Ground loop (electricity)

A ground loop or ground loop by participating in a ground called ground loop is in electronics and sound a closed to a loop ground connection of electrical wiring or wiring in at low frequency noise currents due to the impedance ( resistance> 0 ) of the loop an unwanted voltage drop signal path generated. Thus, an interference signal to the useful signal is added. The interference can be transmitted over a common impedance with an interference circuit or inductively by magnetic field coupling in utilizing circuit. The effect manifests itself in the sound as annoying buzzing sound, often at twice the frequency of the power supply network, which can then connect to the usual rectification ( Graetz bridge or rectification).

• 4.1 Avoiding a loop
• 4.2 reduction in the equalizing currents
• 4.3 Reduction of interference
• 4.4 reduction in the resistance of a part of the loop
• 4.5 Separation of the signal path from the loop
• 4.6 Digital and optical connections

Occurrence

The interference signal can express, for example, in audio engineering equipment as unwanted, annoying buzzing noise. For encoders and sensors distort the low frequency noise signal oscillograms or signals.

Depending on the cause of the noise signal contains the network frequency (50 or 60 Hz) and a more or less high proportion of its harmonics or harmonic. This occurred even harmonics ( in Europe 150 Hz, 250 Hz, etc. ), and (eg participation of network rectifiers ) and even harmonics ( in Europe 100 Hz, 300 Hz, etc.). Often the grid frequency is hardly or not be heard even in audio systems, since it lies at the lower limit of the AF frequency range.

( DC ) tram wires and their supply lines and railroads can cause interference magnetic fields at 300 Hz, resulting in sub-station in the rectifying of the three-phase supply ( ripple current ).

Causes

The transmission of electrical signals, whether analog or digital, requires the co-transfer of a base or reference potential. This is for balanced signals the other wire ( analog telephone: a- to b-core ) or asymmetric signals, the so-called mass. Are not equal to the reference potential to be connected to equipment, a fault signal is produced at the level of the reference potential difference.

If you connect, for example, the mass of two devices via a cable shield and the device masses at the same time via the protective contacts of the sockets (protection class I ), so results in a closed circuit (in the picture highlighted in yellow ), which on all alternating magnetic sources (transformers acts of power supplies, electric motors, inductors, etc. ) of the environment as a shorted secondary winding of a transformer.

The current flowing through the ground connection is generated across the resistance of the ground connection ( contact resistance wire resistance) voltage. The reference potential of the receiver so that differs by just this power from the transmitter reference potential. is applied to the received signal as an interference signal.

Ground loops occur without connection to the protective conductor eg in metal cabinets. Even there, they can cause interference if ground connections are made several times and in different ways, or even grounded antennas and telephone cables are repeatedly linked.

Large equalizing currents can flow in particular between different points of a protective earth connection (different circuits in domestic installations ) or between those and a grounded antenna or cable TV system.

Furthermore, flow AC and DC currents in the milliamp range in the protective conductor between buildings.

Remedy differential signal transmission with two wires that are connected to the grounding of the equipment does not, or at most in a single place ( principle of an analog telephone ).

Examples

Video and sound

Single audio equipment (amplifier, mixer, but also computers) are often grounded to earth and have a connection of the protective conductor to the signal ground. Another ground connection via the signal cable then leads to a closed ground loop and cause interference. Between different ground connections ( for example, the antenna cable and the protective conductor ) caused by transient currents due to minor differences in ground potentials. They cause voltage drops on the ground connections of the signal ground. These voltage drops add up directly to the useful signal ( NF) or cause in ferrite filter coils amplitude modulation in the ( amplitude modulated ) television signal. In the second case caused by the image current horizontal bars, which may also have missing line sync when the sync pulses are lost due to saturation of the ferrites.

Audio devices (internal)

Ground loops in audio equipment caused by improper design: the mass points of the input jacks and other points of the inner circuit are connected in several ways with each other or even with the protective conductor, this case is. Often the supply current flows to the rectification section of a common ground connection. This leads to hum - depending on the cause of the line frequency or twice the line frequency and harmonics thereof.

Computer networks

In particular, older, asymmetric standards of electronic data transmission (RS -232, parallel port, 10BASE2 ) have problems with ground loops. The mostly with protective earthing ( grounding, protection class I ) provided computer equipment caused together with the outer conductors of the shielded cable between the devices ground loops that could interfere with data transmission. Today's network connections for long distances contain isolation transformers ( Ethernet " Magnetics "). In the USB cable differential signal transmission is used to reduce possible interference with the costs associated with operating voltage and ground connection and the shield ground loops.

Sensors

In machinery and equipment ground loops occur frequently in via coaxial cable connected sensors, if they are both connected at various points on the equipment grounding conductor or to the chassis of the machine. Remedy differential, floating inputs (see pseudo-differential signal transmission ) or connect the shield only at one end of the cable connection.

Circuit boards, motherboards, switching power supplies

For PCBs, and wiring devices occur two mutually contradictory requirements:

• The low-induction possible training ground connections ( ground plane )
• Avoiding ground loops by neutral earthing

These mutually exclusive claims are handled differently depending on the module or device:

• For motherboards, and often even with switching power supplies using a closed ground surface as a separate layer ( Layer)
• If analog and digital signals on mixed, separate ground planes are used, which are connected together at one point only
• At low frequency devices ( amplifiers) it is possible to apply substantially neutral earthing and optionally occupy portions of a ground plane, which are not of large currents ( rectifier outputs) may be flowed through.

