Hey, I'm trying to build a floating probe, this design is based on the tek A6902B, for the optocoupler LF insulator. Tweaking the design a bit I was able to get 230kHz BW from it using a TL082, a NE5532 and two 4N35. The cost I mention is for those four ICs, of course, the complete build cost will be higher but still quite cheap compared to anything on the market. 200kHz BW is nothing to write home about but quite useful for any audio applications among others.
I leave you a schematic and the rising edges with two different compensations, for 100kHz and 230kHz -3dB bandwidth. As you can see for the 230kHz BW there's a bit of a jiggle so I tweaked the filter to get the other compensation. Also, for the 230kHz the delay in the signal makes way over 90º phase shift at -3dB point. I didn't tried to trim for gain or offset, that shouldn't be too hard to make and having a spare amplifier in each side leaves the option for tweaking. I didn't tested linearity either, but it doesn't look too bad from the waveforms I've observed.
There are faster linear optocouplers, like the 6N136 I'd like to test but I don't have the sim model and can't source the part locally. I'd like to run a test with it!
I think I could find this quite useful, as for higher frequencies using a transformer is not too hard to get around. As you could observe in the 6902 schematics, TEK is using a transformer to get from the optocoupler (which goes up to a few kHz) to 20 MHz, so something similar could be added, but I can't source HF opamps locally either so I gave up about that for now. It doesn't seem to have trouble running from 6V or even lower (other than the clipping level) so it could very well run from two 9V batteries to makes things easier, or build a few channels with a 9V battery for each input and a dedicated PSU for the output. The battery on the floating side should last about 100 hours in this condition, I guess I could try to get that longer using lower DC currents for the optocouplers or use two 18650 for the input and a dedicated supply for the output, which could also work without much hassle.
JS