This Letter proposes a novel, to the best of our knowledge, matrix digitization method for a photonic analog-to-digital converter with phase-shifted optical quantization (PSOQ-ADC). This method overcomes the issues of excessive bit width of the output code and the generation of invalid codes encountered by the traditional direct digitization method. A PSOQ-ADC was fabricated on a lithium niobate on insulator (LNOI) platform, and an experimental platform was built. The results show that RF signals at 1/2/5 GHz, which were sampled by a 50GS/s optical pulse train, were digitized successfully with the matrix digitization method, producing 5-bit codes without invalid codes. In comparison, the direct digitization method yields 10-bit codes, and as the optical signal-to-noise ratio (OSNR) decreases, the ratio of invalid codes increases in the direct digitization method; even with Hamming distance correction, its effective number of bits (ENOB) remains smaller than that of the matrix digitization.