FIELD OF THE INVENTION 
   The present invention relates generally to signal peak detectors and more particularly to variable threshold bipolar signal peak detectors and associated methods of comparing an input signal to a predetermined value. 
   BACKGROUND OF THE INVENTION 
   Disc drive storage systems are commonly used for storing data in electronic products ranging from digital cameras to computer systems. A disk drive comprises a head assembly and an electronics assembly conventionally attached to the head assembly for controlling head operation and providing a communication link between the head assembly and a host device served by the disk drive. The disk drive further comprises a disk having a data-recording surface for storing information therein. The disk is rotated at a constant speed while a closed loop servo system controls head position. The head assembly further comprises a read head for reading data from the recording surface and a write head for writing data to the recording surface. 
   An exemplary disk drive storage system  10  (see  FIG. 1 ) comprises a disk  12  comprising magnetic material within a recording surface region for storing information in the form of binary bits for later retrieval and processing by the host device. Information bits are written to the recording surface of the disk  12  by magnetizing magnetic material domains to represent a binary zero or a binary one. The domains retain the magnetization for later retrieval during a read operation. 
   A spindle motor  13  rotates the disk  12  (typically at speeds up to 10,000 revolutions per minute) as a read/write head  14  disposed over an upper surface of the disk  12  writes data to or reads data from the disk  12 . The read/write head  14  is affixed to a suspension arm  16  controlled by a voice coil motor  18  for moving the suspension arm  16  across the upper surface of the disk  12  along an arc extending between a disk circumference  24  and a hub  26 . The physical features of the read/write head  14  and the suspension arm  16  allow the read/write head  14  to ‘fly’ over the disk upper surface at a distance referred to as a fly height. Contact between the head and the disk is to be avoided. The fly height is established to optimize performance of the read/write head  14  during read and write operations. An abrupt reduction in the operating fly height, for example, when the read/write head  14  encounters an asperity in the disk  12 , may reduce the probability of returning correct read data to the host device. 
   The disk  12  comprises a plurality of concentric tracks  30  (typically 20,000 or more per radial inch) for storing binary data in fields  32  and head location information in servo bursts  34 , where the latter provide feedback to the voice coil motor  18  for controlling head position along the tracks  30  (referred to as track following) and for moving the read/write head  14  rapidly and accurately between the tracks  30  (referred to as track accessing). 
   To write data to the disk  12 , the voice coil motor  18  moves the suspension arm  16  to a desired radial position above the disk  12  while the disk  12  is rotating to position the region to be written under the read/write head  14 . An electronics module  40  produces write current representing the data bits, as supplied by the host system, to be written to the disk  12 . The write current is supplied to a coil (magnetically coupled to a magnetically permeable core) of an inductive writer in the read/write head  14  for producing a magnetic field. The magnetic field extends from the core across an air gap between the read/write head  14  and the disk&#39;s upper surface to magnetize a region of magnetic domains in the recording surface, thereby storing a data bit in the region. 
   The read/write head  14  comprises a read head including, in one embodiment, a magnetoresistive sensor that undergoes a change in electrical resistance in the presence of magnetic fields of selected orientation produced by the magnetic domains in the recording surface. During a read operation, a DC (direct current) bias voltage, e.g., of between about 0.025 V and 0.3 V, is supplied to the magnetoresistive sensor from the electronics module  40  to bias the read head. 
   During a data read or a servo read operation the suspension arm  16  moves while the disk  12  rotates to position the read/write head  14  (and thus the magnetoresistive sensor) above the magnetized region to be read. The magnetized disk region alters a resistance of the magnetoresistive sensor, generating an output signal comprising a relatively small AC (alternating current) voltage imposed on the DC bias voltage. The AC voltage, which is supplied to the electronics module  40 , represents the magnetization of the read region of the disk  12 , i.e., either a data one or a data zero. A preamplifier within the electronics module  40  receives and amplifies the AC voltage for further processing and decoding in a read channel of the module  40 . 
