In this paper, we present a 4-bit FLASH A/D converter. The converter is based on a current mode comparator. In order to increase its sensitivity, the latch following the comparator is in an unstable state during the evaluation phase. The circuit is designed in a 0.18 /spl mu/m CMOS process, with a supply voltage of 1.8V. Simulation results show that, with a power consumption of 3mW, the sampling frequency reaches 500 MHz.