Editing Ground loop (electricity) (section)

===Ground currents on signal cables===
[[Image:Signal cable with ground current.svg|thumb|upright=1.5|Fig. 1: A typical signal cable ''S'' between electronic components, with a current ''I'' flowing through the shield conductor]]

The symptoms of a ground loop, ground noise and hum in electrical equipment, are caused by current flowing in the ground or {{em|shield}} conductor of a cable.  Fig. 1 shows a signal cable ''S'' linking two electronic components, including the typical [[line driver]] and receiver amplifiers ''(triangles)''.<ref name="Robinson" />  The cable has a ground or shield conductor which is connected to the chassis ground of each component.   The driver amplifier in component 1 ''(left)'' applies signal ''V''<sub>1</sub> between the signal and ground conductors of the cable.  At the destination end ''(right)'', the signal and ground conductors are connected to a [[differential amplifier]].  This produces the signal input to component 2 by subtracting the shield voltage from the signal voltage to eliminate [[common-mode interference|common-mode noise]] picked up by the cable
<math display="block">V_2 = V_\text{S2} - V_\text{G2} \,</math>

If a current ''I'' from a separate source is flowing through the ground conductor, the resistance ''R'' of the conductor will create a voltage drop along the cable ground of ''IR'', so the destination end of the ground conductor will be at a different potential than the source end 
<math display="block">V_\text{G2} = V_\text{G1} - IR \,</math>
Since the differential amplifier has high impedance, little current flows in the signal wire, therefore there is no voltage drop across it: <math>V_\text{S2} = V_\text{S1} \,</math>  The ground voltage appears to be in series with the signal voltage ''V''<sub>1</sub> and adds to it
<math display="block">V_2 = V_\text{S1} - (V_\text{G1} - IR)\,</math>
<math display="block">V_2 = V_1 + IR\,</math>

If ''I'' is an AC current this can result in noise added to the signal path in component 2.