   Continual advancements have led to higher storage capacity disks, higher data transfer rates during read and write operations and smaller magnitude read head signals representing the stored data bits. The use of magnetoresistive (MR) heads, giant magnetoresistive (GMR) heads and tunneling magnetoresistive (TMR) heads permits reduction of the fly height and correspondingly higher data storage densities. However, the reduced fly height introduces an increased likelihood of read signal distortion due to thermal interaction between the head and the disk. More particularly, changes in head temperature as the head flies over the disk surface, referred to as thermal asperities, introduce distortion in the read signals. Such thermal asperities are typically caused by physical contact between the head and contaminating oil or a contaminating particle on the disk  12 . Thermal asperities can also be induced by flying height changes as the read/write head  14  passes over “hills” and “valleys” present on an irregular disk surface. 
   The effects of an asperity event on the magnetoresistive read head and on an output signal of a read channel responsive to the head signal are well known. When the head strikes a particle or magnetic media defect, referred to as a thermal asperity event, the head temperature can increase by more than 120° C. Due to the resistance temperature coefficient of the magnetoresistive sensor (about 0.02% per degree C.), the thermal asperity can cause a significant voltage transient or a DC baseline shift in the read signal amplitude, followed by a decaying signal amplitude having a duration of about two to five microseconds while the heat is released to the local environment as the head cools. 
     FIG. 2  illustrates a quiescent DC bias voltage baseline  42  and a read signal  43  for decoding within the read channel to detect the read bit. A thermal asperity event occurs at a time t 1  and the resulting thermal effects on the read/write head  14  cause a baseline shift to a transient baseline  42 A, increasing a DC level of the read signal  43 . The baseline voltage shift resulting from the thermal asperity event may be in the range of about 5 mV to about 25 mV. 
   The thermal asperity event usually does not damage the head  14 , but during the event the read signal is distorted as illustrated in  FIG. 2 , jeopardizing successful recovery of the data bits. Although the read channel includes error detection and correction features, a data re-read operation must be performed if the bit error rate during the event exceeds those error correction capabilities. As the magnetoresistive head cools, the baseline offset  42 A decays back to the nominal baseline  42  as illustrated in  FIG. 2 . 
   In addition to data bit detection difficulties, the increasing read signal amplitude at the inception of the thermal asperity event can be coupled to the adjacent write head. The resulting write head current may saturate the magnetic recording medium in the area of the write head, erasing any recorded data in that region. 
   A thermal asperity event length can span the reading of a significant number of bytes from the disk-recording surface. For example, in a disk drive having a data transfer rate of 200 megabits per second (Mbits/sec), uncompensated thermal asperities have a duration from about two to five microseconds, distorting about 50 to 125 bytes of read data. As contaminates and disk surface corrosion build-up with use of the disk drive, the event duration lengthens. These effects further reduce the capabilities of a disk drive to reliably store and retrieve user data over its operational life. 
   Localized media anomalies can also cause problems in the storage and recovery of data from the disk  12 , preventing the head  14  from recovering stored data. Typically these media anomalies generate a read signal disturbance signature similar to a thermal asperity event. 
   Various data and synchronization block formats have been utilized in an effort to compensate for thermal asperity effects, localized media anomalies and other anomalous disk conditions. For example, one format utilizes redundant first and second synchronization fields in each data block so that the stored data can be recovered if an anomalous condition prevents the read channel from correctly decoding one of the two synchronization fields. Further, the two synchronization fields in the data block are separated by a sufficient distance such that a thermal asperity coincident with a read operation on one of the synchronization fields does not interfere with a read operation on the other synchronization field. 
   According to another known technique, a thermal asperity detector determines when the read signal exceeds a predetermined threshold, ignores the read data during the asperity event and signals a disk drive controller to reread the sector after the asperity event has ended. The occurrence of second asperity event during the reread operation is not likely because the collision between the head and the particle usually moves the particle off the disk sector. Permanent, immobile defects in the disk storage media are rare. 
