This paper reports a 4-bit flash ADC with a supply range from 0.35 to 1 V. The corresponding sampling rates are 0.2 to 3 GS/s. This low-voltage low-power ADC is a feasible building block for a solar-powered wireless module. Passive subtraction by resistor ladders replaces active subtraction by 4-input preamplifiers since the linearity of ladders is independent of supply voltage. Passive subtraction and low-threshold devices enable low-supply operation. At 1.2 GS/s, the ADC consumes 1.93 mW from a 0.6-V supply. The ENOB is 3.60 bit and ERBW is 550 MHz, resulting in an FOM of 145 fJ/conversion-step.