Disk drive calibrating bias signal for touchdown sensor

A method of operating a disk drive, the disk drive comprising a head actuated over a disk, the head comprising a touchdown sensor comprising a resistance, the method comprising:

• applying a fly height control signal to a fly height actuator to control a fly height of the head;
• configuring the fly height control signal to substantially zero and applying a bias signal at a reference value to the touchdown sensor at a reference temperature and measuring a corresponding reference resistance of the touchdown sensor;
• and
• estimating an operating resistance of the touchdown sensor at a predetermined operating temperature of the touchdown sensor based on the reference resistance, the reference temperature, and a thermal coefficient of resistance (TCR) of the touchdown sensor, wherein the TCR specifies a change in temperature of the touchdown sensor relative to a change in the resistance of the touchdown sensor;
• and
• configuring the fly height control signal to achieve a target fly height of the head and generating an operating value for the bias signal by adjusting the bias signal until a measured interim resistance of the touchdown sensor substantially equals the operating resistance.