Circuitry, systems, and methods for measuring the fly height of a read/write head in a magnetic data storage system. The circuitry generally includes a measurement circuit configured to determine an electrical current of a periodic signal applied to the read/write head, wherein the periodic signal has a generally constant voltage slew rate, and a fly height converter configured to produce an output signal based on a value of the electrical current, wherein the output signal corresponds to the fly height. The present invention advantageously provides for electronically measuring the fly height of a read/write head in situ in a magnetic storage system. The sensed fly height may then be used by the system to control fly height actuation.