Yeah, i've looked at other solution, and taken apart the popular 100:1 HV diff probe re-branded and sold by various peeps including dave.
But i wanted to also approach from first principals, and ask what others have done or would do. I have the 100:1 compensated attenutator from the existing design, which i've tuned up, and now has roughly a 20Mhz bandwidth (-3dB), so was wondering if there was an off-the-self INA that i can just slap on after it, or if it's worth the trouble to role my own. Component Cost is (relatively) un-important.
The Common mode signal is typically a square wave at around 20 Khz fundamental, but of course, having fast edges (think 500V in <1uS) it contains a significant high frequency energy. It is this energy that is currently causing issues with the INA128 i use at the moment, driving it into saturation / overshoot and then the long (relatively) recovery period during which time it's output cannot reflect the true differential input signal.