An apparatus and a method for compensating signal attenuation based on an equalizer. The apparatus Includes an auto gain controller, an analog-to-digital converter and an auto-gain-control mapping circuit. The auto gain controller is used for receiving an incoming analog signal and amplifying the received analog signal. The analog-to-digital convertor is used for converting the output of the auto gain controller into a digital signal. The output of the auto-gain-control mapping circuit is used to control the gain of the auto gain controller. The equalizer is connected to the output of the analog-to-digital converter to eliminate signal attenuation in the digital output of the analog-to-digital converter. The output of the auto-gain-control mapping circuit controls the gain of the auto gain controller based on the primary weight of the equalizer.