A Time-over-Threshold asynchronous front-end in 28 nm CMOS for the readout of pixel detectors in extreme radiation environments

This work describes the design, in a 28 nm CMOS technology, of a front-end channel for the readout of pixel sensors in future particle accelerators. The channel being developed leverages the Time-Over-Threshold technique for the numerical conversion of the detector signal amplitude, and includes a low-noise charge sensitive amplifier featuring a compact gain stage architecture. The front-end circuit features an equivalent noise charge not exceeding 70 electrons r.m.s. for a detector capacitance of 50 fF, with a threshold dispersion of the order of 20 electrons r.m.s. A prototype chip including a matrix of 8x32 readout channels has been submitted for fabrication. The design of the front-end channel, together with the main simulation results, is discussed in the paper.