High-sensitivity optical receivers enable energy-efficient solutions for high-speed optical interconnects. In this article, an analog front-end (AFE), employing a sub-Nyquist CMOS-based trans-impedance amplifier (TIA) with post-TIA equalization, is characterized to give an insight on how to design the AFE for optimizing receiver sensitivity and energy-efficiency. Thus, maximizing SNR is targeted. Sub-Nyquist TIA design trade-offs were analyzed to study the BW limitation impact on linearity and noise. The characterization utilizes idealized NRZ/PAM-4 samplers with DFE/FFE to analyze the equalization complexity. Optimum SNR is reported for different equalizers, specifying the relaxation achieved in TIA BW requirement with respect to the desired data-rate.