# Patent

## Title
Programmable high pass amplifier for perpendicular recording systems

## Abstract

A system comprises a read element, a first differential amplifier, and a second differential amplifier including inputs that communicate with outputs of the first differential amplifier. First and second resistances communicate with the inputs of the first differential amplifier, respectively, and communicate with the outputs of the first differential amplifier, respectively. First and second capacitances communicate with the inputs of the first differential amplifier, respectively. Third and fourth resistances communicate with the first and second capacitances and with the magneto-resistive read element, respectively, and communicate with the outputs of the second differential amplifier, respectively.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser. No. 10/876,216 filed on Jun. 24, 2004, which application is related to U.S. patent application Ser. Nos. 10/072,843, filed on Feb. 6, 2002, 10/838,040, filed May 3, 2004, 10/459,731, filed Jul. 11, 2003 and 10/814,534, filed Mar. 31, 2004. The disclosures of the above applications are incorporated herein by reference.


FIELD OF THE INVENTION
The present invention relates to amplifiers, and more particularly to high pass amplifiers for read elements in perpendicular recording systems.
BACKGROUND OF THE-INVENTION
Referring now to FIG. 1, an exemplary magnetic storage system 10 such as a hard disk drive is shown. A buffer 14 stores data that is associated with the control of the hard disk drive and/or buffers data to optimize block sizes for increased transfer speed. The buffer 14 may employ SDRAM or other types of low latency memory. A processor 16 performs processing that is related to the operation of the hard disk drive. A hard disk controller (HDC) 18 communicates with the buffer 14, the processor 16, a spindle/voice coil motor (VCM) driver 20, and/or a read/write channel circuit 24. A host 26 sends data read/write requests to the HDC 18.
During a write operation, the read/write channel circuit (or read channel circuit) 24 encodes the data to be written onto the storage medium. The read/write channel circuit 24 processes the signal for reliability and may include, for example error correction coding (ECC), run length limited coding (RLL), and the like. During read operations, the read/write channel circuit 24 converts an analog output from the medium to a digital signal. The converted signal is then detected and decoded by known techniques to recover the data written on the hard disk drive.
One or more platters 28 include a magnetic coating that stores magnetic fields. The platters 28 are rotated by a spindle motor that is schematically shown at 30. Generally the spindle motor 30 rotates the platter 28 at a fixed speed during the read/write operations. One or more read/write arms 34 move relative to the platters 28 to read and/or write data to/from the platters 28. The spindle/VCM driver 20 controls the spindle motor 30, which rotates the platter 28. The spindle/VCM driver 20 also generates control signals that position the read/write arm 34, for example using a voice coil actuator, a stepper motor or any other suitable actuator.
A read/write device 36 is located near a distal end of the read/write arm 34. The read/write device 36 includes a write element such as an inductor that generates a magnetic field. The read/write device 36 also includes a read element (such as a magneto-resistive (MR) sensor) that senses the magnetic fields on the platter 28. A preamplifier (preamp) 40 amplifies analog read/write signals. When reading data, the preamp 40 amplifies low level signals from the read element and outputs the amplified signal to the read/write channel circuit 24. The preamp 40 may include a high pass amplifier. While writing data, a write current that flows through the write element of the read/write channel circuit 24 is switched to produce a magnetic field having a positive or negative polarity. The positive or negative polarity is stored by the platter 28 and is used to represent data.
Referring now to FIG. 2, the read/write channel circuit 24 outputs write signals wdx and wdy to the preamp 40 when writing data. The preamp 40 amplifies the write signals using a write amplifier 42. The amplified write signals are output to the read/write device 36. When reading data, the preamp 40 receives signals from the read/write device 36, amplifies the signals using a read amplifier 44, and outputs amplified read signals rdx and rdy to the read/write channel circuit 24. Other types of the read elements of the read/write device 36 include tunneling magneto-resistive (TMR) and giant magneto-resistive (GMR) sensors.
SUMMARY OF THE INVENTION
A high pass amplifier according to the present invention comprises a first amplifier including an input and an output. A second amplifier includes an input that communicates with the output of the first amplifier and an output. A first resistance includes a first end that communicates with the input of the first amplifier and an output that communicates with the output of the first amplifier. A first capacitance includes a first end that communicates with the input of the first amplifier. A second resistance includes a first end that communicates with a second end of the first capacitance and a second end that communicates with the output of the second amplifier.
In other features, the first resistance is a variable resistance. The first resistance includes a resistive network including a plurality of resistances and a switch that communicates with at least one of the plurality of resistances. A first resistance value of the first resistance is greater than a second resistance value of the second resistance. The first resistance value is approximately one order of magnitude higher than the second resistance value.
In still other features, the first and second amplifiers include an operational amplifier, a voltage mode amplifier and/or a transimpedance amplifier.
In other features, the high pass amplifier is a differential high pass amplifier. A system comprises the high pass amplifier having an input coupled to a magneto-resistive sensor. A flex cable couples the magneto-resistive sensor to the high pass amplifier. A characteristic impedance at the one end of the flex cable substantially matches a characteristic impedance at an input of the differential high pass amplifier. A characteristic impedance at an opposite end of the flex cable substantially matches a characteristic impedance of the read element.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
FIG. 1 is a functional block diagram of an exemplary data storage device according to the prior art;
FIG. 2 is a functional block diagram of a read channel circuit and preamp according to the prior art;
FIG. 3 is an electrical schematic and functional block diagram of a high pass amplifier according to the present invention coupled to a read element;
FIG. 4 is an electrical schematic of a differential high pass amplifier according to the present invention that is coupled to a read element;
FIG. 5 is a graph illustrating gain as a function of frequency for the high pass amplifier of FIG. 4;
FIG. 6 is an electrical schematic an alternate high pass amplifier;
FIGS. 7A and 7B show exemplary resistive networks; and
FIG. 8 is a functional block diagram illustrates a characteristic impedance at opposite ends of a flex cable, at the read element and at the input of the high pass amplifier.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify the same elements. As used herein, “characteristic impedance” shall refer to a ratio of a complex voltage between conductors of a transmission line to a complex current on the conductors in the same transverse plane with the sign chosen such that the real part is positive, as defined by “The Authoritative Dictionary of IEEE Standards Terms”, Seventh Edition (2000).
Referring now to FIG. 3, a read/write device 15 is shown to include a resistance 50, which represents an MR sensor, a TMR sensor, a GMR sensor and/or other suitable read element. A first current source IMR1 biases one end of the resistance 50. A second current source IMR2 biases an opposite end of the resistance 50. A differential high pass amplifier 54 includes a first input that is connected to the one end of the resistance 50 and an opposite and that is connected to the opposite end of the resistance 50.
Referring now to FIG. 4, the differential high pass amplifier 54 according to the present invention is shown. The high pass amplifier 54 includes a first operational amplifier (opamp) 110 having first and second outputs that communicate with a second opamp 114. First and second inputs of the opamp 110 are coupled by input capacitance CIN to opposite ends of the resistance 50, which represents the read element of the read/write device 15. First and second variable feedback resistances RF1 have first ends that communicate with the first and second inputs of the opamp 110 and second ends that communicate with first and second outputs of the opamp 110.
Third and fourth feedback resistances RF2 have first ends that communicate with opposite ends of the resistance 50 and second ends that communicate with first and second outputs of the second opamp 114. In one implementation, RF1>>RF2. For example, RF1 may have a resistance value between 100 k and 200 k and RF2 has a resistance value between 2 k and 10 k.
The impedance looking into the input of the first opamp 110 or




