Isolated signal measurement

It's often useful to measure voltages that are floating at a high voltage. One technique is to use analog to digital converters, feed the digital signals into fiber optic links, then digital to analog converters (or just use the digital signals) at ground level. This can also be done with voltage-to-frequency converters and frequency-to-voltage converters. Other techniques include capacitive pick up, transformers, etc. Linear optoisolators can be used in some applications. There are a few manufacturers that can make them, or you can assemble your own. There is an application note from Agilent that can provide additional information.

For less voltage isolation, other techniques can be used, such as isolation amplifiers.

There are a few commercially available devices provide isolated voltage measurement.