A differential amplifier is usually used in frequency inverters and such applications to measure high DC and AC voltages. They do not provide perfect isolation, but if one or two megohms is adequate, they can be used.
There are also dual optocouplers that can be used in linear applications and make around one percent untrimmed accuracy possible. And then, there is the PWM technique or V/f and transmission via a small HF transformer or opto coupler.
And, of course, the Burr Brown piezoelectric isolator can also be used in circuits where the du/dt isn't too high.
Hmm, i never thought some of these techniques. Thanks for the help. What products are available for dual optocouplers? I think it may be fit and easy to use.
You are right. They are not linear. But they are consistent and two optocouplers made from the same batch of silicon are (almost) identical. So you can use one optocoupler for the isolation and one in the feed-back path of an auxiliary amplifier. Both emitters carry the same current. The output from the HV side opamp follows input linearly. Since both optocouplers are identical, the "close to ground" opamp outputs a voltage that tracks the HV opamp closely.
