There are several methods of isolation. The method used depends largely on what exactly is being isolated, the bandwidth of any signals being isolated and whether or not power must be transferred between the two sides of the isolation barrier.
Some of the common methods are:
– These work by combining a LED and a phototransistor in the same pack age. As the LED emits light, the phototransistor will change states, these are well suited for isolating digital signals, but cannot transfer power across the barrier.
– These work by using magnetic coupling. Transformers are very good for passing power and fast switching signals across the isolation barrier.
Differential Capacitor Coupling (DCC)
– This method makes use of a capacitors ability to pass AC signals while blocking DC. DCC can be used for high speed digital data.
Whether or not you need to isolate your USB device from the computer is dependant on the application. For something like connecting a flash drive to a computer, the answer is almost always no, however if it’s medical equipment being connected to the computer, the answer is almost always yes. If you’re connecting industrial/remote I/O equipment to your computer,there is no easy answer, you must evaluate the environment in which the
device(s) will be installed.
Things to consider are whether or not the sensors or device could be subjected to ESD (Electrostatic Discharge), Lightening, Power Surges, Ground Loops, differing ground potentials and/or EMI/RFI (Electromagnetic Interference/RadioFrequency Interference), if the answer to any of these is yes, then you need to isolate the USB connection. In the case of ESD, Lightening and Power Surges, having a non-isolated USB connection can permit dangerous voltages to travel from the sensor, through the USB device, through the cable and into the computer and possibly into other peripherals connected to the computer. Best case is the computer shuts down and needs to be restarted, worst case is complete system failure with many hours or days of down time for device repair or replacement. Even if the transient is small enough to not damage your equipment, the data can contain large errors.