METHOD AND APPARATUS FOR READING RECORDED DATA BY A MAGNETORESISTIVE HEAD

Abstract: Method and apparatus for magnetoresistive (MR) head biasing and output signal detection for reading data recorded in magnetic media. The MR head may be biased by constant current or constant voltage, and an output signal, proportional to the ratio of the instantaneous value of the change in MR head resistance to total MR head resistance when exposed to the magnetic field of the media, is detected.

Description: The present invention relates to retrieval of previously stored data, including data recorded in magnetic media and data stored in magnetic bubble memories, by a magnetoresistive (MR) read head or sensor. An MR head is an active or parametric transducer which requires an electrical current through its resistive sensor stripe to be active. The current serves as a sensing current for converting the stripe resistance variations produced by the field, Hy, emanating from the medium into voltage variations across the stripe. The higher the current applied to the head, the larger the readout voltage. The magnitude of the current, however, must be limited to avoid overheating the sensor stripe, and to avoid electromigration of the stripe material. This current should be provided from a low noise source to minimise noise injected into the read channel electronics. Most existing MR head preamplifiers bias the sensor stripe of the head at a constant bias current, I b , and detect the signal voltage, V s , developed at the head terminals. Thus, V s = I b ΔR h , (A) where ΔR h is the absolute change in the head resistance, R h , owing to the magnetic input signal, H y , from the media being read. Stripe height, the dimension of the sensing stripe perpendicular to the media will vary from device to device because of variations in the lapping process. Stripe height also changes as much as 50% over the life of an MR head in contact recording applications. Since both R h and, more importantly, ▵R h are proportional to stripe height prior art preamplifiers which detect V s of equation (A) provide different sensistivity if switched from one head to another in a multihead storage device or, in contact recording devices, as the stripe height is worn away. Similarly, different sensitivities would be encountered when switching from one sensor to another in bubble memory systems. Moreover, R n typically exhibits a temperature coefficient of about 0.3 to 0.5% per degree C. Thus, as temperature varies, R h varies according to its particular temperature coefficient. The corresponding variation in ▵R h causes low frequency noise modulation of the output signal, V s . US-A-3,814,863 describes one prior art configuration which proposes biasing an MR head using resistors having large values with respect to the value of R h , and an AC coupled differential voltage amplifier . IN another described in US-A-4,040,113 a current source used for biasing a centre-tapped MR element and an AC coupled differential voltage amplifier for detecting the output signal produced by the head. Finally, US-A-4,191,977 described a method of biasing a centre-tapped MR head using two inductors in series with the head and a voltage source with a DC coupled differential voltage amplifier. The prior art hereinbefore described teaches biasing the MR head with a constant bias current and detecting the signal produced across the head with a differential voltage amplifier. The detected signal, therefore, which is proportional to ▵R h , is sensitive to production tolerances, contact recording wear, and temperature variations. GB 2 023 326 describes a circuit for detecting magnetic fields having encoded information to produce electrical signals representing the encoded information. The circuit comprises a magnetoresistive element and detection means. The detection means detects the ratio of the instantaneous value of the change in the resistance of the magnetoresistive element to the absolute magnetoresistive element resistance. In accordance with the present invention, there is now provided a circuit for detecting magnetic fields having encoded information to produce electrical signals representing the encoded information, the circuit comprising: a magnetoresistive element; and detection means for detecting the ratio of the instantaneous value of the change in the resistance of the magnetoresistive element to the absolute magnetoresistive element resistance; characterised in that the detection means comprises: first and second current sources connected in series with the magnetoresistive element for generating a bias current in the magnetoresistive element; a capacitor connected across the magnetoresistive element; a