For the next generation of wireless communications (6G and beyond), peak data rates of several Tbit/s are foreseen. To keep up with such ever-rising demands, photonics-assisted signal processing has been considered a viable solution where conventional copper-based electronics suffers from excess power dissipation issue. This is also attributed to its high bandwidth, energy efficiency, CMOS compatibility, and compact design. Photonics-assisted analog-to-digital converters (PADCs) have been regarded as an alternative to bandwidth and signal integrity-limited electronic analog-to-digital converter (EADC). For the comparison of the quality, different error metrics such as SINAD, ENOB, and root mean square error (RMSE) are used. In this chapter, we begin by describing and discussing these various metrics, followed by using them to evaluate the performance of PADC and compare it with EADC. We address different error sources including time jitter and laser relative intensity noise (RIN). As we further validate by simulation and experiment, PADC may provide better signal-to-noise and distortion ratio (SINAD), effective number of bits (ENOB), and RMSE than state-of-the-art EADC.