Just sharing my project.
My fastest scope so far is 23 MS/s and just a single channel.
I use Analogue Devices AD9283 which comes in versions up to 100MS/s.
The weakest link in the chain is the microcontroller which runs at 72MHz but cant clock out the A2D data as fats as it will run.
The analogue bit is just some fast op amps with a multiplexor for switching in different op amp feedback resistors?to set the gain.
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