A readout integrated circuit of a FMCW LiDAR system comprising: a balanced PIN photo diode configured to: receive an optical signal; and convert the optical signal into a current signal; an optical mixer configured to: receive the current signal from the balanced PIN photo diode; and convert the current signal into a mixed signal of high and low frequencies; a transimpedance amplifier configured to: receive the converted current signal from the optical mixer; and convert the converted current signal into a voltage signal including a low frequency sinusoidal waveform after filtered high frequency; a comparator configured to: receive the voltage signal from the transimpedance amplifier; and generate a pulse signal from the voltage signal; a primary counter configured to: receive a primary signal corresponding to the pulse signal generated by the comparator; and count a number of pulses of the primary signal during a certain time period to generate primary counter data; a reference counter configured to: receive a reference signal; and count a number of pulses of the reference signal during a time period to generate reference counter data; and an arithmetic logic unit configured to: receive the primary counter data and the reference counter data; and calculate depth information corresponding to a distance of a target based on the primary counter data and the reference counter data. a balanced PIN photo diode configured to: receive an optical signal; and convert the optical signal into a current signal; receive an optical signal; and convert the optical signal into a current signal; an optical mixer configured to: receive the current signal from the balanced PIN photo diode; and convert the current signal into a mixed signal of high and low frequencies; receive the current signal from the balanced PIN photo diode; and convert the current signal into a mixed signal of high and low frequencies; a transimpedance amplifier configured to: receive the converted current signal from the optical mixer; and convert the converted current signal into a voltage signal including a low frequency sinusoidal waveform after filtered high frequency; receive the converted current signal from the optical mixer; and convert the converted current signal into a voltage signal including a low frequency sinusoidal waveform after filtered high frequency; a comparator configured to: receive the voltage signal from the transimpedance amplifier; and generate a pulse signal from the voltage signal; receive the voltage signal from the transimpedance amplifier; and generate a pulse signal from the voltage signal; a primary counter configured to: receive a primary signal corresponding to the pulse signal generated by the comparator; and count a number of pulses of the primary signal during a certain time period to generate primary counter data; receive a primary signal corresponding to the pulse signal generated by the comparator; and count a number of pulses of the primary signal during a certain time period to generate primary counter data; a reference counter configured to: receive a reference signal; and count a number of pulses of the reference signal during a time period to generate reference counter data; and receive a reference signal; and count a number of pulses of the reference signal during a time period to generate reference counter data; and an arithmetic logic unit configured to: receive the primary counter data and the reference counter data; and calculate depth information corresponding to a distance of a target based on the primary counter data and the reference counter data. receive the primary counter data and the reference counter data; and calculate depth information corresponding to a distance of a target based on the primary counter data and the reference counter data.