   A thermal asperity detector for use with a conventional disk drive  10  comprises a signal peak detector for determining when a read signal has exceeded a predetermined threshold, including thresholds indicating the possible occurrence of a thermal asperity event or another media anomaly. Various known peak detectors can detect either a positive polarity peak, a negative polarity peak, or both positive and negative polarity peaks. Also, the peak detector can detect cooling thermal asperities that are of the opposite polarity to the positive going heating thermal asperities described above. 
   Peak detectors for determining the occurrence of a thermal asperity event may not be able to accurately detect signal peaks in lower magnitude read signals produced by advanced-technology heads. False peak detections are also to be avoided. 
     FIG. 3  illustrates, in block diagram form, a prior art differential-input peak detector  50  capable of detecting both positive and negative polarity peaks in excess of a threshold value. A differential input signal is supplied from the read head  14  on conductors  51  and  52  to a high pass filter  54  for removing DC offset or DC bias levels in the input signal. An inverting amplifier  56  amplifies and inverts the differential signals input thereto from the filter  54  to produce a differential output signal supplied as inputs to comparators  58  and  60 . Specifically, a first signal on an output conductor  61  is supplied to an inverting input terminal of the comparator  58 . A second signal on an output conductor  62  is supplied to a non-inverting input terminal of the comparator  60 . 
   A reference threshold voltage VTH is supplied to a non-inverting input terminal of the comparator  58  and to an inverting input terminal of the comparator  60 . To limit variations in the reference voltage VTH with temperature, in one embodiment VTH is produced by a zero temperature coefficient current (i.e., the current does not vary with a temperature) flowing through a resistor (neither shown in  FIG. 3 ). 
   The comparators  58  and  60  produce an output signal at respective output terminals  70  and  72  in response to a relationship between the signal present at the inverting terminal (for the comparator  60 ) or the non-inverting input terminal (for the comparator  58 ) and the reference voltage. The comparator  58  produces a high logic state output signal at the output terminal  70  when the threshold voltage is greater than the signal at the inverting input terminal. The comparator  60  produces a high logic state output signal on the output terminal  72  when the signal at the non-inverting input terminal is greater than the threshold voltage. Recognizing that the amplifier  56  is an inverting amplifier, input signal excursions above VTH are indicated by a high logic state of the comparator  58  and input signal excursions less than a negative VTH are indicated by a high logic state of the comparator  60 . Thus the comparators  58  and  60  indicate the occurrence of a positive or negative signal peak having a magnitude greater than the threshold voltage VTH. 
   A gain of the amplifier  56  is typically between about 0.5 and 2; the gain is selected based on the expected amplitude range of the differential-input signals on the conductors  51  and  52  and on operating head room of the comparators  58  and  60 . When using an amplifier with a relatively low gain, the reference voltage must be reduced and the comparators  58  and  60  must have the capability to process small amplitude signals, and to detect smaller differences between the input signal and the threshold to accurately detect a thermal asperity event. The use of a higher gain amplifier may appear to resolve this difficulty, but higher gains can introduce signal nonlinearities into the amplification process. For accurate detection of a thermal asperity event by peak signal detection, the effects of gain nonlinearities in the amplified differential-output signals produced by the amplifier  56  and in the reference voltage VTH must be minimized. 
     FIG. 4  illustrates in schematic form additional details of one embodiment of the amplifier  56 . The reference voltage VTH is generated by a current supplied from a current source  96  through a resistor R 4  and supplied to the non-inverting input terminal of the comparator  58  and to the inverting input terminal of the comparator  60 . In another embodiment, the signals supplied to the inverting and non-inverting input terminals of the comparator  58  (and the comparator  60 ) are reversed with a corresponding reversal in the interpretation of the comparator output signal. 
   Bipolar junction transistors (BJTS) Q 1  and Q 2 , each operative with a respective load resistor R 2  and R 3 , operate as a bipolar differential amplifier, producing differential-output signals Vo 1  and Vo 2  in response to the differential input signals supplied from the high pass filter  54 . The output signal Vo 1  is supplied to the inverting input terminal of the comparator  58 . The output signal Vo 2  is supplied to the non-inverting input terminal of the comparator  60 . In a preferred embodiment, the resistors R 2  and R 3  have the same resistance. 
