Abstract: Apparatus and method for asynchronous gain adjustment are provided for data detection in a partial-response maximum-likelihood (PRML) data channel. The PRML data channel includes an analog to digital converter (ADC) having a normal operating range and a filter, gain and timing control coupled to the ADC. A plurality of samples are detected from the ADC. Each of the detected samples are sequentially compared with predetermined threshold values. The predetermined threshold values include a zero value, and a minimum value and a maximum value of the normal operating range of the ADC. An absolute value of each of the detected samples are sequentially compared with a forth predetermined threshold value. A gain adjustment correction value is determined utilizing the sequentially compared values.