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At first, I couldn't think of any mechanism to get great isolation between the front-end and the sense amps because hundreds of volts of isolation sounds like capacitive isolation. Capacitive coupling wouldn't allow DC to pass normally. If some sort of dynamic sensing was done, I would think the accuracy would drop greatly. I guess the alternative here is that you simply lose the DC component in this case.

Then I thought maybe you could have a front-end be a MOSFET common-drain/voltage follower with a special FET that has a thick gate oxide increasing the breakdown voltage. Is that all it is? Is it multiple stages of that with each stage on a separate isolated power rails?

Spehro's answer here created the question for me: Can an op amp Vin+ and Vin- be outside the supply voltage so long as their difference is inside it?

This doesn't answer the question, but it's related: Why so few osilloscope with dedicated isolated channels?

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The isolation could be achieved by performing the analog to digital conversion before the isolation but it often isn't.

Here is a portion of the block diagram of the Tektronix TPS2000B isolated oscilloscope from their service manual. Note how the analog to digital conversion is performed after the isolation barrier.

The exact method that is used is not described but it does involve modulating the input signal onto an AC carrier frequency that is transferred across the isolation boundary using a transformer.

On the other side of the transformer the signal is demodulated before being fed to the analog to digital converter.

Tektronix have decided that for the particular bandwidth and accuracy requirements for this oscilloscope this method was better/cheaper etc than putting the analog to digital conversion before the isolation.

Tektronix TPS2000B block diagram

Image from Tektronix.

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  • \$\begingroup\$ Wow, that's a lot more engineering than I was expecting. Modulating the signal allows DC or low frequency signals to get passed through the isolation transformers and allows them to be much smaller. Now I know why they're expensive. \$\endgroup\$
    – horta
    Jan 20, 2022 at 22:05
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    \$\begingroup\$ And if you want very high bandwidth very high isolation you can get one of these tek.com/en/products/oscilloscopes/probes/isovu-isolated-probes for not much over $10,000. Again the isolation happens in the analog domain. \$\endgroup\$ Jan 20, 2022 at 22:44
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The actual solution is much simpler: The ADC sits on the isolated section of the circuit and only digital data is transmitted across the isolation barrier. This is much easier and cheaper than isolating high-speed analog signals with any kind of acceptable signal quality.

The isolated supply voltage for each front-end is usually created with a DC/DC converter brick to provide minimal coupling capacitance between the main circuitry and the isolated section. You need one DC/DC converter per front-end.

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