Flash ADC is the fastest method of conversion. But it has demerits. it requires much opamps.
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for 8-bit conversion we need 255 opamps. which is impossible to construct. where would you place this freaky circuit just for 8 bit. so this is the major disadvantage of the flash adc.
The fastest ones, you'll probably never see; indium phosphide
stuff in fairly narrow flash architectures was running 30+ GSPS,
five years ago. Northrop Grumman and NTT and folks like that
doing their own ICs in their own fabs for in-house consumption.