Galvanic Isolation
Galvanic isolation is breaking the direct electrical connection between two circuits with an optocoupler, isolation IC or transformer; passing the information through while not passing the current path. This way the ground difference and noise between the two sides are not carried into the sensitive circuit.
Isolation is typically used for three reasons:
- Ground difference: there can be a measurable voltage difference between two distant points. A direct connection leads to current circulating over the signal line and to corrupted readings.
- Noise separation: when the references of the switching power stage and the measurement or communication stage are separated, the fast current changes on the power side affect the logic side less.
- Protection: it becomes harder for a fault voltage occurring on the field side to pass to the controller side.
Isolation has a price: a separate supply source is required, a small delay appears in the signal and the cost increases. For this reason the decision is selective; usually the communication line, the field inputs or the feedback path are isolated, not the whole board. It is also a known point that the current transfer ratio of optocouplers changes with temperature and over time, and that margin must be left in the design.
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