BIAS SERVO LOOP FOR MAGNETO-RESISTIVE READ/WRITE HEAD

A bias current servo loop for a magneto-resistive recording head detects distortion in the read signal from the MR head, and adjusts the operating point for the head to minimize the distortion. The distortion is detected by monitoring the second harmonic of a digitized sample of the read signal. A running average of the digitized amplitude of the second harmonic is generated by a digital bandpass filter. The root mean square of the running average of the digitized amplitude of the second harmonic is estimated. The estimated RMS value is compared against a reference value indicative of minimum distortion and the difference between the estimated RMS value and the reference value is used to adjust the bias current.