators|linear voltage regulator]] with 2 decoupling capacitors]]

[[File:Photo-SMDcapacitors.jpg|thumb|right|Capacitor packages: [[Surface-mount technology|SMD]] ceramic at top left; SMD tantalum at bottom left; [[through-hole]] tantalum at top right; through-hole electrolytic at bottom right. Major scale divisions are cm.]]

AIn [[electronics]], a '''decoupling capacitor''' is a [[capacitor]] used to [[Decoupling_Decoupling (electronics)|decouple]] (i.e. prevent [[electrical energy]] from transferring to) one part of ana [[electrical network|circuit]] (circuit) from another. [[Noise (electronics)|Noise]] caused by other [[circuit elementselement]]s is shunted through the capacitor, reducing theits effect it has on the rest of the circuit. An{{anchor|Bypass capacitor}}For higher frequencies, an alternative name is '''bypass capacitor''' as it is used to bypass the [[power supply]] or other high-[[Electrical impedance|impedance]] component of a circuit.

==Discussion==

Active devices of an [[electronic system]] (e.g. [[transistor]]s, [[integrated circuit]]s, [[vacuum tube]]s) are connected to their [[power supplies]] through [[Electrical conductor|conductors]] with finite [[Electrical resistance and conductance|resistance]] and [[inductance]]. If the [[Electric current|current]] drawn by an active device changes, the [[voltage drop]] from the power supply to the device will also change due to these [[Electrical impedance|impedance]]s. If several active devices share a common path to the power supply, changes in the current drawn by one element may produce voltage changes large enough to affect the operation of others – [[voltage spike]]s or [[ground bounce]], for example – so the change of state of one device is coupled to others through the common impedance to the power supply. A decoupling capacitor provides a bypass path for [[Transient response|transient]] currents, instead of flowing through the common impedance.<ref name=TTL75> Don Lancaster, ''TTL Cookbook', Howard W. Sams, 1975, no ISBN, pp.23-24 </ref>

For example, if the voltage level for a device is fixed, changing power demands are manifested as changing current demand. The power supply must accommodate these variations in current draw with as little change as possible in the power supply voltage. When the current draw in a device changes, the power supply cannot respond to that change instantaneously. As a consequence, the voltage at the device changes for a brief period before the power supply responds. The [[voltage regulator]] adjusts the amount of current it is supplying to keep the output voltage constant but can only effectively maintain the output voltage for events at frequencies from DC to a few hundred kHz, depending on the regulator (some are effective at regulating in the low MHz). For [[Transient (oscillation)|transient events]] that occur at frequencies above this range, there is a time lag before the voltage regulator responds to the new current demand level. This is where the decoupling capacitor comes in. The decoupling capacitor works as the device’s local [[Electric field|energy storage]]. The capacitor is placed between power line and ground to the circuit that current is to be provided. According to capacitor equation, ''i(t) = CdV(t)/dt'', voltage drop between power line and ground results in current draw out from the capacitor to the circuit and when capacitance ''C'' is large enough, sufficient current is supplied with acceptable range of voltage drop (however, as described below, [[Parasitic element (electrical networks)|parasitic inductance]] is typically high for large capacitor so small and large capacitors are usually placed together in parallel to fully cover circuit bandwidth). The capacitor cannot provide DC power because it stores only a small amount of energy but this energy can respond very quickly to changing current demands. The capacitors effectively maintain power-supply voltage at [[Frequency response|frequencies]] roughly from hundreds of kHz to hundreds of MHz (in the milliseconds to nanoseconds range). Decoupling capacitors are not useful for events occurring above or below this range: at the low end, they don't store enough energy and at the high end (at or above the self-resonant frequency) their parasitic inductance dominates their (high) impedance.

