Ground loop (electricity)

Ground loops are a major cause of noise, hum, and interference in audio, video, and computer systems.

A common example is two electrical devices each connected to a mains power outlet by a three-conductor cable and plug containing a protective ground conductor for safety.

This forms a closed loop through the ground conductors of the power cords, which are connected through the building wiring.

In the vicinity of electric power wiring there will always be stray magnetic fields, particularly from utility lines oscillating at 50 or 60 hertz.

The ground loop acts as a single-turn secondary winding of a transformer, the primary being the summation of all current-carrying conductors nearby.

Since the conductors comprising the ground loop usually have very low resistance, often below one ohm, even weak magnetic fields can induce significant currents.

Ground loops can also exist within the internal circuits of electronic equipment, as design flaws.

Proper design of such a system will satisfy both safety grounding requirements and signal integrity.

For this reason, in some large professional installations such as recording studios, it is sometimes the practice to provide two completely separate ground connections to equipment bays.

Although ground loops occur most often in the ground conductors of electrical equipment, similar loops can occur wherever two or more circuits share a common current path, which can cause a similar problematic voltage drop along the conductor if enough current flows.

[3][4][5][6] The voltage drops in the ground system caused by these currents are added to the signal path, introducing noise and hum into the output.

1 shows a signal cable S linking two electronic components, including the typical line driver and receiver amplifiers (triangles).

At the destination end (right), the signal and ground conductors are connected to a differential amplifier.

Since the differential amplifier has high impedance, little current flows in the signal wire, therefore there is no voltage drop across it:

of 1 mA at the local power frequency is induced in the ground loop, and the resistance

Attempting to cure these problems by removing the protective ground conductor creates a shock hazard.

Isolation is the quickest, quietest and most foolproof method of resolving hum problems.

Balanced connections see the spurious noise due to ground loop current as common-mode interference while the signal is differential, enabling them to be separated at the destination by circuits having a high common-mode rejection ratio.

This rejection can be accomplished with transformers or semiconductor output drivers and line receivers.

With the increasing trend towards digital processing and transmission of audio signals, the full range of isolation by small pulse transformers, optocouplers or fiber optics become more useful.

Standard protocols such as S/PDIF, AES3 or TOSLINK are available in relatively inexpensive equipment and allow full isolation, so ground loops need not arise, especially when connecting between audio systems and computers.

In instrumentation systems, the use of differential inputs with high common-mode rejection ratio, to minimize the effects of induced AC signals on the parameter to be measured, is widespread.

It may also be possible to introduce narrow notch filters at the power frequency and its lower harmonics; however, this can not be done in audio systems due to the objectionable audible effects on the wanted signal.

Ground loop issues with television coaxial cable can affect any connected audio device such as a receiver.

In digital systems, which commonly transmit data serially (RS-232, RS-485, USB, FireWire, DVI, HDMI etc.)

the signal voltage is often much larger than induced power frequency AC on the connecting cable screens.

Of those protocols listed, only RS-232 is single-ended with ground return, but it is a large signal, typically + and - 12V, all the others being differential.

Other systems break the ground loop at data frequencies by fitting small ferrite cores around the connecting cables near each end or just inside the equipment boundary.

These form a common-mode choke which inhibits unbalanced current flow, without affecting the differential signal.

Coaxial cables used at radio frequencies may be wound several times through a ferrite core to add a useful amount of common-mode inductance.