« Previously: Easily isolate circuits with optical fibre  

The low cost of plastic optical fibre is a real plus. You can get a 6m cable with Versalink connectors for less than 4 bucks. Since Versalink is an industrial standard, you can expect to pay more for transceivers, but cable cost is often a major factor in the BOM (bill of material) cost. One thing to keep in mind is that POF is simplex, or one-way in nature. When you order and use duplex POF cable, it is simply a pair of optical fibres molded into one jacket and terminated by two separated fiber ends.

An even slower optical isolation is MIDI (musical instrument digital interface). The standard’s data-rate is 32kbps, but the transceivers will handle faster frequencies should you be willing to push it. Here the cables are copper, but they feed into an optocoupler on the receiver end. All the current is differential. There is no current that can exchange between the connected circuits, and therefore ground loops are eliminated. You would not use this for high-voltage isolation. The copper in the cable will carry any kilovolt potentials into your experiment where it would short into your circuits. Still, MIDI is a great way to use cheap consumer technology to isolate to low-voltage circuits, just what it does in musical instruments.

[MIDI schematic cr] Figure 4: MIDI standard uses copper cables but relies on an optocoupler to provide isolation. (Source: Wikimedia user Dr. Crash)  

[MIDI instrument cr] Figure 5: MIDI interface connects musical instruments and audio equipment. (Source: YouTube/Michael Capoccia)  

Don’t discount physical connections. While wireless technology is ascendant, it can never be as reliable or as secure as a physical connection. As more and more wireless gizmos invade the home, office, and factory, there is more likelihood of interference. Physical connections are naturally more secure since you are not broadcasting the data over a large area. Besides EMP resistance, the Navy loves fibre optics since it is much harder to snoop on an optical connection. So if you need a way to make an isolated system, keep in mind using cheap plastic optical fibres. It won’t run a kilometre, but it might stretch far enough to solve your problem, even if that is only a few inches on a circuit board.

First published by EDN