User:Hddharvey/GroundLoopDraft

In an electrical system, a ground loop (or earth loop) is a scenario where multiple ground paths exist between two devices, forming a closed loop of ground conductors. Ground loops formed between devices are known for their tendency to cause interference and noise problems due to effects such as electromagnetic induction and common-impedance coupling.

The term "ground loop" has also been used to refer to any scenario where there are undesirable variations in ground potential between two points, whether or not this involves a closed loop of ground conductors. Variations in ground potential can result from any of the non-ideal properties of conductors, such as parasitic resistance and self-inductance, as well as inductive and capacitive coupling with nearby conductors.

Ground loops are a major cause of noise, hum, and interference in audio, video, and computer systems. While these phenomena are not unique to conductors intended to ground or earth a circuit, the terms "ground loop" and "earth loop" are reserved for those cases.

Inductive coupling
Changing currents, such as in nearby mains wiring, will create time-varying electromagnetic fields in the environment. These fields can create an electromotive force in nearby circuits and conductors, in a form of inductive coupling. Long spans of conductor over large areas are especially susceptible to this effect since the fields accumulate to produce a non-negligible electromotive force. If this happens across ground conductors that connect the grounds of two devices, the resulting electromotive force can cause differences in ground potential that interfere with the signal.

Example
A simple ground loop scenario is illustrated involving a transmitter sending a single-ended signal to a receiver over an unbalanced line. Both devices are earthed via the mains earthing system. Time-varying electromagnetic fields from the environment can result in an electromotive force along conductors.

The induced electromotive force along each conductor is as if a parasitic voltage source was present along it, representing noise from the environment. These voltages are less likely to have an effect on the receiver if it was isolated from ground, since both the ground shield and signal line would experience approximately the same noise, cancelling out. However, if the devices are also connected through the mains earth wiring, this symmetry is broken, and circulating ground loop currents create voltage drops along the ground shield that are different from the signal line. This could also be regarded as a specific form of common-impedance coupling.

How the voltage drops due to the noise voltages are distributed will depend on the impedance around the loop. Higher impedances will tend to attract a more significant voltage drop (see voltage divider). If these voltage drops occur across the signal cable, they will contribute to noise seen at the receiver. These voltage drops can also cause interference in places other than the signal cable itself (see ).

Coupling at higher frequencies
Higher frequency noise may be able to induce non-negligible currents through ground conductors, even when there is no closed conductive path for them to flow. This is due to parasitic capacitances that "close the loop".

Common-impedance coupling
Ground conductors are often intended to be the return path for the power supply. Due to the parasitic impedance of the ground conductors, these return currents will result in voltage drops occurring along the return path. If multiple devices or sub-circuits share ground conductors, then current sunk by one can cause a change in the ground potential seen by all. This is a specific form of common-impedance coupling, and can create interference. Any conductor that is accidentally placed across distinct points in this return path, such as a signal cable, will carry some of these currents.

The earthing systems in buildings are intended for electrical safety, and are not usually supposed to carry the return current from the power supply, which is the purpose of the neutral wire. However, ground loops formed by connections between devices can put the mains earth wiring in parallel with the return path of a device, causing return currents to flow through it. This particular effect can be avoided with good design practices (see ).

Example
An example is illustrated of a ground loop causing common-impedance coupling over a signal cable that connects two devices. Normally, the return currents of device A will flow along its own local device ground back to its power supply.

If a signal cable connects the two devices, then a ground loop is formed. The rest of the ground loop is now in parallel with the normal return path in device A, which will result in them carrying some of the current. This will result in some of device A 's return currents flowing through the signal cable, and potentially also through the local circuit ground of device B, resulting in potentially problematic voltage drops. This effect will tend to be worse if device A is a high-current device.

How much (if any) of device A 's ground current flows through the rest of the ground loop depends on the impedance of the ground loop, and how the ground points of device A are configured (see ).

Ground currents
Whatever their source may be, unintended currents flowing through ground conductors can produce undesirable voltage drops due to the parasitic impedance of the ground conductors. Even smaller voltage drops within the signal ground of sensitive circuitry (e.g., that involve high gains or small signals) can introduce non-negligible interference into systems, even those that are able to handle ground potential differences across the cable itself (e.g., systems using balanced lines).

Sources of ground currents
Ground conductors within an individual circuit are often used as the return path for the power supply, meaning that any current drawn by the circuit will return through these ground conductors. Any conductor inadvertently connected between two ground points on the circuit will conduct some of these ground currents.

