A decoupling capacitor is a
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
used to
decouple one part of an
electrical network
An electrical network is an interconnection of electrical components (e.g., batteries, resistors, inductors, capacitors, switches, transistors) or a model of such an interconnection, consisting of electrical elements (e.g., voltage sources ...
(circuit) from another. Noise caused by other circuit elements is shunted through the capacitor, reducing its effect on the rest of the circuit. For higher frequencies an alternative name is bypass capacitor as it is used to bypass the power supply or other high impedance component of a circuit.
Discussion
Active devices of an electronic system (transistors, ICs, vacuum tubes, for example) are connected to their power supplies through conductors with finite resistance and inductance. If the current drawn by an active device changes, voltage drops from power supply to device will also change due to these impedances. 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
In electrical engineering, spikes are fast, short duration electrical transients in voltage (voltage spikes), current (current spikes), or transferred energy (energy spikes) in an electrical circuit.
Fast, short duration electrical transients ( ...
s or
ground bounce
In electronic engineering, ground bounce is a phenomenon associated with transistor switching where the gate voltage can appear to be less than the local ground potential, causing the unstable operation of a logic gate.
Description
Ground bounc ...
, 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 currents, instead of flowing through the common impedance.
[ Don Lancaster, ''TTL Cookbook', Howard W. Sams, 1975, no ISBN, pp.23-24 ]
The decoupling capacitor works as the device’s local
energy storage. The capacitor is placed between the power line and the ground to the circuit the current is to be provided. According to the
capacitor current–voltage relation,
, a voltage drop between power line and ground results in current draw out from the capacitor to the circuit. When capacitance ''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
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...
effective series inductance, small and large capacitors are often placed in
parallel
Parallel is a geometric term of location which may refer to:
Computing
* Parallel algorithm
* Parallel computing
* Parallel metaheuristic
* Parallel (software), a UNIX utility for running programs in parallel
* Parallel Sysplex, a cluster of ...
, adjacent to individual integrated circuits (see ).
In digital circuits, decoupling capacitors also help prevent radiation of
electromagnetic interference from relatively long circuit traces due to rapidly changing power supply currents.
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 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 stability of power supply.
Decoupling
A bypass capacitor is often used to decouple a subcircuit from AC signals or
voltage spike
In electrical engineering, spikes are fast, short duration electrical transients in voltage (voltage spikes), current (current spikes), or transferred energy (energy spikes) in an electrical circuit.
Fast, short duration electrical transients ( ...
s on a power supply or other line. A bypass capacitor can
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 to circuit ground instead of to the harder path of the decoupled circuit, but DC cannot go through the capacitor and 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 subcircuit, switching will change the load current drawn from the source. Typical power supply lines show inherent
inductance, which results in a slower response to change in current. The supply voltage will drop across these parasitic inductances for as long as the switching event occurs. This transient 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 of the power supply.
To decouple other subcircuits from the effect of the sudden current demand, a decoupling capacitor can be placed in parallel with the subcircuit, across its supply voltage lines. When switching occurs in the subcircuit, the capacitor supplies the transient current. Ideally, by the time the capacitor runs out of charge, the switching event has finished, so that the load can draw full current at normal voltage from the power supply and the capacitor can recharge. The best way to reduce switching noise is to design a
PCB
PCB may refer to:
Science and technology
* Polychlorinated biphenyl, an organic chlorine compound, now recognized as an environmental toxin and classified as a persistent organic pollutant
* Printed circuit board, a board used in electronics
* ...
as a giant capacitor by sandwiching the power and ground planes across a
dielectric
In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...
material.
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.
Transient load decoupling
Transient 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 appropriate type if switching occurs very fast.
Logic
Logic is the study of correct reasoning. It includes both formal and informal logic. Formal logic is the science of deductively valid inferences or of logical truths. It is a formal science investigating how conclusions follow from premise ...
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 poor
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 supply voltage to change. 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 is placed as close as possible to the device requiring the decoupled signal. This minimizes the amount of line
inductance and series
resistance between the decoupling capacitor and the device. The longer the conductor between the capacitor and the device, the more inductance is present.
