Envelope detector architecture with noise cancelation

A pre-amplifier circuit of a hard disk drive, the pre-amplifier circuit comprising:

• a sensor configured to measure a fly height of the hard disk drive;
• a half-wave rectifier configured to:
• receive a differential voltage from the sensor indicating the fly height of the hard disk drive, and
• generate a pair of single-ended output waveforms based on the differential voltage, each of the pair of single-ended output waveforms having a positive polarity for a half-cycle it passes through;
• a low-pass filter comprising a first low-pass filter and a second low-pass filter, the low-pass filter being configured to allow low-frequency signals from the pair of single-ended output waveforms to pass through while attenuating or blocking higher-frequency signals;
• and
• a differential full-wave rectifier configured to reconstruct a differential signal from the low-pass filter while removing DC rectified components.

A method for detecting an envelope of a signal from a sensor in a pre-amplifier circuit of a hard disk drive, the method comprising:

• receiving, by a half-wave rectifier, a differential voltage from the sensor indicating a fly height of the hard disk drive;
• generating, by the half-wave rectifier, a pair of single-ended output waveforms based on the differential voltage, each of the pair of single-ended output waveforms having a positive polarity for a half-cycle it passes through;
• filtering, by a low-pass filter, high-frequency signals from the pair of single-ended output waveforms while passing through low-frequency signals;
• and
• reconstructing, by a differential full-wave rectifier, a differential signal from the low-pass filter while removing DC rectified components.