Adaptive pole-zero cancellation for wide bandwidth magnetoresistive pre-amplifier

#The present invention generally relates to magnetic storage systems. More particularly, the present invention relates to storage devices which employ magnetoresistive heads.

#The trend in the data storage industry is to continually increase the data rate at which information is read from magnetic storage surfaces such as disk drives. Designers are developing high performance disk drives which tend to have magnetic domains positioned close together on the magnetic storage media, creating a high density of data recording. Additionally, these drives have greater rotation rates than lower performance drives, resulting in higher data rates. Higher data rates require faster voltage transitions at the input to the channel circuitry used to read data from the disk surface. Faster voltage transitions produce higher frequency components in the voltage input signal, thereby requiring a greater bandwidth from the channel circuitry.

#Until recently, designers primarily used inductive heads to read data from these disk surfaces. Unfortunately, inductive heads are limited in the bandwidth at which they may function. Designers, accordingly, developed magnetoresistive (MR) heads which do not have the large inductance associated with previous heads. These MR heads, therefore, can accommodate higher data rates. Data is read from a disk surface by monitoring the changing resistivity of the MR head. The resistance of MR heads is a function of the strength of the magnetic field to which it is exposed. Since the resistance of the head varies with magnetic flux, the current through, or the voltage across the head is a function of the data written on the magnetic media.

#FIG. 1 is a block diagram depicting a disk drive head and read channel incorporating features of the present invention;

#FIG. 2 is a simplified input model of the CBCS MR head and preamplifier of FIG. 1;

#FIGS. 3A and 3B are diagrams depicting the response of a CBCS MR head and preamplifier without the compensation features of the present invention; and

#Referring now to FIG. 1, one specific embodiment of a system 10 implementing the adaptive compensation features of the present invention is shown. In this specific embodiment, a preamplifier 12 is coupled to receive inputs from a MR head 20. The head 20 has a resistance R.sub.MR associated with it. This resistance varies as a result of changes in the magnetic flux of the magnetic media positioned proximate the head 20. Current source 18 serves to bias the preamplifier 12. A constant current I.sub.BIAS (typically on the order of 10 mA) is provided from the current source 18. This constant current, in conjunction with the changing resistance .DELTA.R.sub.MR of the MR head, results in a voltage .DELTA.V proportional to the resistivity of the head. Preamplifier 12, in one specific embodiment, senses the signal current through the MR sensor. The preamplifier 12 may be a current preamplifier with a low input impedance and virtually shorted inputs. An amplified signal proportional to the resistivity of the MR head is input to the channel 14. Channel 14 provides data to a servo signal processor 28 and to a controller 16.

#Disk systems typically include a plurality of magnetic disks that rotate about a common axis. A number of MR heads and their associated preamplifiers are positioned proximate the surfaces of these disks using one or more actuator arms. The actuator arms are positioned by the controller 16 using information received from the channel 14 and the servo processor 28. The preamplifier 12 for each MR head is positioned as closely as possible to the head to minimize the length of the leads or wires used to connect the preamplifier 12 to the head 20. Minimizing the lead lengths tends to reduce the inductance associated with the lead; however, there is typically some inductance associated with these leads. This inductance will be termed L.sub.IN. The value of L.sub.IN is generally known, e.g., by measurement at the factory or by using a known wire size and length. The inductance L.sub.IN contributes to pole frequencies which tend to erode the useful bandwidth of the system.

#CBCS-type MR preamplifiers have a single pole. The elements contributing to this pole are shown in FIG. 2 where a simplified input model 30 of a CBCS MR preamplifier is shown. The bandwidth response of this preamplifier is a function of the resistivity of the head (R.sub.MR), the inductance associated with the head leads (L.sub.IN), and a small resistance associated with the amplifier (R.sub.AMP). The pole (-3 dB roll-off) frequency is determined as follows: f.sub.-3dB c[R.sub.MR +R.sub.AMP ]/[2*PI*L.sub.IN ].