Abstract: Methods for determining a variable data frequency for recording data on a zone of a magnetic storage disc, each zone including a plurality of tracks and each track including a plurality of sectors, the method includes measuring a signal to noise ratio (S/N) around at least a first track in a first zone; and modulating a data frequency based on the measured S/N around the first track.

Abstract: A head testing apparatus is provided for testing an output of a magnetic data recording head. The apparatus includes first and second tester input terminals for coupling to the head, first and second input stages and an AC-coupled differential amplifier. The first input stage has a first buffer input, which is coupled to the first tester input terminal, a first buffer output and a plurality of parallel-connected buffer amplifiers coupled between the first buffer input and the first buffer output. The second input stage has a second buffer input, which is coupled to the second tester input terminal, a second buffer output and a plurality of parallel-connected buffer amplifiers coupled between the second buffer input and the second buffer output. The AC-coupled differential amplifier has first and second amplefier inputs coupled to the first and second buffer outputs through first and second capacitors, respectively, and has a measurement output.

Abstract: A balanced bi-directional current source is provided, which includes first and second current output terminals, a current control circuit, first and second amplifiers and first and second sense impedances. The current control circuit has a reference input, a feedback input, and a control output, which is based on the reference input and the feedback input. The first amplifier has a first amplifier input, which is coupled to the control output, and a first amplified output. The second amplifier has a second amplifier input, which is coupled to the control output, and a second amplified output, which is inverted relative to the first amplified output. The first sense impedance is coupled in series between the first amplified output and the first current output terminal and has a feedback output which is coupled to the feedback input. The second sense impedance is matched with the first impedance and is coupled in series between the second amplified output and the second current output terminal.

Abstract: A balanced bi-directional current source is provided, which includes first and second current output terminals, a current control circuit, first and second amplifiers and first and second sense impedances. The current control circuit has a reference input, a feedback input, and a control output, which is based on the reference input and the feedback input. The first amplifier has a first amplifier input, which is coupled to the control output, and a first amplified output. The second amplifier has a second amplifier input, which is coupled to the control output, and a second amplified output, which is inverted relative to the first amplified output. The first sense impedance is coupled in series between the first amplified output and the first current output terminal and has a feedback output which is coupled to the feedback input. The second sense impedance is matched with the first impedance and is coupled in series between the second amplified output and the second current output terminal.

Abstract: A voltage measurement circuit is provided, which is has an AC-coupled differential amplifier coupled to first and second input terminals for measuring a voltage across the first and second input terminals. The AC-coupled differential amplifier blocks a DC component of the voltage and presenting a low input bias current to the first and second input terminals while isolating the first and second input terminals from charge stored in capacitances in the AC-coupled differential amplifier.

Abstract: A lapping system data acquisition unit is disclosed for use in measuring resistances of multiple resistors embedded in a structure to be machined. Each of multiple independent current sources directs a current through one of the multiple resistors embedded in the structure during machining of a surface of the structure. Each of multiple voltage sensing devices couples to a separate one of the multiple resistors and provides a separate analog voltage output signal indicative of the resistance of the resistor. Each of multiple filters is coupled to a separate one of the multiple voltage sensing devices Analog-to-digital conversion circuitry coupled to the multiple filters generates a digitized signal for each of the filtered output signals.