   Positive and negative rail voltages (the latter can be ground in one embodiment) are identified as VPOS and VNEG in  FIG. 4 . 
   The resistor R 4  and the current source  96  comprise a single-ended reference voltage generator producing the reference voltage VTH. In one embodiment the current source  96  comprises a zero temperature coefficient DAC (digital-to-analog converter) current source (referred to as an IDAC) responsive to a digital value that determines the current generated by the current source. The digital value is selected during manufacture of the of the disk drive  10  and stored in the electronics module  40 , allowing the disk drive manufacturer to establish the reference voltage VTH and thereby determine the input signal level at which the comparators  58  and  60  trigger to indicate a signal peak greater than the threshold, e.g., to indicate the occurrence of a thermal asperity event. 
   Current sources  102  and  104  provide drive current through the resistors R 2 , R 3  and R 9 , with the resistor R 9  operative as a degeneration resistor for improving linearity (i.e., minimizing distortion) of the differential amplifier&#39;s response. To minimize nonlinearities in the differential amplifier&#39;s output signals, the input voltage differential between the input signals supplied to Q 1  and Q 2  should be small, permitting the bipolar junction transistors Q 1  and Q 2  to operate in a narrow linear region. The degeneration resistor R 9  connected between the emitters of Q 1  and Q 2  drops a majority of the input differential voltage, reducing the input voltage differential between Q 1  and Q 2 , increasing linearity in the operation of Q 1  and Q 2  and therefore increasing linearity in the operation of differential amplifier. 
   The affect of the resistor R 9  on the linearity of the amplifier  56  is determined by a relationship between R 9  and the transconductance parameter, gm, of the transistors Q 1  and Q 2 . The amplifier  56  exhibits relatively good gain linearity for 1/gm&lt;&lt;R 9 . Ideally, it is desired that the bipolar differential amplifier  56  exhibit perfect gain linearity so as not to introduce nonlinear effects during the amplification process, as such nonlinear effects can impair the detection of positive and negative signal peaks by the comparators  58  and  60 . 
   A gain of the bipolar differential amplifier  56  is approximated by a ratio 2R 2 /R 9  (or 2R 3 /R 9  since in a preferred embodiment where R 2 =R 3 ). More precisely, the gain equals 2R 2 /(R 9 +2/gm)=2R 3 /(R 9 +2/gm), which simplifies to 2R 2 /R 9 =2R 3 /R 9  for R 9 &gt;&gt;2/gm. As is known, these gain equations are valid only for relatively low voltage or low magnitude input signals. 
   For high voltage input signals, the gain is not constant (i.e., not a constant function of R 2  and R 9 ) and therefore the voltages Vo 1  and Vo 2  input to the comparators  58  and  60  may not be as linear as desired. Thus a magnitude of the differential input signals supplied on the conductors  51  and  52  may cause a gain-induced nonlinearity in the detection process. Any such nonlinearity in the operation of the peak detector  50  may falsely indicate the occurrence of a thermal asperity event or fail to detect a thermal asperity event. All nonlinearities should be minimized. 
   Undesired signal nonlinearities may also occur in the generation of the reference threshold voltage VTH due to nonlinear operation of the current source  96 , which may be caused by random process variations encountered while fabricating the individual components of the current source  96 . 
   The BJTS Q 1  and Q 2  of the amplifier  56  illustrated in  FIG. 4  may be replaced by respective MOSFETS (metal-oxide semiconductor field-effect transistors) M 1  and M 2  cooperatively operating as a MOSFET differential amplifier responsive to differential input signals supplied to MOSFET gate terminals from the filter  54 . The MOSFET embodiments may be preferred for cost reasons when otherwise fabricating integrated circuits (such as the electronics module  40 ) with MOSFETS. That is, the incorporation of process steps for fabricating BJTS introduces cost and complexity into a MOSFET fabrication process. 
