Methods for removing ground loops

Removing accidental short: If the ground loop is caused by an unintentional connection of a cable shield to ground, then the ground loop is easily removed by eliminating the unintentional connection to ground. 

Isolation transformer, optical link or differential input:
A ground loop that is intrinsic to the circuit design and so not accidental can be removed by inserting an isolation transformer or an optical break in the offending cable. A differential input arrangement could also be tried if the two cables in a twisted pair system each pick up the ground loop signal Vloop, so that by subtracting the signal on one cable from the other, Vloop is removed. This requires a differential input with enough common mode rejection to do this subtraction. In general using isolation transformers, optical breaks or differential amplifiers requires financial outlay and introduces signal degradation.  Signal degradation occurs because transformers operate over a finite bandwidth so that signals outside this bandwidth will be lost. Differential inputs can degrade signals because each of the two differential inputs functions over a limited voltage range and saturates if the common mode signal, i.e., Vloop, is outside of this range. For example, if Vloop =10 volts and the differential amplifiers saturate at 1 volt then the signal will be corrupted because the subtraction will fail.  Optical isolation involves converting the signal voltage to an optical signal that is then converted back to an electrical signal. Both the up- and down-conversion involve nonlinearities which can lead to distortion. Also, up- and down- conversion typically involves a requirement for isolated electrical power being supplied on both sides.

Removing linked magnetic flux:
The ground loop signal could be eliminated without removing the loop if the magnetic flux linking the loop is removed. For example, if the ground loop is due to an electric motor, the motor could be moved to a different location. If the source is a switching power supply, the power supply could be replaced by a linear power supply since the latter does not produce time-dependent magnetic fluxes. Magnetic shielding could be used to divert the magnetic flux away from the ground loop. The entire circuit could be surrounded by a flux-conserving conducting shield, an arrangement that differs from the electrostatic shields discussed earlier. The flux conserving shield has image electric currents that produce a magnetic field equal and opposite to the offending magnetic field. The shield must be thick enough so that the L/R decay time of these currents is much longer than the characteristic time scale of the spurious signal. In contrast an electrostatic shield could have gaps and need not be made of a thick conductor because all that is required is a means to drain off capacitively coupled electric current.

Increasing inductance of loop path to decrease ground loop current:
The inductance of the ground loop current path can be increased without affecting the inductance of the path of the desired signal by wrapping a portion of the coaxial or twisted pair cable around shield beads, ferrite cores, or an iron-core transformer. This has the effect of increasing the impedance seen by the ground loop current without increasing the impedance seen by the signal current.   The coaxial or twisted pair cable could be wrapped one or more times around the ferrite or iron core which then acts as an inductor in series with the common mode signal. Because inductance increases as the square of the number of turns, wrapping a large number of turns around a core can be very effective. While this method does not affect the desired signal, it becomes progressively less effective at attenuating the spurious signal as the frequency of the spurious signal is lowered. It is most suitable for protecting against fast transient glitches but would be ineffective for low frequency hum.

Eliminating area linked by ground loop:
The area effectively linked by the ground loop could be made arbitrarily small by twisting signal cables around the power cables and have all instruments plugged into adjacent mains sockets. As an example,  the signal cable interconnecting instruments A and B would start at A, be twisted around the A power cable going back to the mains outlet, then twist around the B power cable and then connect to its appropriate terminal on B. This scheme eliminates the area linked by the loop so no stray magnetic flux is linked. The ground loop area is minimized by having instruments A and B plugged into adjacent power outlets.






Troubleshoot ground loops with ease
and
without disconnecting anything!
US Patent 7,791,353 B2