  
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where A1 is the gain of the first opamp 110. The impedance at





  
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Therefore, the high pass amplifier 54 provides a relatively small impedance relative to the read element without adversely impacting the location of the pass bandwidth of the high pass amplifier 54. The high pass corner is related to Zin1∥Cin.

Referring now to FIG. 5, a comparison is made between a bandwidth 120 of the high pass filter with the feedback as shown in FIG. 4 and a bandwidth 122 of a high pass filter having feedback provided by RF1 connected on the opposite side of CIN. The corner of the high pass amplifier 54 occurs at a lower frequency. If RF1 changes, the high pass corner also changes as shown in FIG. 5.
Referring now to FIG. 6, an alternate or nondifferential embodiment of a high pass amplifier 128 is shown. Inverting opamp 130 has an inverting input coupled to the input capacitance CIN and to one end of the variable feedback resistance RF1. An opposite end of variable feedback resistance RF1 communicates with an output of the opamp 130. The output of the opamp 130 also communicates with the input of an opamp 140. A feedback resistance RF2 has one end that communicates with an opposite end of the input capacitance CIN and an opposite end that communicates with the output of the opamp 140.
As can be appreciated, the high pass amplifiers according to the present invention have an improved high pass corner frequency. The feedback resistance RF2 can be set relatively low without significantly adversely impacting the pass bandwidth of the high pass amplifier. The high pass amplifiers are single pole high pass amplifiers from input to output. The resistances disclosed herein can be discrete resistors, poly resistors, transistor-based resistors and/or other suitable resistances.
Referring now to FIGS. 7A and 7B, the variable resistances RF1 can also be parallel, serial or hybrid resistive networks with switches or transistors 200 that are used to switch resistances 202 to change RF1 and to program or adjust the high pass corner.
As can be appreciated, while operational amplifiers are shown and described above, voltage mode amplifiers and transimpedance amplifiers can also be used.
Referring now to FIG. 8, a functional block diagram illustrates the characteristic impedance ZfI at one end of a flex cable 118, the characteristic impedance ZfO at the opposite end of the flex cable 118, the characteristic impedance Zr at the inputs of the read element 15, and the characteristic impedance Zin2 at the input of the high pass amplifier 54. In a preferred embodiment, ZfI is substantially equal to Zr and ZfO is substantially equal to Zin2, which provides maximum power transfer from the read element 15 through the flex cable 118 to the high pass amplifier 54. In some embodiments, ZfO and ZfI are also substantially equal.
Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.

