Low Voltage and High Frequency Signal

At what resolution do you want to get? Are you looking to create a square wave then no a comparitor would do it, if you are looking to build an 8-bit representation then yes!

Im interesting.

200Mhz sample, how mcuh bits? and s/n?

what do you want to do?

Are you looking for an integrated circuit or a PC plug in board (or what?).

National semiconductors do a 170 MS/s 3.3 V 12 bit ADC; that would give you 3 bits of resolution but not quite at 200 MHz.

There are lots of PC plug in boards that do 12 bit, +/- 200 mV at 200 MS/s. That would give you 6 bits of resolution.

What are you trying to do?

What about a cheap DSO?

if 10mv at 10bits converter, noixe will be less than 10uv.

12bit less than 5uv.

it may be a hard work

If you are only dealing with 200 MHz, dont forget about undersampling.