   Gain of a MOSFET differential amplifier is determined according to an equation that has the same form as the equation for the BJT differential amplifier of  FIG. 4 . That is, 2R 2 /(R 9 +2/gm) or 2R 3 /(R 9 +2/gm). Since a MOSFET transconductance is generally much smaller than a BJT transconductance, gain nonlinearities are greater in the MOSFET differential amplifier than in the corresponding BJT differential amplifier. Also, a pair of MOSFETS may have larger parameter offsets (random parameter mismatches due to the fabrication process variations, such as a threshold voltage mismatch) than a pair of bipolar junction transistors. 
   Given the relatively smaller transconductance and likelihood of larger parameter offsets, the MOSFET differential amplifier may introduce more nonlinear effects over a range of expected input signals than the BJT differential amplifier. The MOSFETS may also introduce greater nonlinearities into the amplification process than those attributable to the reference voltage VTH. 
   The output voltages of a differential amplifier can be mathematically expressed as an output voltage term plus an error term that accounts for parameter offsets between the two transistors comprising the differential amplifier. Thus the comparators  58  and  60  of  FIG. 4  compare the threshold reference voltage VTH with Vo 1 ±Verror 1  and Vo 2 ±Verror 2 , where Verror 1  and Verror 2  represent the transistor parameter offsets that contribute to undesired differences between Vo 1  and Vo 2 . The error terms cause nonlinearities and asymmetries in the comparison process performed by the comparators  58  and  60 , possibly generating comparator output signals that do not accurately represent the relationship between Vo 1  and the threshold voltage VTH and between Vo 2  and VTH. 
   Preferably, the peak detector  50  is a component of the preamplifier of the electronics module  40  for receiving signals representing the read data bits and for detecting a thermal asperity event in response thereto as described above. 
   BRIEF SUMMARY OF THE INVENTION 
   According to one embodiment, the present invention comprises a signal peak detector. The signal peak detector further comprises a differential amplifier responsive to first and second differential input signals to produce first and second differential output signals, a reference voltage generator configured to provide a third output signal representative of a common mode voltage of the first and second input signals and a comparator connected to determine a relation between the first and the second output signals and the third output signal. 
   Another embodiment of the present invention comprises a method for determining when a signal of a pair of differential input signals exceeds a predetermined signal threshold. The method further comprises amplifying the differential input signals to produce differential output signals, producing a reference voltage indicative of the signal threshold and related to a common mode voltage of the differential input signals and determining a relation between the differential output signals and the reference voltage, wherein the relation indicates if the signal of the pair of differential input signals exceeds the predetermined signal threshold. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the present invention will be apparent from the following more particular description of the invention as illustrated in the accompanying drawings, in which like reference characters refer to like features throughout the different figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
       FIG. 1  is a partial block diagram of a prior art disk drive storage system to which the teachings of the present invention can be applied. 
       FIG. 2  is a time line illustrating a baseline shift due to a thermal asperity event in a disk drive system of  FIG. 1 . 
       FIG. 3  is a partial block diagram and partial schematic diagram of a prior art differential peak detector. 
       FIG. 4  is a schematic illustration of additional details of an amplifier of the differential peak detector of  FIG. 3 . 
       FIGS. 5 and 6  are schematic illustrations of peak detector circuit embodiments of the present invention. 
   


   DETAILED DESCRIPTION OF THE INVENTION 
   Before describing in detail the particular variable peak detector circuit and method related thereto according to the present invention, it should be observed that the present invention resides in a novel and non-obvious combination of structural elements and process steps. Accordingly, these elements and process steps have been represented by conventional elements and steps in the drawings and specification, wherein elements and process steps conventionally known in the art are described in lesser detail and elements and steps pertinent to understanding the invention are described with greater detail. 
   The present invention is useful for detecting signal excursions that exceed a predetermined threshold, including excursions caused by media anomalies and thermal asperity events, while reading data from a disk drive data storage system. 
