This paper presents a fully integrated and battery-free 90 nm interface circuit for ac/dc conversion and step up of low-voltage ac signals generated by electromagnetic (EM) energy harvesters. The circuit is composed of two stages: The rectifier in the first stage utilizes an improved ac/dc doubler structure with active diodes internally powered by a passive ac/dc doubler and custom-designed comparators to minimize the voltage drops. With this, the efficiency is enhanced to 67% while providing 0.61 V to 40 mu A load. The second stage is a dc/dc converter utilizing a low-voltage charge pump with an on-chip ring oscillator for further voltage step up. The rectifier stage is functional down to 125 mV input peak voltage, and the full interface circuit can maintain more than 1 V dc at 1 M Omega load for input peak voltages higher than 0.4 V. The circuit delivers 2.48 V to a 4.4 M Omega load, when interfaced to an in-house EM harvester, operating under 10 Hz, 0.5g vibration.