We have developed a 6-bit flash ADC powered from a single 1.8 V supply, using a bipolar based BiCMOS process. Measurements revealed that it operates up to 340 Msps at 1.26 V power supply consuming 36 mW. The conversion rate per power performance index of 9.4 Msps/mW is the highest in the fast 6-bit ADCs reported to date. To operate at this low supply voltage, a new encoder scheme, together with the unique layout, was devised which also substantially improved sparkle error rate. The encoder circuits was synthesized in a new logic topology that we named "folding logic. " This new logic topology is not only suitable for low-voltage operation but also intrinsically fast.