Proton Sound Detectors sense the weak acoustic signal emitted by a proton beam that collides with an energy absorber. This paper presents the complete design and electrical characterization of a 28 nm CMOS Analog-Front-End (AFE) for detection of proton-induced acoustic signals generated by a 20 MeV acceleration beam. The proposed AFE is the core of a cross-domain detector composed by the Acoustic Sensor (feeding the AFE) and the Digital Signal Processing (driven by the AFE). The device consumes 4.9 mW and performs <3 nV/√Hz Input Referred Noise Power Spectral Density. Total Harmonic Distortion is -40 dB at 2 MHz and 1.16 dBm single tone, for 37.5 dB final Signal-to-Noise-Ratio (SNR).