In pipelined-successive approximation register analog-to-digital converter (pipelined-SAR ADC), it is necessary to use large-open-loop gain operational amplifiers to improve the gain accuracy of closed-loop residual amplifications. The proposed ring amplifier uses a gain-enhanced output stage to improve the open-loop gain and stability, achieving an open-loop gain of over 90 dB and significantly reducing the residue gain errors without any calibration techniques, meeting the accuracy requirement of a 16 bit ADC. The ADC is implemented in the 65 nm CMOS process with an active area of 0.256 mm². At a sampling rate of 25 MS/s and with Nyquist-rate input, the proposed ADC achieves simulated signal-to-noise distortion ratio (SNDR) and spurious free dynamic range (SFDR) of 77.8 dB and 96.8 dB, respectively, with a power consumption of 2.8 mW. The proposed ADC achieves Walden and Schreier figure-of-merit (FoM) of 18.0 fJ/conversion-step and 174.3 dB, respectively.