Ground loops are created when the source of a signal and the driven instrument have their common inputs tied to the earth at each device or the common lead is used to carry AC current for some other device. AC current flowing in the common lead drops a small AC voltage due to the wire resistance. The voltage appears in series with the desired signal and creates "noise" on the desired signal. The Isolator breaks the common lead and the AC current can no longer flow.
A common mode voltage is a voltage that is connected to the + and - inputs of a device. Example - A small resistor in series with the + lead of a 138VDC battery and the - side of the battery is the "common" lead for the system. The resistor drops a small voltage proportional to the battery current. The Isolator can measure the small voltage across the resistor even though it is floating 138VDC above the system circuit common.
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