current probe for detecting a current deviation from the bias current, the deviation being proportional to the relative resistance change of the magnetoresistive element produced by exposing the magnetoresistive element to the magnetic field, the current probe generating a signal voltage proportional to the deviation. Viewing the present invention from a second aspect, there is provided a circuit for detecting magnetic fields having encoded information to produce electrical signals representing the encoded information, the circuit comprising: a magnetoresistive element; and detection means for detecting the ratio of the instantaneous value of the change in the resistance of the magnetoresistive element to the absolute magnetoresistive element resistance; characterised in that the detection means comprises: a constant voltage source connected to first and second matched resistors in series with the magnetoresistive element to generate a bias current through the magnetoresistive element; and a ferrite toroid having primary and secondary windings, the primary winding being connected in series with the magnetoresistive element to generate current deviations in the primary winding, deviations being proportional to the relative resistance change of the magnetoresistive element produced, the deviations generating a signal voltage across the secondary winding, the signal voltage being proportional to the deviations. Viewing the present invention from a third aspect, there is provided a circuit for detecting magnetic fields having encoded information to produce electrical signals representing the encoded information, the circuit comprising: a magnetoresistive element; detection means for detecting the ratio of the instantaneous value of the change in the resistance of the magnetoresistive element to the absolute magnetoresistive element resistance; characterised in that the detection means includes: a current source connected in series with the magnetoresistive element having a resistance Rh for generating a bias current Ib in the magnetoresistive element; and a detector for detecting current deviations from the bias current, the deviations being proportional to the relative resistance change ▵R h of the magnetoresistive element produced by exposing the element to the magnetic fields, the detector comprising a differential amplifier for amplifying the instantaneous value of the current deviations, Is, of the current deviations, substantially according to the relation where Ib is the bias current, R h is the absolute resistance of the magnetoresistive element, and ▵R h is the instantaneous value of the change in the resistance of the magnetoresistive element in the presence of the magnetic field, the differential amplifier having one input coupled to a reference voltage and the other input connected in series with magnetoresistive element to generate an output signal voltage which is proportional to the deviations, and further including a capacitance having one terminal connected to the magnetoresistive element and said other input of the amplifier and the other terminal connected to the reference voltage. Viewing the present invention from a fourth aspect, there is provided a circuit for detecting magnetic fields having encoded information to produce electrical signals representing the encoded information, the circuit comprising: a magnetoresistive element; detection means for detecting the ratio of the instantaneous value of the change in the resistance of the magnetoresistive element to the absolute magnetoresistive element resistance R h ; characterised in that the detection means includes: a controlled constant current source connected to the magnetoresistive element to generate a current, I b' therethrough; a detector for detecting deviations of the voltage across the magnetoresistive element from the constant voltage, V b , provided by a constant voltage source, the deviations being proportional to the relative resistance change of the magnetoresistive element produced by exposing the element to the magnetic fields, the detector comprising a voltage amplifier for amplifying the instantaneous value, V s , of the deviations substantially according to the relation where V b is the voltage provided by the constant voltage source, R h is the absolute resistance of the magnetoresistive element, and ▵R h is the instantaneous value of the change in the resistance of the magnetoresistive element in the presence of the magnetic field, and further comprising a bias feedback loop for minimising the steady state voltage across the inputs of the voltage amplifier. Biasing and detection of the signal produced by an MR head, centre-tapped or not, according to