Easily isolate circuits with optical fibre

Article By : Paul Rako

You never regret adding isolation between your circuits. And that is true not just for power supplies, but for isolating analog or different digital sections of your circuits.

Alan Martin [applications manager at Texas Instruments] once told me, “You never regret adding isolation between your circuits.” While he was speaking in the context of power supplies, the same goes for isolating analogue or different digital sections of your circuits. There are many accepted ways to achieve isolation. These days, rather than try to make an isolated analogue signal chain, many engineers send isolated power to an ADC (analogue to digital converter). They then use a digital isolator to allow the serial digital output to cross the boundary back to the main circuit.

Don’t overlook using fibre optics to achieve isolation. The Navy learned to love optical fibres since they are insensitive to EMP pulses from nuclear weapons. Glass fibres are expensive, and you can expect to pay $25 to $100 for the transmitters and receivers. Then again, glass fibres work to GHz frequencies. A much more cost-effective solution is plastic optical fibre (POF). Here data rates are limited to 10MHz or 100MHz, but the costs are closer to $5 for transceivers.

Another major advantage is that you can use existing consumer fibre, like the TOSLINK cables used in high-end audio receivers. Most new televisions provide an audio output using the optical TOSLINK standard. PC laptops sometimes combine the 3.5mm audio jack with an LED, so you can use a small adapter to convert to the TOSLINK connector. There are also cables available with Mini TOSLINK on one end and a regular TOSLINK on the other. Toshiba has a good overview of its plastic optical fibre systems (PDF).

 

Figure 1: Toshiba Link (TOSLINK) is a plastic optical fibre used in audio and TV equipment. (Source: Wikimedia user Hustvedt)
 

 

Figure 2: Plastic optical fibre can be used with a 3.5mm audio jack, to allow use of a TOSLINK cable. (Source: Monoprice)
 

Combine a Toshiba TOTX1350 transmitter and a TORX1350 receiver and you can send data at 10Mbps. The modules cost about 7 bucks, and you can plug in any TOSLINK cable, including one from the Radio Shack down the street. That $15 cable could provide kilovolts of isolation as long as you ensure no creepage down the length of the cable.

The TOSLINK standard uses the JIS F05 connector. While cheap, it also has no positive retention. So you can use an SMA plastic optical fibre cable. This is used in industrial standards like Sercos. This standard also supports glass fibres that can carry much higher speeds. Glass fibres will take the price from 5 bucks a cable to 50 bucks. You can also adapt automotive plastic optical fibre designs as used in standards like MOST bus that carry 50 to 150Mbps traffic. With these speeds you can transmit digital video signals over 20 metres.

 

_Figure_3: The Sercos industrial standard allows use of plastic optical fibre that employs SMA-style threaded connectors. (Source: Industrial Fiber Optics)
 

 
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