An ultra-low current sensing digitizer circuit was designed for radiation monitoring for personnel and environmental safety at CERN. The Ultralow Picoammeter 2 (Utopia 2) ASIC includes some key functionalities like on-chip active leakage current compensation, charge balancing and range changing. It was designed in AMS 0.35 μm technology that was selected for its low leakage current performance. The architecture is based on the asynchronous current to frequency converter (CFC) and the front-end can digitize input currents over a wide dynamic range of 9 decades. If the measurement time is equal to 100 s, the ASIC can measure current as low as 1 femtoampere (fA). The calibration procedure and the measurements of the Utopia 2 ASIC are summarized in this article. The ASIC has been characterized for its ultra-low current performance at the Swiss Federal Institute of Metrology (METAS)