The decoupling capacitor works as the device’s local [[Electric field|energy storage]]. The capacitor is placed between the power line and the [[Ground (electricity)|ground]] to the circuit the current is to be provided. According to the [[Capacitor#Current–voltage relation|capacitor current–voltage relation]]

==Decoupling==

:<math>i(t) = C \frac{d\,v(t)}{dt},</math>

One common kind of decoupling is to protect a powered circuit from signals in the [[power supply]]. Sometimes, for various reasons, a power supply supplies an [[Alternating current|AC]] signal superimposed on the [[Direct current|DC]] power line. Such a signal is often undesirable in the powered circuit. A decoupling capacitor can prevent the powered circuit from seeing that signal, thus decoupling it from that aspect of the power supply circuit.

a voltage drop between a power line and the ground results in a current drawn out from the capacitor to the circuit. When capacitance {{mvar|C}} is large enough, sufficient current is supplied to maintain an acceptable range of voltage drop. The capacitor stores a small amount of energy that can compensate for the voltage drop in the power supply conductors to the capacitor. To reduce undesired [[Parasitic element (electrical networks)|parasitic]] [[equivalent series inductance]], small and large capacitors are often placed in [[Parallel circuit|parallel]], adjacent to individual integrated circuits (see [[#Placement|§ Placement]]).

In digital circuits, decoupling capacitors also help prevent radiation of [[electromagnetic interference]] from relatively long circuit traces due to rapidly changing power supply currents.

Another kind of decoupling is stopping a portion of a circuit from being affected by switching that occurs in another portion of the circuit. Switching in subcircuit A may cause fluctuations in the power supply or other electrical lines, but you do not want subcircuit B, which has nothing to do with that switching, to be affected. A decoupling capacitor can decouple subcircuits A and B so that B doesn't see any effects of the switching.

Decoupling capacitors alone may not suffice in such cases as a high-power amplifier stage with a low-level pre-amplifier coupled to it. Care must be taken in the layout of circuit conductors so that heavy current at one stage does not produce power supply voltage drops that affect other stages. This may require re-routing printed circuit board traces to segregate circuits, or the use of a [[ground plane]] to improve the stability of power supply.

To decouple a subcircuit from AC signals or [[voltage spike]]s on a power supply or other line, a bypass capacitor is often used. A bypass capacitor is to [[Shunt (electrical)|shunt]] energy from those signals or transients past the subcircuit to be decoupled, right to the return path. For a power supply line, a bypass capacitor from the supply voltage line to the power supply return (neutral) would be used.

==Decoupling==

High frequencies and transient currents flow through a capacitor, in this case in preference to the harder path through the decoupled circuit, but DC cannot go through the capacitor, so continues on to the decoupled circuit.

[[File:MLCC-Imp-versus-Freqenz.engl.png|thumb|right|Typical impedance curves of X7R and NP0 [[Ceramicceramic capacitor|MLCC chip capacitors]].s]]

[[File:Al-Elko-Impedanzverläufe-Elko-Polymer-Wiki-07-02-19.jpg|thumb|right|Impedance curves of aluminum [[electrolytic capacitor]]s (solid lines) and [[polymer capacitor]]s (dashed lines)]]

ToA bypass capacitor is often used to decouple a subcircuit from AC signals or [[voltage spike]]s on a power supply or other line, a bypass capacitor is often used. A bypass capacitor is tocan [[Shunt (electrical)|shunt]] energy from those signals, or transients, past the subcircuit to be decoupled, right to the return path. For a power supply line, a bypass capacitor from the supply voltage line to the power supply return (neutral) would be used.

High frequencies and transient currents can flow through a capacitor, into thiscircuit caseground ininstead preferenceof to the harder path throughof the decoupled circuit, but DC cannot go through the capacitor, soand continues on to the decoupled circuit.

Another kind of decoupling is stopping a portion of a circuit from being affected by switching that occurs in another portion of the circuit. Switching in subcircuit A may cause fluctuations in the power supply or other electrical lines, but you do not want subcircuit B, which has nothing to do with that switching, to be affected. A decoupling capacitor can decouple subcircuits A and B so that B doesn't see any effects of the switching.

