You have to set up your TIA circuit correctly in LTSpice according to the schematic provided in the article you referenced. This includes the photodiode, feedback resistor, and operational amplifier (Op-Amp) configuration.Before analyzing the AC behavior, it's essential to verify that the DC operating point of your circuit is stable and within acceptable ranges. This involves checking voltages and currents at different nodes in the circuit. Perform an AC analysis to determine the frequency response of your TIA. Plot the magnitude and phase of the gain (trans-impedance) across a range of frequencies.