Utopia: A Nine Decade Femtoampere Sensitivity Current Digitizer and its Application in Ionizing Radiation Monitoring

Radiation measurements and monitoring are very important for particle physics accelerators, hadron therapy institutes, and nuclear facilities. An Application Specific Integrated Circuit (ASIC) able to digitize current generated from ionization chambers was designed and characterized in order to be used as the front-end of the new radiation monitoring system at CERN. The design of the Utopia 2 ASIC was motivated by the need to measure input currents as low as 2 fA and over a wide dynamic range of 9 decades. This paper presents the challenges, the design procedure, the architecture, and the measurement results of the front-end that was fabricated in AMS 0.35-$\mu$m technology. The main limitation was related to the leakage currents that are injected into the input of the ASIC from various sources and are added to the signal of the detector. By active leakage current compensation, the ASIC can measure current down to 1 fA, and by introducing a multiple range architecture, the ASIC can digitize current up to 5 $\mu$A .