choosing adc for digital oscilloscope

hi all.

I am designing a digital oscilloscope and i am searching for a 1 or 2 channel12 or 14 bit ADC with 40M sample rate  or higher.

Power Dissipation is an important factor because the power will supply from USB and there is another chips in my circuit that will draw current!

can any one guide me for selecting ADC?

finally

can any one guide me for measuring -+50v with differential ADC's?!

thanks