In the case of a building's earth wiring, there are also multiple possible sources of ground currents:


 * Inductive coupling, as discussed previously (see ).
 * Unintentional return currents due to ground loops, as discussed previously (see ).
 * Ground fault currents due to electrical faults.
 * Stray currents due to parasitic capacitances, such as in power supply transformers.
 * Electrostatic discharge, for example due to the triboelectric effect.

Good design practices can ensure that currents through a building's earth wiring do not flow through the local ground of a device, even if ground loops are formed involving the device (see ).

Differential receiver
A differential receiver takes two inputs and produces a signal equal to their difference. If noise that occurs along the signal line presents itself to the receiver as a common-mode voltage, then it will be be cancelled out. In general, a balanced line is required to ensure that noise becomes common-mode. Higher receiver input impedances will allow a greater degree of mismatch to be tolerated between the impedances of each line.

Galvanic isolation
There are several forms of galvanic isolation that allow a signal to be passed between two points that operate at different ground potentials and that prevent ground loop currents from flowing. An example that works for analog signals is the isolation transformer, which passes signals via inductive coupling. Specialized digital isolators are also available that provide galvanic isolation through inductive coupling.

Several non-ideal properties of transformers, such as parasitic capacitances between the primary and secondary, or distortion, can reduce their effectiveness. The transformer chosen for a specific application should minimize these effects.

Optical coupling is another form of galvanic isolation that is effective in preventing ground loops. Examples include opto-isolators and fiber-optic communication. These allow the ground potential between transmitter and receiver to differ without affecting signal transmission.

Suitable isolation does not guarantee that ground loops cannot arise through other means. For example, MIDI uses optical coupling to transmit signals, but if two devices are connected through their power supply, then there can still be problematic ground loops.

Ground lift
A ground lift is a mechanism to prevent ground loops by disconnecting a ground connection between two points that would normally both be connected, allowing them to float at different ground potentials and preventing ground currents from flowing.

In systems with balanced lines, ground lifts can be applied to the shield of balanced lines to prevent ground currents that can cause undesirable voltage drops in other cables and circuitry present in the system. When doing this, it is recommended to that the cable shield is lifted on the receiver side, to minimize the filtering effect due to the parasitic capacitance of the ground shield. Having the ground shield still connected on one side (TODO why is it good - it helps ESD I guess? - I've seen some sources dispute the usefulness of shielding for balanced lines though...).

"Ground lifting" can also refer to the use of a cheater plug, or the act of removing the earth pin from a device's power cable. While these techniques can prevent ground loops, they also increase the risk of electrical shock since they also remove the protection that the earthing system is supposed to provide.

Common-mode choke
Common-mode chokes can be used to inhibit unbalanced current flow. They can be applied to signal cables to present a high impedance to ground loop currents while still allowing the signal to pass.

Wiring practices
Inductive coupling in particular can be mitigated by grouping the cables involved in the ground loop into a bundle or "snake". The ground loop still exists, but the two sides of the loop are close together, so stray electromagnetic fields produce an equal electromotive force in both sides, which cancel out.

Circuit design
Ground loop currents can cause undesirable voltage drops within signal circuitry and across signal cable shields. This issue can be mitigated by creating an alternate low-impedance path for the currents to flow through. This issue can also be avoided altogether by ensuring that a device's circuitry is externally grounded at a single point. This ensures that:


 * ground loop currents from other sources (e.g., electromagnetic induction, leakage currents, other devices, etc.) do not flow through the local device ground, possibly creating undesirable voltage drops (see ); and
 * a device's own ground currents (e.g., back to its own power supply) do not flow through the rest of the ground loop, possibly creating undesirable voltage drops in other circuits or across interconnecting cables (see ).

An example is illustrated where the ground plane of a receiving device's circuitry is externally grounded at only one point. The incoming signal cable has its ground shield connected to chassis ground, but not directly to circuit ground. Ground loop currents will flow along the chassis instead of circuit ground, and the circuit's own ground currents will not flow across the rest of the ground loop.

Audio systems
TODO

Analog video systems
In analog video, mains hum can be seen as hum bars (bands of slightly different brightness) scrolling vertically up the screen. These are frequently seen with video projectors where the display device has its case grounded via a 3-prong plug, and the other components have a floating ground connected to the CATV coax. This problem can not be solved by a simple isolating transformer in the video feed, as the video signal has a net DC component, which varies. The isolation must be put in the CATV RF feed instead. The internal design of the CATV box should have provided for this.

TODO image of hum bars on video if possible

Digital systems
TODO discuss the effects of ground loops

In the case of Ethernet 10BASE-T, 100BASE-TX and 1000BASE-T, where the data streams are Manchester encoded to avoid any DC content, the ground loops that would occur in most installations are avoided by using signal isolating transformers, often incorporated into the body of the fixed RJ45 jack.