     FIG. 5  illustrates an embodiment of a signal peak detector circuit  140  comprising a differential input amplifier  142 , a differential reference voltage generator  143  and comparators  58  and  60  (as described above with respect to  FIG. 3 ) for receiving amplified version of the input signals. 
   A high pass filter  144  comprising capacitors C 0  and C 1  receives and filters differential read signals supplied on the conductors  51  and  52 . 
   A center tap circuit  145  receives filtered differential signals Vin 1  and Vin 2  to determine a common mode voltage signal VCM (and its polarity) in the input signals. One example of a common mode voltage is a common mode DC bias voltage due to a thermal asperity event as described above in conjunction with  FIG. 2 . 
   The common mode voltage VCM, produced at a node  146  is supplied to the differential reference voltage generator  143 , as further described below, for use in producing the threshold voltage VTH. Using the common mode voltage to produce the reference voltage causes the reference voltage to track the common mode component of the differential input voltages Vin 1  and Vin 2 . 
   Since the components (e.g., MOSFETS) of the differential input amplifier  142  and the differential reference voltage generator  143  are fabricated according to the same process steps on the same integrated circuit, thus exhibiting substantially similar performance characteristics, process-induced nonlinearities in the output signals Vo 1  and Vo 2  of the differential input amplifier  142  would also appear in the output signal VTH (the reference voltage) of the differential reference voltage generator  143 . Common fabrication of the differential input amplifier components and the differential reference voltage generator components minimizes nonlinearity mistracking between the input signals Vin 1  and Vin 2  and the threshold voltage VTH, permitting the comparators  58  and  60  to more accurately determine the relationship of the differential output signals Vo 1  and Vo 2  and the threshold voltage VTH, for example, to detect the occurrence of a thermal asperity event. 
   The center tap circuit  145  for determining the common mode voltage VCM (both magnitude and polarity of the common mode voltage) comprises MOSFETS M 4 , M 5 , M 6  and M 7 , each having a gate G biased by the negative rail voltage VNEG. The MOSFETS M 4 , M 5 , M 6  and M 7  have a relatively large channel length and a relatively small channel width, causing each to operate in a triode region of its characteristic curve and to exhibit a relatively high channel resistance. In an alternative embodiment the MOSFETS M 4 , M 5 , M 6  and M 7  are replaced by conventional resistors, although when the center tap circuit  145  is implemented in an integrated circuit, the MOSFETS occupy less circuit area than the resistors. 
   The differential input amplifier  142 , comprising input MOSFETS M 0  and M 3 , respective drain resistors R 10  and R 12  and degeneration resistor R 11 , receives the filtered input signals Vin 1  and Vin 2  at respective gates G of the MOSFETS M 0  and M 3  for amplifying a difference between the input signals. In an ideal differential amplifier the difference between the two input signals is amplified and the common mode signal is rejected, i.e., the common mode signal has no effect on the output signals. The differential output signals Vo 1  and Vo 2  (representing a scaled up version of the input signals Vin 1  and Vin 2 ) at the drain of each of the MOSFETS M 0  and M 3 , are supplied to respective inverting and non-inverting input terminals of the comparators  58  and  60 . Drive current for the MOSFETS M 0  and M 3  is supplied by current sources  122  and  124 , respectively. 
   The differential reference voltage generator  143  comprises MOSFETS M 8  and M 9 , respective drain resistors R 15  and R 16 , a gate resistor R 13  and a degeneration resistor R 14 . Drive current for the MOSFETS M 8  and M 9  is supplied by current sources  152  and  154 , respectively. 
   The voltage VCM, determined by the center tap circuit  145 , is supplied to a gate G of the MOSFET M 8 . A voltage at a gate G of the MOSFET M 9  is function of the current supplied by the IDAC  96  and the voltage VCM according to the equation (in an embodiment where the negative rail voltage VNEG is at ground potential):
 
 M 8 gate voltage= VCM −( IDAC*R 13).
