A new on-chip 4-bit transient-to-digital converter for system-level electrostatic discharge (ESD) protection design is proposed. The proposed converter is designed to detect ESD-induced transient disturbances and transfer different ESD voltages into digital codes under system-level ESD tests. The experimental results in a 0.13-μm CMOS integrated circuit with 1.8-V devices have confirmed the detection function and digital output codes. The proposed on-chip transient-to-digital converter can be codesigned with firmware operations to effectively enhance immunity of display systems against system-level ESD stresses.