Radiation measurements for high energy physics experiments, nuclear facilities, hospitals and hadron therapy institutes require precise low current sensing. This paper provides a methodology for experimental characterisation of the leakage current sources present at the input of a current-to-digital converter. The limitations in sub-picoampere current measurements are presented along with the design of an ASIC that can accurately measure currents in the femto-ampere range after integrating over sufficient time. Proposals to minimise the subthreshold leakage current of the switches connected to the input, the leakage current of the package, the leakage of the PCB and the leakage related to the ESD protection diodes are shown. The remaining leakage current can be measured and subtracted. The front-end can digitise currents over 8 decades produced by a radiation detector. These guidelines were established during the current design and testing and will be used for the second version of the front-end electronics that will be installed at CERN for radiation protection and monitoring.
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