We propose and demonstrate a performance-enhanced optical quantizer by inverse design. An adjoint shape co-optimization method is used to optimize the boundaries of the optical quantizer, aiming to reduce the insertion loss (IL), improve the uniformity, and increase the bandwidth of the effective number of bits (ENOB). Meanwhile, the optimized shape maintains its deep ultraviolet (DUV) photolithography fabrication capability. We fabricate the device on a commercial silicon-on-insulator (SOI) platform. Measurement results show that the IL is reduced from 0.85 to 0.35 dB, and the uniformity is optimized from 1.21 to 0.24 dB at 1550 nm. The maximum ENOB increases to 3.31 bit, which is very close to the ideal value of 3.32 bit, and the bandwidth of the ENOB > 3 bit is expanded to more than 50 nm.