OK, so you want to "design" it rather than buy a
USB DAQ unit off eBay for a few tens of dollars.
Not that the low end gear will hang with your
goal of (accurate) acquisition of a 2MHz baseband
signal.
Design always starts with "what do you want?".
That is barely formed, here. You talking 2MHz signal
at what resolution, accuracy (voltage and time)?
8 bits time resolution means 512MHz sample rate.
Not gonna find too many of those kind of ADCs
out there and the ones you do, are for RF with
very limited signal max range. So you'd have to
scale the input probably (range, unspecified at
present) to make any use, and your sample rate
exceeds USB2.0 bit rate (forget word rate).
The USB interface is digital and you can buy chips
for it, although the state machine is probably on
you to design (pump data from ADC to serializer).
Using a good-to-go USB interface chip (like FTDI)
gets you low level drivers and maybe even some
VI code in the vendor's or contrib-space to rip off.