==Switching subcircuits==

In a switching subcircuit, switching noisewill mustchange be suppressed. When athe load iscurrent applieddrawn tofrom a voltagethe source, it draws a certain amount of current. Typical power supply lines show inherent [[inductance]], which results in a slower response to changechanges in current. ThisThe in turn affects the transientsupply voltage levels,will sincedrop ifacross thethese loadparasitic currentinductances isfor zeroas thelong voltage acrossas the loadswitching isevent zero as welloccurs. This suddentransient voltage drop would be seen by other loads as well if the inductance between two loads is much lower compared to the inductance between the loads and the output capacitors of the power supply. This is only temporary; the inductor ultimately saturates (that is the magnetic field around the conductor reaches its max), the voltage drop across the inductor reaches zero, and the supply voltage comes back to normal. But even a temporary reduction in voltage can disturb adjacent subcircuits. Decoupling caps provide instantaneous current jolt which helps maintain constant voltage across a subcircuit (or provide a low impedance path for the transient currents; different descriptions are used by different industries).

To decouple other subcircuits from the effect of the sudden current demand, a decoupling capacitor can be placed betweenin theparallel supply voltage line and its reference (ground) next towith the switched load. While the load is switched outsubcircuit, theacross capacitor charges up to full powerits supply voltage andlines. otherwiseWhen doesswitching nothing.occurs Whenin the load is appliedsubcircuit, the capacitor initiallysupplies suppliesthe demandedtransient current. Ideally, by the time the capacitor runs out of charge, the powerswitching supplyevent linehas inductancefinished, isso saturated, andthat the load can draw full current at normal voltage from the power supply (and the capacitor can recharge too). Note that the voltage dip is reduced but not eliminated; i.e., the decoupling is not perfect and sometimes parallel combinations of caps are used to improve response. The best way to reduce switching noise is to design a [[Printed circuit board|PCB]] as a giant capacitor by sandwiching the power and ground planes across a [[dielectric]] material.{{Citation needed|date=August 2018}}

Sometimes parallel combinations of capacitors are used to improve response. This is because real capacitors have parasitic inductance, which causes the impedance to deviate from that of an ideal capacitor at higher frequencies.<ref>{{Cite web|url=http://www.cypress.com/file/135716/download|title=Using Decoupling Capacitors|date=2017-04-07|website=Cypress|access-date=2018-08-12}}</ref>

The size of the capacitor must be reasonable, and there is a tradeoff between capacitor size and signal quality at a given frequency. If a cap is too large it would distort the signal by charging too slowly and filtering out the signal's most needed high-frequency components.

==Transient load decoupling==

[[Transient (oscillation)|Transient]] [[Electrical load|load]] decoupling as described above is needed when there is a large load that gets switched quickly. The parasitic inductance in every (decoupling) capacitor may limit the suitable capacity and influence the appropriate type if switching occurs very fast.

[[Logic]] circuits tend to do sudden switching (an ideal logic circuit would switch from low voltage to high voltage instantaneously, with no middle voltage ever observable). So logic circuit boards often have a decoupling capacitor close to each logic IC connected from each power supply connection to a nearby ground. These capacitors decouple every IC from every other IC in terms of supply voltage dips.

These capacitors are often placed at each power source as well as at each analog component in order to ensure that the supplies are as steady as possible. Otherwise, an analog component with a poor [[power supply rejection ratio|power supply rejection ratio (PSRR)]] will copy fluctuations in the power supply onto its output.

In these applications, the decoupling capacitors are often called ''bypass capacitors'' to indicate that they provide an alternate path for high-frequency signals that would otherwise cause the normally steady suppliessupply voltage to movechange. Those components that require quick injections of current can ''bypass'' the power supply by receiving the current from the nearby capacitor. Hence, the slower power supply connection is used to charge these capacitors, and the capacitors actually provide the large quantities of high-availability current.

