Characterization of a 9-decade femtoampere ASIC front-end for radiation monitoring

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 $\mu$m 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).