1. Interference
   Interference (communication)
   In communications and electronics, especially in telecommunications, interference is anything which alters, modifies, or disrupts a signal as it travels along a channel between a source and a receiver. The term typically refers to the addition of unwanted signals to a useful signal...
   
    that appears on both signal leads (signal and circuit return), or the terminals of a measuring circuit
   Electrical network
   An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...
   
   , and ground
   Ground (electricity)
   In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
   
   .
2. A form of coherent
   Coherence (physics)
   In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
   
    interference that affects two or more elements of a network
   Telecommunications network
   A telecommunications network is a collection of terminals, links and nodes which connect together to enable telecommunication between users of the terminals. Networks may use circuit switching or message switching. Each terminal in the network must have a unique address so messages or connections...
   
    in a similar manner (i.e., highly coupled) as distinct from locally generated noise or interference that is statistically independent between pairs of network elements.

Techniques for dealing with common-mode interference

• Common mode noise may be sensed and fed back negatively into object providing the signals (see Driven Right Leg).
• Both signal and signal return may be applied to the primary of a transformer
  Transformer
  A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
  
  , with the signal taken from the secondary. As common mode interference will not cause current to be induced in the primary, no signal from this source will be seen in the secondary, while differential signals on the primary will cause current in the primary and so cause induced voltage in the secondary.
• A signal transformer may have a center tap
  Center tap
  In electronics, a center tap is a connection made to a point half way along a winding of a transformer or inductor, or along the element of a resistor or a potentiometer....
  
  ped primary to ground, with the signal and signal return operating as a balanced line
  Balanced line
  In telecommunications and professional audio, a balanced line or balanced signal pair is a transmission line consisting of two conductors of the same type, each of which have equal impedances along their lengths and equal impedances to ground and to other circuits. The chief advantage of the...
  
   (push-pull technique). This is advantageous; its resistance to signals raised on ground due to ground loop
  Ground loop (electricity)
  In an electrical system, a ground loop usually refers to a current, almost always unwanted, in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. Ground loops created by improperly designed or improperly installed...
  
   induction and ground circuit resistance.
• The signal (line and return) may be used to drive the LED
  LEd
  LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....
  
   in an opto-isolator
  Opto-isolator
  In electronics, an opto-isolator, also called an optocoupler, photocoupler, or optical isolator, is "an electronic device designed to transfer electrical signals by utilizing light waves to provide coupling with electrical isolation between its input and output"...
  
  .