Controlled impedance amplifiers

Details Controlled impedance amplifiers Controlled impedance amplifiers

2000-11-15

2004-01-06

Holder, Regina N. (Department: 2651)

Dynamic magnetic information storage or retrieval

General recording or reproducing

Specifics of the amplifier

C360S046000

Reexamination Certificate

active

06674595

ABSTRACT:

FIELD OF THE INVENTION
The present invention is generally related to mass storage devices including hard disk drive (HDD) systems, and more particularly to impedance mismatches between sensing heads on a flexible printed circuit (FPC) and a differential amplifier on a separate circuit board.
BACKGROUND OF THE INVENTION
In a HDD system, sensing heads adapted to read data from a magnetic storage media are typically mounted on a flexible printed circuited (FPC), and are usually electrically connected to a differential amplifier on a fixed printed circuit board as shown in FIG.
1
. The sensor preamp connection is the connection between the MR sensor and the preamp, which has an input impedance of Rin. The FPC usually has a characteristic impedance of 50-150 ohms. Due to the impedance mismatch of the FPC impedance and the input impedance of the differential amplifier, the frequency response is not very flat, and in many cases peaking is present at a high frequency. The peaking in gain response is not favorable since it degrades group delay flatness, and generates high frequency noise. The peaking may also trigger an oscillation and makes the signal read back process unstable.
SUMMARY OF THE INVENTION
The present invention achieves technical advantages by adding feedback to the input of the differential amplifier to set the amplifier input impedance close to the FPC characteristic impedance. A matched source and input impedance produces a generally flat gain frequency response. As the result of flat response and wider gain bandwidth, white noise without high frequency peaking is obtained.
Conventionally, the input impedance may be set to a desired value by adding a termination resistor (R
T
) in parallel with the input as shown in FIG.
2
. However, the termination resistor attenuates the input signal but not the differential amplifier's noise. Thus, input noise referred to the sensing device increases. Providing feedback according to the present invention is superior in terms of the input referred noise since the feedback attenuates both the input signal from the sensor and the noise from the amplifier. The input referred noise to the sensor does not degrade. In general, the sensing devices are set close to ground potential, while the differential amplifier's outputs are usually close to positive supply voltage due to a nature of a differential amplifier. The feedback resistors may be connected through AC coupling capacitors to compensate the voltage differences between input and output. However, the AC coupling capacitors causes settling issues and increases physical die size. The circuit of the present invention provides a method to connect the feedback resistors directly from outputs to inputs which have different potential.
The advantages of the present invention include matched impedance results in better system level frequency response, the direct feedback connection eliminates costly on-chip capacitors, and the input impedance can be adjusted by the amount of feedback.


REFERENCES:
patent: 5168397 (1992-12-01), Iwamura et al.
patent: 5717361 (1998-02-01), Saito
patent: 5793551 (1998-08-01), Ngo et al.
patent: 6111711 (2000-08-01), Barber et al.
patent: 6341046 (2002-01-01), Peterson

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