 
   The reference threshold voltage VTH, at node  146  (between the resistor R 15  and drain D of the MOSFET M 8 ), is supplied to the non-inverting input terminal of the comparator  58  and to the inverting input terminal of the comparator  60 . Since the gate voltage of each of the MOSFETS M 8  and M 9  is responsive to the common mode voltage VCM, and further since the MOSFETS M 8 , M 9 , M 0  and M 2  are concurrently fabricated according to the same fabrication processes, the differential pair of MOSFETS M 8  and M 9  impose substantially similar nonlinearities in the threshold voltage VTH as the differential pair of MOSFETS M 0  and M 2  impose in generating the output signals Vo 1  and Vo 2 . Since the threshold voltage VTH and the output signals Vo 1  and Vo 2  all exhibit substantially similar nonlinearities, the nonlinear effects effectively cancel, permitting the comparators  58  and  60  to more accurately determine the relationship between the output signals Vo 1  and Vo 2  and the reference threshold VTH, for example to detect a thermal asperity event. Thus MOSFET operational nonlinearities due to the relatively small MOSFET transconductance and fabrication-induced parameter offsets and operational nonlinearities are compensated. 
   The peak detector circuit  140  “pre-distorts” the threshold voltage VTH to compensate for nonlinear effects introduced by the differential pair comprising the MOSFETS M 0  and M 3 , where the nonlinear effects are caused by large input signal magnitudes that cause the MOSFETS to operate in their nonlinear region (obviated by using the common mode voltage VCM to generate the threshold voltage VTH), by parameter offsets due to random fabrication anomalies (obviated by common fabrication of the differential amplifier MOSFETS M 0  and M 3  and the threshold generator MOSFETS M 8  and M 9 ) and/or by the relatively small MOSFET transconductance parameter. 
   When the peak detector circuit  140  operates to detect a thermal asperity event, the current supplied by the IDAC  96  is based on expected excursions in the input signals Vin 1  and Vin 2  caused by thermal asperity events during disk drive operation. As explained above, the IDAC current, in addition to the common mode voltage VCM, determines the threshold voltage VTH and thus the input signal level at which the comparators  58  and  60  indicate the occurrence of a thermal asperity event. During manufacture of the disk drive system the manufacturer may store a binary value for controlling the IDAC  96  in the electronics module  40 . This value is supplied to the IDAC  95  during operation for use in generating the IDAC current. 
   A primary operational concern according to the prior art peak detectors was the introduction of nonlinearities in the differential output signals Vo 1  and Vo 2  by the input MOSFET pair M 0  and M 3 . Such nonlinearities were primarily caused by large magnitude excursions of the input signals Vin 1  and Vin 2  on the conductors  51  and  52 , in particular during a thermal asperity event, such that the input signals drive the MOSFETS M 0  and M 3  into nonlinear operation, producing nonlinearities in the output signals Vo 1  and Vo 2 . Nonlinearities could also be introduced by parameter offsets, such as differences in gain or threshold voltage between the MOSFETS M 0  and M 3 . Such parameter offsets are typically introduced by fabrication process anomalies. 
   According to the present invention, with the reference threshold voltage VTH derived from the common mode signal of Vin 1  and Vin 2 , the same error terms and nonlinearities present in Vo 1  and Vo 2  are present in VTH, thus avoiding the effects of such nonlinearities during the comparison process performed by the comparators  58  and  60 . Performance of the peak detector  140  is also improved since the MOSFETS M 0 , M 3  of the differential input amplifier  142  and the MOSFETS M 8  and M 9  of the differential reference voltage generator  143  have similar operating characteristics, gain and performance offsets due to the common processes employed in their fabrication. 
   According to one embodiment, the differential reference voltage generator  143  provides an amplification factor substantially identical to that provided by the differential input amplifier  142 . According to another embodiment, the components of the generator  143  are scaled relative to the components of the differential input amplifier  142  to reduce power consumption in the generator  143 . For example, in one embodiment the differential reference voltage generator  143  is scaled by a factor of four with respect to the differential input amplifier  142 , that is, the current sources  152  and  154  are scaled to deliver ¼ of the current of the current sources  122  and  124  and the channel of each MOSFET M 8  and M 9  is scaled by ¼ relative to the channel of each MOSFET M 0  and M 3 . To produce a correctly proportioned reference voltage VTH from the generator  143  relative to the output signals Vo 1  and Vo 2 , the resistor values R 15  and R 16  in the generator  143  are increased by a factor of four over the resistor values R 10  and R 12  in the differential input amplifier  142 . 