Telephone and communication equipment

Experiments in the 19th century, telephone calls unbalanced ( that is, with only one line and with the Earth as a counterpoint ) to be transferred, remained at a few kilometers limited - too large disturbances were. So it was recognized in communications technology first, that the transmission of signals over long distances is only possible, if not just a reference signal is carried as a second line, but this mass also unaffected and remain unused by other signals. The principle of differential signal transmission was obtained. See also Balanced signal transmission.

Countermeasures

Avoiding a loop

Remedy for protective conductor ground loops can create the so-called protective isolation. Therefore, many audio devices are insulated and have - even with metal housings - no connection to the protective conductor. A ground connection between devices is only in the form of the shield of the signal lines. However, once devices are connected through multiple signal ground paths, ground loops can also occur here.

Therefore, the Balanced signal transmission is used, among others, in professional audio equipment. Any potential equalization currents are hereby kept away from the useful signal and differences in the level of the signal grounds between different devices do not cause problems. The mass will only have a shielding effect; Voltage drops to her do not reach the signal. Often in audio systems also exchangers are inserted. Characterized the connection of the mass signal is separated.

Audio engineering devices will come equipped with a so-called ground-lift switch. With this, the connection between the earth pin of the device and the signal ground can be separated in the device. Thus, a possibly existing ground loop can be separated, although this may result in disadvantages for the insensitivity to radio frequency interference and other capacitive interference. A good solution may be a capacitor ( about 0.1 uF ) between signal ground of a device and its protective conductor / casing be: the capacitor is compared to the ripple current has a high resistance, but also ensures the shielding effect of the housing relative to higher-frequency disturbances.

Prerequisite for these two measures ( ground lift switch or condenser ) is a so-called safe separation between parts carrying mains voltage on the one hand and audio signals as low voltage transmitting parts on the other hand: The devices must meet the requirements of safety extra-low voltage ( PELV - Protective Extra Low Voltage) meet.

The separation of the ground connections may be made by interrupting the protective contacts of the mains connection of the devices, however: Here, the protection ( protective earthing) is released, which can lead to dangerous voltages to the device housings in case of error.

See also: functional grounding

Reduce the transient currents

Often devices are used, the signal ground is connected to the protective conductor ( antenna or cable systems, computers ), so ground loops are unavoidable. Here often help ground breaker, isolating transformer or optical data transmission methods. For high-frequency and low-frequency noise useful signals, eg on aerial lines, it may be sufficient to disrupt the outer and inner conductors or even the outer conductor of the antenna line using a capacitor electrically. High-frequency information signals are then transmitted capacitive. For low-frequency noise, the ground loop is broken. Such intermediate connectors are sometimes referred to as a sheath current filter, but they only reduce low-frequency sheath currents.

Reduction of interference

Typically, the enclosed area of the signal lines is kept low by twisted pair cables or coaxial cables are used. The interference in ground loops can be done by reducing the enclosed area (parallel or even twisted laying signal cables separately from power cables). In special cases, a hum compensating succeed by laying in a figure eight.

The source of the interference can not be eliminated often, however, the impact may be in the case of transformers or chokes reduced eg by its change in position. Magnetic shielding around the source can also help, but are costly for low-frequency magnetic fields and therefore are usually found only in tape players or turntables.

Uniformly wound toroidal core transformers have the slightest stray magnetic fields of all transformer designs.

Reducing the resistance of a portion of the loop

The revolving in the ground loop noise current affects the useful signal because it flows at least partly in the same wiring as the useful signal. In general, it then flows into the connection of the signal masses of two devices. Since the signal ground as the reference point for the transmission of the desired signal from one device to another, every difference of the reference point between the devices will act as interference in appearance. If, therefore, ensures that these reference points are as equal as possible, then circulating noise currents will have less impact on the useful signal.

It is therefore advantageous if the ground connection between the devices has a low resistance as possible. This can be achieved ( high cross-section of the screens, low contact transition resistance) or high by additional ground connections cross-section of cables and connectors with little resistance. Some devices have this additional mass screws. Then while still balancing current is obtained ( or possibly increase ), but the voltage drop shifts to areas of the loop, cause the no signal ground.

Separation of the signal path from the loop

Better results can be obtained if the interference does not flow via the signal ground. About the cable shield then flow though possibly interference, the signal ground but is managed separately within this shield. The prerequisite is the galvanic isolation of the signal ground from the shield ground like the gauges plug of telecommunications. Both are connected to a maximum at a single point.

This is equivalent to balanced signal transmission, here no signal ground connection is required - the signal is the differential voltage between two oppositely phased signals. In addition, therefore, stand out in this solution also capacitive couplings - they act on both anti-phase signal lines alike and have no effect on the differential voltage.

Digital and optical connections

Digital compounds in which the audio signals are transmitted as binary coded packets, eliminate the problem of ground loops, as on the receiving end, the digital signals must be decoded and the ( analog ) ripple component is not considered. The S used for digital transmission / PDIF standard provides various types of cables. Wherein the coaxial cable connections or jack is in poor isolation on the receiver side of the opportunity of influencing the analog portions of the signal processing. A complete remedy optical TOSLINK cable. These fiber optic cables are electrically non-conductive and can not be influenced by magnetic or electric fields. Disadvantages are their elaborate optical signal converter.