Patent ID: 6900959

Claim:
A disk drive comprising: (a) a disk; (b) a head; (c) a voice coil motor (VCM) comprising a voice coil for actuating the head radially over the disk, the voice coil comprising a first end and a second end; (d) a first offset-nulling amplifier for detecting a voice coil voltage across the voice coil, the voice coil voltage comprising a component due to a resistance of the voice coil and a component due to a velocity of the VCM, the first offset-nulling amplifier comprising: a differential amplifier comprising a first input and a second input and a first output and a second output; an input stage responsive to the first and second ends of the voice coil, the input stage comprising a plurality of input switches for periodically configuring the input stage between a first state and a second state, wherein: while in the first state the first end of the voice coil is connected to the first input of the differential amplifier and the second end of the voice coil is connected to the second input of the differential amplifier; and while in the second state the first end of the voice coil is connected to the second input of the differential amplifier and the second end of the voice coil is connected to the first input of the differential amplifier; and an output stage comprising a single-ended amplifier and a plurality of output switches, the plurality of output switches for: connecting an input of the single-ended amplifier to the first output of the differential amplifier while the input stage is in the first state; connecting the input of the single-ended amplifier to the second output of the differential amplifier while the input stage is in the second state; wherein an output of the single-ended amplifier represents the voice coil voltage; (e) a current detector for detecting a coil current flowing through the voice coil, (f) an IR voltage detector, responsive to the coil current, for detecting a resistive voltage due to the resistance of the voice coil; (g) a voltage compensator for subtracting the resistive voltage from the voice coil voltage to generate a back EMF voltage; and (h) a control voltage generator, responsive to a command input and the back EMF voltage, for generating a control voltage applied to the voice coil.