Not many people are kind enough to provide a circuit diagram of this unfortunately : (
For example, https://www.fpga4fun.com/digitalscope.html have a project, but it is closed-source, and
they don't supply the code for the FPGA, nor the circuit diagram for the input stage and ADC, even if you
purchase the boards from them, so it is virtually impossible to improve on it, e.g. implement a better trigger, etc.
I would avoid it personally.
You can use Spartan-3E starter Kit to develop osscilloscope on FPGA. It has 14-bit ADC but maximum sampling rate will 1.4 M-samples/sec. You can use FMC ADC cards for high speed data acquisition. check out different ADC Module cards at
The problem is, although in theory you can do this with Spartan starter kit, not many people tend to publish such designs. And a whole lot more needs to be developed than just the data aquisition. The trigger functionality would need to be programmed in too, for a start. Also, a decent analog interface that has the bandwidth, and at least some basic protection and voltage scaling, so that it can be used beyond 0-3.3V or something.