==Placement==

A transient load decoupling capacitor should usually beis placed as close as possible to the device requiring the decoupled signal. The goal is toThis minimizeminimizes the amount of line [[inductance]] and series [[Electrical resistance|resistance]] between the decoupling capacitor and thatthe device,. and theThe longer the conductor between the capacitor and the device, the more inductance there is present.<ref>[http://www.interfacebus.com/Design_Capacitors.html Capacitor Design Data, and Decoupling Placement, How-to] on [http://www.interfacebus.com/ Leroy's Engineering Web Site]</ref>

Since capacitors differ in their high-frequency characteristics (and capacitors with good high-frequency properties are often types with small capacity, while large capacitors usually have worse high-frequency response), decoupling oftenideally involves the use of a ''combination'' of capacitors. For example in logic circuits, a common arrangement is ~100 nF ceramic per logic IC (multiple ones for complex ICs), combined with [[Electrolytic capacitor|electrolytic]] or [[tantalum capacitor]](s) up to a few hundred μF per board /or board section.

==Example uses==

These photos show old [[printed circuit board]]s with through-hole capacitors, where as modern boards typically have tiny [[Surface-mount technology|surface-mount]] capacitors.

<gallery widths="220" heights="165" mode="packed">

File:0431 - C64 Mainboard ASSY250407 RevA.jpg|1980s [[Commodore 64]] main board. Most of the "orange" round disc parts are decoupling capacitors.

File:Cromemco XXU 32-bit 68020 S-100 microcomputer CPU.jpg|1980s [[Cromemco]] XXU, a [[Motorola]] [[68020]] processor [[S-100 bus]] card. The axial parts between the ICs are decoupling capacitors.

File:Cromemco 16KZ S-100 Board.jpg|1970s [[Cromemco]] 16KZ, a 16KB [[DRAM]] memory [[S-100 bus]] card. The green round disc parts are decoupling capacitors.

File:Interface I1.JPG|1970s I1 [[Parallel communication|parallel]] interface board for [[Electronika 60]]. The green rectangular parts are decoupling capacitors.

</gallery>

==See also==

* [[Ceramic capacitor]]

* [[Equivalent series inductance]]

* [[Equivalent series resistance]]

* [[Film capacitor]]

* [[E-series of preferred numbers]]

==References==

{{Reflist}}

<references/>

==External links==

* [https://web.archive.org/web/20120213101028/http://www.ultracadintersil.com/mentordata/esr%20and%20bypass%20capsan/an1325.pdf ESRChoosing and Bypass Capacitor Self Resonant Behavior: How to SelectUsing Bypass CapsCapacitors] &ndash; article writtenapplication bynote Douglasfrom BrooksIntersil

* [https://web.archive.org/web/20200224172143/http://www.hottconsultants.com/techtips/decoupling.html Decoupling] &ndash; decoupling guide for various frequencies by Henry W. Ott

* [http://www.ultracad.com/articles/todd_h.pdf Bypass Capacitors, an Interview With Todd Hubing] &ndash; by Douglas Brooks, President, UltraCAD Design, Inc.

* [http://www.intersil.com/data/an/an1325.pdf Choosing and Using Bypass Capacitors] &ndash; application note from Intersil

* [http://www.designers-guide.org/Design/bypassing.pdf Power Supply Noise Reduction] &ndash; how to design effective supply bypassing and decoupling networks by Ken Kundert

* [https://web.archive.org/web/20200518044500/https://www.ultracad.com/mentor/esr%20and%20bypass%20caps.pdf ESR and Bypass Capacitor Self Resonant Behavior: How to Select Bypass Caps] &ndash; article written by Douglas Brooks

* [http://learnemc.com/circuit-board-decoupling-information Circuit Board Decoupling Information] &ndash; decoupling guidelines for various types of circuit boards

* [https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/wp/wp_sgnlntgry.pdf Basic Principles of Signal Integrity] &ndash; Altera whitepaper

* [http://postreh.com/vmichal/papers/high_voltage_active_decoupling_capacitor.pdf Active Decoupling Capacitor] &ndash; Circuit allowing reduce volume of high voltage electrolytic capacitors

* [httphttps://web.archive.org/web/20200518044459/https://www.ultracad.com/articles/todd_h.pdf Bypass Capacitors, an Interview With Todd Hubing] &ndash; by Douglas Brooks, President, UltraCAD Design, Inc.

[[Category:Capacitors]]