## Claims

1. A system comprising:
a read element;
a first differential amplifier;
a second differential amplifier including inputs that communicate with outputs of said first differential amplifier;
first and second resistances that:
communicate with respective inputs of said first differential amplifier; and
communicate with respective ones of said outputs of said first differential amplifier;

first and second capacitances that communicate with respective ones of said inputs of said first differential amplifier; and
third and fourth resistances that:
communicate with said read element;
communicate with respective ones of said first and second capacitances; and
communicate with an output of said second differential amplifier,

wherein one of said first differential amplifier and said second differential amplifier is a transimpedance amplifier.

2. The system of claim 1, wherein said first and second resistances include variable resistances.

3. The system of claim 1, wherein:
said first resistance comprises a resistive network including a plurality of resistances and a switch; and
said switch communicates with at least one of said plurality of resistances.

4. The system of claim 1, wherein first resistance values of said first and second resistances are greater than second resistance values of said third and fourth resistances.

5. The system of claim 4, wherein said first resistance values are approximately one order of magnitude higher than said second resistance values.

6. The system of claim 1, wherein said first and second capacitances respectively include first and second capacitors.

7. The system of claim 1, wherein said read element includes at least one of a magneto-resistive (MR) sensor, a tunneling MR sensor and a giant MR sensor.

8. A perpendicular recording system comprising the system of claim 1.

9. The system of claim 1, wherein said first and second differential amplifiers are selected from a group including operational amplifiers, voltage mode amplifiers, and transimpedance amplifiers.

10. The system of claim 1, further comprising a flex cable that communicates with said read element and with said first and second capacitances.

11. The system of claim 1, wherein said read element includes a magneto-resistive read element.

12. The system of claim 1, wherein said first differential amplifier detects current at an input and at an output of said read element.

13. The system of claim 1, wherein said first and second capacitances are at least part of a high pass filter.

14. The system of claim 1, wherein said third and fourth resistances are connected to said first and second capacitances and to said read element.

15. The system of claim 1, wherein said third and fourth resistances are connected to said output of said second differential amplifier.

16. The system of claim 1, wherein:
said third and fourth resistances comprise inputs;
said inputs of said third and fourth resistances are connected to said first and second capacitances and to said read element; and
said third and fourth resistances comprise outputs that are connected to said output of said second differential amplifier.

17. The system of claim 1, wherein said first differential amplifier comprises a non-inverting input and an inverting input, and
wherein said first and second capacitances communicate with respective ones of said non-inverting input and said inverting input.

18. A system, comprising:
a read element;
a first amplifier;
a second amplifier including an input that communicates with an output of said first amplifier;
a first resistance that communicates with an input of said first amplifier and with said output of said first amplifier;
a first capacitance that communicates with said input of said first amplifier; and
a second resistance that:
communicates with said first capacitance and said read element; and
communicates with an output of said second amplifier,

wherein one of said first amplifier and said second amplifier is a transimpedance amplifier.

19. The system of claim 18, wherein a first resistance value of said first resistance is greater than a second resistance value of said second resistance.

20. The system of claim 19, wherein said first resistance value is approximately one order of magnitude higher than said second resistance value.

21. The system of claim 19, wherein said first and second amplifiers are selected from a group including operational amplifiers, voltage mode amplifiers and transimpedance amplifiers.

22. A system, comprising:
a read element;
a first amplifier;
a second amplifier including an input that communicates with an output of said first amplifier;
a first resistance that communicates with an input of said first amplifier and with said output of said first amplifier;
a first capacitance that communicates with said input of said first amplifier; and
a second resistance that:
communicates with said first capacitance and said read element; and
communicates with an output of said second amplifier,

wherein:
said first resistance comprises a resistive network including a plurality of resistances and a switch; and
said switch communicates with at least one of said plurality of resistances.

23. A system, comprising:
a read element;
a first amplifier;
a second amplifier including an input that communicates with an output of said first amplifier;
a first resistance that communicates with an input of said first amplifier and with said output of said first amplifier;
a capacitance that communicates with said input of said first amplifier;
a second resistance that:
communicates with said capacitance and said read element; and
communicates with an output of said second amplifier; and

a flex cable comprising:
a first end that communicates with said read element; and
a second end that communicates with said capacitance,

wherein a first impedance of said first end is equal to an input impedance of said read element;
wherein a second impedance of said second end is equal to an input impedance of a filter; and
wherein said filter includes said first amplifier and said capacitance.

24. The system of claim 23, wherein said first impedance is equal to said second impedance.

25. A hard disk drive comprising:
the system of claim 23;
a read and write head that comprises said read element; and
a high pass filter that comprises said first amplifier, said second amplifier, said first resistance, said capacitance, and said second resistance,
wherein said flex cable is connected between said read and write head and said high pass filter.