Thanks for the info.
The datasheet for ADG612 says the following:
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Peak Current, S or D 20 mA (pulsed at 1 ms, 10% duty cycle maximum)
Continuous Current, S or D 10 mA
Overvoltages at IN, S, or D are clamped by internal diodes. The current should be limited to the maximum ratings given.
With a 820 K series resistor you need to apply 8200 volts to get up to the 10 mA that the clamps in the chip can safely handle - but that would cause the resistor to drop 82 watts - that's maybe a bit over the rating of the resistor :-)
But at a more reasonable 500 volts input there would only be a 0.6 mA current into to clamping diodes and the resistor would only need to dissipate 300mW - still at least three times a normal 0402-sized smd can handle continously, but for short term it probably would be ok.So if we disregard any potential problems with too small isolation distances between tracks the oscilloscope inputs would most likely handle mains (230 volts) just fine. I think. Maybe.....