Capacitor Design Data, and Decoupling Placement, How-to
o
Leroy's Engineering Web Site
/ref>
Since capacitors differ in their high-frequency characteristics, decoupling ideally 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
An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon di ...
or tantalum capacitor
A tantalum electrolytic capacitor is an electrolytic capacitor, a passive component of electronic circuits. It consists of a pellet of porous tantalum metal as an anode, covered by an insulating oxide layer that forms the dielectric, surrounde ...
(s) up to a few hundred μF per board or board section.
Example uses
These photos show old printed circuit boards with through-hole capacitors, where as modern boards typically have tiny surface-mount capacitors.
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
Cromemco was a Mountain View, California microcomputer company known for its high-end Z80-based S-100 bus computers and peripherals in the early days of the personal computer revolution.
The company began as a partnership in 1974 between Harry ...
XXU, a Motorola
Motorola, Inc. () was an American multinational telecommunications company based in Schaumburg, Illinois, United States. After having lost $4.3 billion from 2007 to 2009, the company split into two independent public companies, Motorol ...
68020
The Motorola 68020 ("''sixty-eight-oh-twenty''", "''sixty-eight-oh-two-oh''" or "''six-eight-oh-two-oh''") is a 32-bit microprocessor from Motorola, released in 1984. A lower-cost version was also made available, known as the 68EC020. In keepin ...
processor S-100 bus
The S-100 bus or Altair bus, IEEE 696-1983 ''(withdrawn)'', is an early computer bus designed in 1974 as a part of the Altair 8800. The bus was the first industry standard expansion bus for the microcomputer industry. computers, consisting of p ...
card. The axial parts between the ICs are decoupling capacitors.
File:Cromemco 16KZ S-100 Board.jpg, 1970s Cromemco
Cromemco was a Mountain View, California microcomputer company known for its high-end Z80-based S-100 bus computers and peripherals in the early days of the personal computer revolution.
The company began as a partnership in 1974 between Harry ...
16KZ, a 16KB DRAM memory S-100 bus
The S-100 bus or Altair bus, IEEE 696-1983 ''(withdrawn)'', is an early computer bus designed in 1974 as a part of the Altair 8800. The bus was the first industry standard expansion bus for the microcomputer industry. computers, consisting of p ...
card. The green round disc parts are decoupling capacitors.
File:Interface I1.JPG, 1970s I1 parallel
Parallel is a geometric term of location which may refer to:
Computing
* Parallel algorithm
* Parallel computing
* Parallel metaheuristic
* Parallel (software), a UNIX utility for running programs in parallel
* Parallel Sysplex, a cluster of ...
interface board for Electronika 60
The Electronika 60 (russian: Электроника 60) is a computer made in the Soviet Union by Elektronika in Voronezh.
Overview
Alone the ''Electronika 60'' is a rack-mounted computer with no built-in display or storage devices. It was usu ...
. The green rectangular parts are decoupling capacitors.
See also
* Ceramic capacitor
* Equivalent series inductance
Equivalent series inductance (ESL) is an effective inductance that is used to describe the inductive part of the impedance of certain electrical components.
Overview
The theoretical treatment of devices such as capacitors and resistors tends to ...
* Equivalent series resistance
Practical capacitors and inductors as used in electric circuits are not ideal components with only capacitance or inductance. However, they can be treated, to a very good degree of approximation, as being ideal capacitors and inductors in series w ...
* Film capacitor
Film capacitors, plastic film capacitors, film dielectric capacitors, or polymer film capacitors, generically called film caps as well as power film capacitors, are electrical capacitors with an insulating plastic film as the dielectric, sometime ...
* E-series of preferred numbers
The E series is a system of preferred numbers (also called preferred values) derived for use in electronic components. It consists of the E3, E6, E12, E24, E48, E96 and E192 series, where the number after the 'E' designates the quantity of ...
References
{{Reflist
External links
Choosing and Using Bypass Capacitors
– application note from Intersil
– decoupling guide for various frequencies by Henry W. Ott
Power Supply Noise Reduction
– how to design effective supply bypassing and decoupling networks by Ken Kundert
ESR and Bypass Capacitor Self Resonant Behavior: How to Select Bypass Caps
– article written by Douglas Brooks
Circuit Board Decoupling Information
– decoupling guidelines for various types of circuit boards
Basic Principles of Signal Integrity
– Altera whitepaper
Bypass Capacitors, an Interview With Todd Hubing
– by Douglas Brooks
Capacitors