Ground Loops

Ground loops occur in a system when there are multiple current return paths or multiple connections to "earth ground". Current flowing in a ground loop generates noise (noisy voltage) in the circuit. A straightforward method to eliminate the loop is to sever the connection between the transducer and ground or between the receiver and ground. When this is not possible, isolation of the two circuits is a standard way to break the loop. Isolation prevents ground-loop currents from flowing and effectively rejects ground voltage differences.

A common method of isolation is via the use of signal conditioners based on transformers or optical couplers. Either way, the common-mode noise voltage appears across the isolation device internal to the signal conditioner. Noise coupling is now reduced to the parasitic capacitance across the isolation barrier. Typical coupling capacitance in a optocoupler is 2 picofarads (pF), or 0.5 picofarads (pF) for shielded components. Coupling can be further minimized using photodiodes, LED’s, and fiber-optic cabling. Faraday shielding may also be used in transformers to reduce coupling. Optical couplers are mostly used for digital signals due to the fact that their linearity is not always suitable for use in analog circuits.

Related articles: The Basics of Electrical Grounding and Grounding to Reduce EMI

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