Fly-height control using asynchronous sampling

An apparatus usable to control fly height of a head over a medium in a data storage device, the data storage device having a read path that is configurable in (i) a read-back mode to receive a first signal corresponding to pre-recorded data read from the medium and characterized by at least one periodic pattern and (ii) a loop-back mode to receive a second signal directly from a write path of the data storage device, the second signal bypassing the medium and characterized by the at least one periodic pattern, wherein the apparatus comprises:

• an analog-to-digital converter that (i) samples, in the read-back mode, the first signal at a first sampling frequency and (ii) samples, in the loop-back mode, the second signal at a second sampling frequency different from the first sampling frequency;
• and
• a digital processor that:
• determines, in the read-back mode, a first value based on samples of the first signal;
• determines, in a loop-back mode, a second value based on samples of the second signal;
• generates a third value based on the first and second values, wherein the third value is usable to control the fly height.