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In electronics, ground noise is electronic noise on the ground wires or busses of an electronic circuit. In audio, radio, and digital equipment it represents an undesirable condition since the noise can get into the signal path of the device, appearing as interference in the output. Like other types of electronic noise it can manifest in audio equipment as a hum, hiss, distortion or other unwanted sound in the speakers, in analog video equipment as "snow" on the screen, and in digital circuits and control systems as erratic or faulty operation or computer "crashes". A major source of ground noise is ground loops created by improper interconnection of audio, video or computer components.
Common mode rejection
Most modern audio amplifiers incorporate designs which implement a feature called "common mode rejection". Common mode rejection provides that the amplifier has two input connections: a negative connection (sometimes called "ground" or "signal ground") and a positive input. The amplifier (if properly designed and implemented) will reject signals at its inputs so long as the signals appear at both inputs in equal phase and amplitude, so called common mode signals. Thus, these signals are in "common mode" because they arrive equally common to both inputs. In contrast the amplifier is designed to amplify the differences in input signals applied to its input terminals.
Leveraging common mode rejection is the basis for the methods described here to avoid ground noise and improve rejection of unwanted small signal noise and interference.
Ground noise appears at the negative input of an amplifier for several reasons, including ground loop. Other causes can deliver a different input signal to the amplifier's negative input terminal and not also simultaneously to the amplifier's positive input terminal.
Isolated ground loop
One such source of ground noise is the isolated ground loop. The isolated ground loop differs from the ground loop in that there is no secondary connection to power supply ground or other non-signal ground. Instead two signal ground conductors (such as the outer conductor of stereo coaxial audio wire) are common at the amplifier input (by an internal connection in the multi-channel amplifier), they travel some distance whereupon they experience different induction currents and then are made common at some other secondary location (typically a multi-channel signal source device), as in stereo patch cords. The induction currents being different for each of the two outer coaxial conductors set up a current between the two which results in a difference in potential appearing at the negative input terminals at the amplifier, with respect to the positive input terminals. The problem is made worse by the increasing the length of the cables, and their proximity to devices radiating magnetic noise. One solution to this problem incorporates the use of a single ground conductor as a shield (see coax) and the use of multiple other separated internal conductors as positive signal wires inside a single shield conductor, allowing for several separate and distinct positive signals to all arrive at the amplifier input referenced to a single ground shield conductor. This method also insures that all the signal conductors and the single ground shield conductor (like coax) all experience similar induction of unwanted signals. Another solution is the incorporation of "star topology" signal grounding technique wherein no signal ground ever connects to any other signal ground, except at a single centralized location.
Unequal induced current
Another source of ground noise is also caused by unequal induced current. If a positive signal wire and negative signal wire travel different physical paths to the amplifier input then the differences in induced currents will arrive at the amplifier input terminals as differences which will be amplified. Thus it is imperative that the positive and negative signal wires are dressed out in such a way as to ensure that they both experience equal induction from outside sources. Two popular methods include using coaxial wire (both conductors have the same axis of rotation everywhere they exist, and thus have similar inductive experiences), and twisted pair where the two conductors have the same axis of rotation (but don't always exist in the same path).