the principles of the present invention may be biased by either a constant current or constant voltage source. For a constant current configuration, MR stripe resistance, R h , is biased by a DC current, I b . Current variations, proportional to the instantaneous value of the relative change, ΔR h /R h as the head detects variations in the magnetisation of the recording medium representing data, can then be applied to the input of a current sensing differential preamplifier. In a constant DC voltage configuration instantaneous value of voltage variations produced by the MR head as it detects data recorded in a magnetic medium can be applied to the input of a voltage sensing differential preamplifier. In either the constant current or constant voltage biasing configurations only the instantaneous value of AC signal, ie the change, produced by the MR head is detected and conditioned by the differential preamplifier. It can be seen the instantaneous value of signal produced by the MR head in these configurations is proportional to ΔR h /R h . As both R h and ΔR h are proportional to stripe height and to temperature changes in the same way ΔR h /R h is independent of stripe height and temperature variations. By normalising in this way the signal produced by the MR head, variations in the value of R h arising in such heads because of manufacturing tolerances and temperature sensitivity are therefore intrinsically corrected. It has also been found that detecting ΔR h /R h not only corrects fully, for variations in stripe lengths, but also substantially corrects for variations in stripe thickness. Thus, in a multiple head memory device, for example, switching the signal conditioning circuitry from one head to another, does not require adjustment of the signal conditioning electronics to different levels of gain to accommodate varying detection sensitivities of the heads involved. A number of embodiments of the invention as well as one prior art arrangement are described in the following with reference to the accompanying drawings in which: Figure 1 is a block diagram of one typed prior art MR head biasing and output signal detection; Figure 2A is a block diagram of one embodiment of the present invention employing constant current biasing; Figure 2B is a block diagram of another embodiment employing constant current biasing; Figure 3 is block diagram of an embodiment of the present invention employing a single-ended configuration; Figure 4 is a block diagram of a further embodiment of the invention employing a pseudo-balanced configuration; and Figure 5 is a block diagram of an embodiment of the invention employing constant voltage biasing. Referring first to Figure 1, constant bias current, I b , is supplied to MR head, R h , employed, for example, in a read/write channel for reading data recorded in magnetic media. Impedances Z are the internal impedances of the bias current source. Since voltage amplifier 10 detects the head voltage, V s , given by Equation (A) where ΔR h is the absolute change in head resistance R h . The amplifier output, V s , is therefore proportional to ΔR h . Hence, all other factors causing changes in R h are also detected. Such other factors interfere with detecting only those changes in R h which are caused by data bits recorded on a magnetic medium. The large DC voltage component produced by the constant current flowing through R h may be removed by AC coupling capacitors at the input of amplifier 10. Since the magnitude of ΔR h caused by a certain magnetic excitation is directly proportional to MR head resistance R h , the output signal of amplifier 10 changes as R h changes with temperature. The temperature coefficient of a typical MR head resistance comprising a permalloy stripe is in the range from 0.3 to 0.5% per degree C. Thus, as much as 10% change in output voltage of amplifier 10 is produced for every 20 degrees C of ambient temperature change. Since biasing and sensitivity of the MR head is also dependent on production tolerances in R h , adjustment of the bias current for individual heads to obtain the same specified read mode sensitivity of the channel would be required. Figure 2A illustrates an embodiment of the present invention. In this, the MR head is biased by a constant DC current, I b . Z s is the internal source impedance of the current sources, I b where |Z s |>>R h . AC current variations such as noise, line hum, and the like, produced by the current sources, I b , or supply voltage V+ and V-, are shorted out by capacitor C₁. C₁ is large enough so that, 1/2 Π R h C₁ << F₀ , where F₀ is the lowest frequency of interest in the signal detection channel. Dynamic variation of the head resistance from R