This paper presents the design and the characterization of a front-end circuit, designed in 28 nm CMOS, for real-time dosimetry in radiation diagnostics and radiation therapy. The front-end circuit is optimized for sensors made of hydrogenated amorphous silicon. The scheme is based on a current-to-frequency converter to sustain a large range of input currents. Three different programmable solutions have been investigated to be compatible with a wide variety of sensor sizes and applications. The front-end has been designed around three key specifications: an input capacitance between 1 pF and 50 pF, an input current from 100 pA to 2 µA, and a measurement time ranging from 400 µs to 60 ns. A first prototype has been fabricated and it is being characterized in laboratory.