A "digital" 6-bit ADC in 0.25-μm CMOS

Traditionally, circuit designers have adopted analog techniques to overcome comparator offset in flash converters. These schemes have an adverse effect on area and power consumption and, more seriously, do not scale easily to low-voltage processes. We describe a digital technique which removes the accuracy constraints from the comparators. With no analog matching requirements, the comparators can be small, fast, and power efficient. A 6-bit prototype converter built in a standard 0.25-μm digital CMOS process occupies 1.2 mm₂ and dissipates 150 mW from a 2.2-V supply at 400 MS/s.