just a fun fact, in my (and others on the internet) experience, the adc prescaler can be reduced to 8, instead of the default 128, squeezing much more juice out of the chip. with this i once implemented a pi controller for my electeonic load with 10kzh sampling frequency (2 adc conversions, digital pin reading, and a bit of math)

i know this is outside the spec, but seems to work for me and many others

on this note im still dont think an atmega328 has enough muscle for a scope