An auto gain controller is provided. The auto gain controller includes a variable gain amplification unit configured to receive differential input signals, adjust an amplification gain based on an auto gain control voltage and output differential output signals. The auto gain controller also includes a peak detector configured to detect at least one peak value among the amplified differential output signals, a comparator configured to compare the at least one detected peak value with a reference voltage and generate a comparison signal, and a voltage adjusting circuit configured to adjust the auto gain control voltage and the reference voltage based on the comparison signal.