Ground Loops In Digital And RF Systems PCB

- Feb 28, 2017-

Ground Loops In Digital And RF

Systems PCB

Ground loops in digital and RF systems

In digital systems, which commonly transmit data serially (RS232, RS485, USB, Firewire, DVI, HDMI etc) the signal voltage is often much larger than induced power frequency AC on the connecting cable screens, but different problems arise. Of those protocols listed, only RS232 is single-ended with ground return, but it is a large signal, typically + and - 12V, all the others being differential. Simplistically, the big problem with the differential protocols is that with slightly mismatched capacitance from the hot and cold wires to ground, or slightly mismatched hot and cold voltage swings or edge timing, the currents in the hot and cold wires will be unequal, and also a voltage will be coupled onto the signal screen, which will cause a circulating current at signal frequency and its harmonics, extending up to possibly many GHz. The difference in signal current magnitudes between the hot and cold conductors will try to flow from, for example, item A's protective ground conductor back to a common ground in the building, and back along item B's protective ground conductor. This may involve a large loop area and cause significant radiation, violating EMC regulations and causing interference to other equipment.

As a result of the Reciprocity Theorem the same loop will act as a receiver of high frequency noise and this will be coupled back into the signal circuits, with the potential to cause serious signal corruption and data loss. On a video link, for example, this may cause visible noise on the display device or complete non-operation. In a data application. such as between a computer and its network storage, this may cause very serious data loss.

The "cure" for these problems is different to that for low frequency and audio ground loop problems. For example, in the case of Ethernet 10BASE-T, 100BASE-TX and 1000BASE-T, where the data streams are Manchester encoded to avoid any DC content, the ground loop(s) which would occur in most installations are avoided by using signal isolating transformers, often incorporated into the body of the fixed RJ45 jack.

Many of the other protocols break the ground loop at data baud rate frequency by fitting small ferrite cores around the connecting cables near each end, and/or just inside the equipment boundary. These form a common-mode choke which inhibit unbalanced current flow, without affecting the differential signal. This technique is equally valid for coaxial interconnects, and many camcorders have ferrite cores fitted to some of their auxiliary cables such as DC charging and external audio input, to break the high frequency current flow if the user inadvertently creates a ground loop when connecting external equipment.

RF cabling, usually coaxial, is also often equipped with a ferrite core, often a fairly large toroid, through which the cable can be wound perhaps 10 times to add a useful amount of common mode inductance.

Where no power need be transmitted, only digital data, the use of fiber optics can remove many ground loop problems, and sometimes safety problems too, but there are practical limitations. However, optical isolators or optocouplers are frequently used to provide ground loop isolation, and often safety isolation and the prevention of fault propagation.

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