h to R h +ΔR h produces a signal current, I s , to flow in the R h C₁ circuit. Any current sensor, such as a toroid current probe transformer can be used to detect I s . Thus, from it follows that If ΔR h is small with respect to R h , then I s ≅ I b ΔR h /R h , and output signal, V s , is given by V s ≅ KI b ΔR h /R h , (1) where K is the sensitivity of the current sensor. Thus, the configuration shown in Figure 2A detects a current proportional to the relative resistance change, ΔR h /R h . Since the temperature coefficient is cancelled by this technique, V s is substantially insensitive to temperature changes to which the MR head is subjected. Detection of a voltage proportional to relative resistance change is also insensitive to production tolerances affecting the value of R h . Therefore, I b need not be adjusted for uniform sensitivity from one head to another, and read mode sensitivity is independent of resistance variations arising during the manufacturing process, or owing to wear during the life of the head in contact recording applications. As shown earlier in this specification, C₁ is selected for shorting out noise current produced by the biasing network yet does not limit the bandwidth of the data signals produced by the MR head. Other interference sources at the head/media interface, which cause interference current to flow into or out of the two head terminals, are cancelled by the differential current sensing shown in Figure 2A. Therefore, this arrangement provides high rejection of common-mode interference. In Figure 2B, I b is derived from a reference voltage, V r , via matched resistors, R s . If R s is much greater than R h , the bias current is given by I b = V r /2R s . Toroid, T, is a ferrite toroid having, any suitable number of recording windings, and a primary winding found by the head leads which are fed thru the toroid so that head signal current adds and common mode currents cancel. Thus, for this configuration according to equation (1), if K = 10mv/mA, I b = 10 mA, ΔR h /R h = 0.5%, then V s = 500 µv. Referring now to Figure 3, a single-ended configuration of the present invention is shown. In this circuit, one side of R h is grounded by C₁ at one input of differential amplifier 30. The combination of amplifier 30 and R f is current sensing amplifier 32. Coupling capacitor C₂ removes the DC component from the output signal, V s . Thus, V s = I b R f (ΔR h /R h ). For the configuration of Figure 3, if R h is 50 ohms and ΔR h /R h = 0.5%, R f is 500 ohms, C1 is 0.5 µF, C2 is 0.1 µF and amplifier 30 is a wideband amplifier, then V s is 25 mV at F₀ = 6 kHz. The pseudo-balanced configuration of Fig. 4 is useful in the presence of undesirable stray ground currents which may be injected into the MR head sensing stripe through the slider-to-stripe capacitance. One-half of the injected ground current, I g , will flow through grounding resistor, R g . The other half of I g will flow through feedback resistor, R f . If R g and R f are matched or otherwise substantially equal, the voltage developed by the flow of one-half I g through each of these resistors will be cancelled at the amplifier output since they are of opposite phase. Thus, stray ground current, I g , does not contaminate V s . For the configuration of Fig. 4, if R h changes by an amount ΔR h , the resulting signal current is given by I s = -I b (ΔR h /R h ). Current I s is supplied from feedback resistor R f and flows through R g to ground. Thus, V s = -2I b R f (ΔR h /R h ). For the configuration of Fig. 4, with I b = 10 mA, ΔR h /R h = 0.5%, I s = 50 µA and V s = -50 mV if C₁ = 0.5 µF, C₂ = 0.1 µF, R f = 500 ohms and amplifier 30 is a wideband amplifier, such as an MC1733, manufactured by Motorola. Referring now to Fig. 5, where an embodiment for constant voltage biasing of an MR head according to the present invention is shown, model reference 50 comprises a fixed current source, I r , a resistor, R r and capacitor, C. Voltage V b , given by the product of I r and R r , provides the reference for bias voltage across MR head resistor, R h . Since capacitor C forms a short circuit for all data frequencies, the input of voltage amplifier 70 is effectively coupled across MR head resistance, R h . Capacitor C also shorts out all noise produced by resistor R r . Thus, signal amplifier 70 detects and amplifies the instantaneous value of the voltage, V s , produced by the MR head according to the relation Bias feedback loop 60 offsets V b by driving the potential of R h lead 61 to the same potential as R r lead 62. The operating frequency range of bias feedback loop 60 is determined by low pass filter 63 which is designed to pass only DC and very low frequencies.