   In a peak detector  180  illustrated in  FIG. 6  and constructed according to another embodiment of the present invention, drain or load resistors R 10 , R 12 , R 15  and R 16  in the peak detector  140  of  FIG. 5  are replaced by NMOSFET active loads M 18 , M 19 , M 20  and M 21 , respectively, that exhibit substantially identical operating characteristics to MOSFETS M 16  and M 17  of a differential input amplifier  182  and to MOSFETS M 15  and M 14  of a differential reference voltage generator  184 . The peak detector  180  may be especially advantageous for fabrication according to a CMOS process where the fabrication of resistors is minimized to cost limit fabrication costs. 
   MOSFETS M 10 , M 11 , M 12  and M 13  comprise a center tap circuit  185  that operates similar to the center tap circuit  145  of  FIG. 5 . 
   Drive current for the MOSFETS M 16  and M 17  is supplied by a current source  187  and drive current for the MOSFETS M 16  and M 14  is supplied by a current source  189 . 
   According to one embodiment of the peak detector  180 , channel dimensions of the MOSFETS M 16  and M 17  of the differential input amplifier  182  can be reduced below the channel dimensions of the MOSFETS M 0  and M 3  of the  FIG. 5  embodiment. It may be desired to reduce the channel aspect ratio (W/L) of the MOSFETS M 16  and M 17  to increase the overdrive voltage (Vgs−Vt, where Vgs represents the MOSFET gate-source voltage and Vt represents the MOSFET threshold voltage) while maintaining a constant MOSFET output current. As the overdrive voltage (Vgs−Vt) increases, the output signal linearity improves. However, maintaining linearity according to this technique may not be critical, as the differential input amplifier  182  and the differential reference generator  184  track each other as described above. 
   By using NMOSFET active loads in the  FIG. 6  embodiment, the area consumption and power consumption are reduced compared to the  FIG. 5  embodiment. The amplifier  182  and the reference voltage generator  184  provide a more constant gain over fabrication process and temperature variations because only MOSFET devices are used (i.e., the resistors R 10 , R 12 , R 15  and R 16  are absent in the  FIG. 6  embodiment). Further, the  FIG. 6  embodiment uses only one current source for the input amplifier  182  and one current source for the differential reference voltage generator  184 , unlike the embodiment of  FIG. 5  that uses two current sources for each one of the amplifier  142  and the reference voltage generator  143 . 
   Thus, according to the teachings of the present invention, a simple and reliable apparatus and method to improve the accuracy of MOSFET peak detectors is taught. These advantages are achieved without fabrication cost impact. 
   Although certain embodiments of the present invention depict and describe operation of the invention using NMOSFET devices, PMOSFET devices can be used in lieu thereof with corresponding changes in the polarity of the voltages supplied to the PMOSFET devices. 
   An apparatus and method have been described as useful for detecting bipolar signal peaks with respect to a threshold voltage. Specific applications and exemplary embodiments of the invention have been illustrated and discussed. These provide a basis for practicing the invention in a variety of ways and in a variety of circuit structures. Numerous variations are possible within the scope of the invention. Features and elements associated with one or more of the described embodiments are not to be construed as required elements for all embodiments. The invention is limited only by the claims that follow.
A method and apparatus for detecting signal peaks caused by a thermal asperity event in a magnetic recording media to reduce data reading errors introduced by the thermal asperity event. A common mode voltage is determined for differential signals representing data bits read from the magnetic recording media and a threshold voltage produced responsive to the common mode voltage. A comparator determines if either of the differential signals exceeds the threshold voltage, thereby indicating the occurrence of a thermal asperity event.