In this paper, we have proposed a 4-bit 5-GSample/s flash analog-to-digital converter (ADC) for pulse amplitude modulation (PAM) systems. In order to achieve low-power consumptions, digitalized cells for analogue amplifying are developed in the proposed ADC. Digitalized cells reduce power significantly due to using fewer devices as compared to pure analogue designs. A self-biasing circuit is used in the digitalized amplifier can enhance linear amplifying region. Besides, the digitalized amplifier can achieve high speed according to its bandwidth compensation technique. The test chip is implemented with 90nm CMOS Logic and Mixed-Mode 1P9M Low-K Process. The low-power digitalized ADC is operated under 5GSample/s. All the results of post-simulation are demonstrated in a 16-PAM system, and efficient number of bit is 3.9bit. Moreover, INL and DNL are less than 0.5LSB. The power consumption of the ADC is 33.7mW, and the FoM of energy per conversion step is only 0.45pJ. The overall chip area is 0.873mm2.