Claims: A circuit for detecting magnetic fields having encoded information to produce electrical signals representing the encoded information, the circuit comprising: a magnetoresistive element; and detection means for detecting the ratio of the instantaneous value of the change in the resistance of the magnetoresistive element to the absolute magnetoresistive element resistance; characterised in that the detection means comprises: first and second current sources connected in series with the magnetoresistive element for generating a bias current (I b ) in the magnetoresistive element (R h ); a capacitor (C1) connected across the magnetoresistive element (R h ); a current probe for detecting a current deviation from the bias current, the deviation being proportional to the relative resistance change of the magnetoresistive element produced by exposing the magnetoresistive element to the magnetic field, the current probe generating a signal voltage (V s ) proportional to the deviation (Figure 2A). A circuit for detecting magnetic fields having encoded information to produce electrical signals representing the encoded information, the circuit comprising: a magnetoresistive element (R h ); and detection means for detecting the ratio of the instantaneous value of the change in the resistance of the magnetoresistive element to the absolute magnetoresistive element resistance; characterised in that the detection means comprises: a constant voltage source (V r ) connected to first and second matched resistors (R s ) in series with the magnetoresistive element (R h ) to generate a bias current through the magnetoresistive element (R h ); and a ferrite toroid (T) having primary and secondary windings, the primary winding being connected in series with the magnetoresistive element to generate current deviations in the primary winding, the deviations being proportional to the relative resistance change of the magnetoresistive element produced, the deviations generating a signal voltage (V s ) across the secondary winding, the signal voltage being proportional to the deviations (Figure 2B). A circuit as claimed in claim 2 further including a capacitance (C1) connected across the magnetoresistive element to limit spurious signal current variations at frequencies greater than the break point frequency defined by the the capacitance and the total magnetoresistive element resistance. A circuit for detecting magnetic fields having encoded information to produce electrical signals representing the encoded information, the circuit comprising: a magnetoresistive element; detection means for detecting the ratio of the instantaneous value of the change in the resistance of the magnetoresistive element to the absolute magnetoresistive element resistance; characterised in that the detection means includes: a current source connected in series with the magnetoresistive element having a resistance R h for generating a bias current I b in the magnetoresistive element; and a detector for detecting current deviations from the bias current, the deviations being proportional to the relative resistance change ▵Rh of the magnetoresistive element produced by exposing the element to the magnetic fields, the detector comprising a differential amplifier (30) for amplifying the instantaneous value of the current deviations, Is, of the current deviations, substantially according to the relation where Ib is the bias current, Rh is the absolute resistance of the magnetoresistive element, and ▵Rh is the instantaneous value of the change in the resistance of the magnetoresistive element in the presence of the magnetic field, the differential amplifier having one input connected to a reference voltage and the other input connected in series with magnetoresistive element to generate an output signal voltage which is proportional to the deviations. and further including a capacitance having one terminal connected to the magnetoresistive element and said other input of the amplifier and the other terminal connected to the reference voltage (Figure 3 and 4). A circuit for detecting magnetic fields having encoded information to produce electrical signals representing the encoded information, the circuit comprising: a magnetoresistive element; detection means for detecting the ratio of the instantaneous value of the change in the resistance of the magnetoresistive element to the absolute magnetoresistive element resistance RH; characterised in that the detection means includes: a controlled constant current source connected to the magnetoresistive element to generate a current (Ib) therethrough; a detector for detecting deviations of the voltage across Rh from the constant voltage Vb, provided by a constant voltage source (50), the deviations being proportional to the relative resistance change of the magnetoresistive element produced by exposing the element to the magnetic fields, the detector comprising a voltage amplifier (70) for amplifying the instantaneous value, Vs, of the deviations, and further comprising a bias feedback loop (60) for minimising the steady state voltage across the inputs of the voltage amplifier, substantially according to the relation where Vb is the voltage provided by the constant voltage source (50), Rh is the absolute resistance of the magnetoresistive element, and ▵Rh is the instantaneous value of the change in the resistance of the magnetoresistive element in the presence of the magnetic field (Figure 5). A circuit as claimed in claim 5 wherein the detection means comprises a model reference having a current source connected in series with a resistor (R r ) to generate a reference voltage level (V b ) for the bias voltage across the magnetoresistive element. A circuit as claimed in claim 6 wherein the model reference further comprises a capacitor (C) connected across the resistor to reduce spurious voltage variations produced by the resistor (R r ). A circuit as claimed in claim 7 wherein the voltage amplifier (70) comprises two inputs, and the detection means further comprises a bias feedback loop for minimising the steady state voltage across the inputs of the voltage amplifier (70), the bias feedback loop (60) comprising a variable current source connected in series with the magnetoresistive element, a comparator for varying the current from the variable current source in response to variations in the voltage across the inputs of the voltage amplifier (70), and a low pass filter (63) coupled between the output of the comparator and the control input of the variable current source to limit the frequency response bandwidth of the bias feedback loop (60).

Inventors: KLAASSEN, KLAAS BEREND
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: G11B5
IPC codes: G11B5, G01R33