A Nine Decade Femtoampere Current to Frequency Converter

Various applications require ultra-low current sensing. Some of these applications are related to ionizing radiation detection. Radiation monitoring is important in particle physics experiments, nuclear facilities, hadron therapy institutes and hospitals. In these cases the detectors used are mostly gas-filled detectors like ionization chambers. The output of these detectors is a current that is normally proportional to the energy deposited by the incident radiation. The European Organization for Nuclear Research (CERN) has a legal obligation to comply with the legislation in matters of radiation protection in order to avoid any unjustified dose to people or pollution of the environment. According to the existing detectors, the current output varies from a few femtoamperes up to the microampere range. The scope of this thesis is the design of a microelectronic integrated wide dynamic range front-end for radiation monitoring. Firstly, the state of the art has been investigated and different technologies have been compared. The selected architecture is based on current to frequency conversion with charge balancing. The main limitation in ultra-low current sensing is related to the leakage currents that are present in the front-end input. A demonstrator Application Specific Integrated Circuit (ASIC) named Utopia 1 was built in AMS 0.35 um technology to estimate the different sources of leakage currents and provide guidelines or design solutions for femtoampere measurements. According to the achieved results, a new ASIC named Utopia 2 was designed that has been optimized to minimize the non-ideal effects. The Utopia 2 is able to digitize currents from 1 femtoampere (fA) up to 5 microamperes (uA). To achieve such performance, the ASIC includes an active on-chip leakage current compensation circuit and a multi-range charge balancing circuit. The ASIC integrates the input current in a constant acquisition time, but for the sub-picoampere current measurements the measuring time needs to be increased. The ASIC has been characterized for its low current performance in the Swiss Federal Institute of Metrology (METAS). The ASIC's calibration procedure and qualitative radiation measurements with the detector in the presence of radiation sources have been performed. The designed ASIC is the ultra-low current sensing circuit and digitizer that will be used at CERN for radiation monitoring for personnel and environmental safety.


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