Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
1999-07-19
2001-05-01
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C324S207210, C360S067000
Reexamination Certificate
active
06225802
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a device for measuring the value of a resistance through which a bias current is to be passed and across which a bias voltage is to be generated.
The measurement methods used most frequently are based on the application of Ohm's law, in accordance with which the value of a resistance is equal to the quotient of the value of the voltage appearing across this resistance and the value of the current flowing through it. Therefore, it suffices to measure the bias current and voltage to derive the value of the resistance directly therefrom. Though such a method can be implemented comparatively easily for measuring the resistance of a discrete component, this method is ill-suited for measuring the resistance of a component integrated in a unit, which component may then not be accessible at all from the exterior of the unit. This is the case, for example, for the measurement of the quiescent resistance of a magnetoresistive bar integrated in a read head for reading magnetic information, during which measurement it is obviously out of the question to connect a voltmeter in parallel with the magnetoresistive bar and an ammeter in series with said bar. Moreover, if it is to be utilized, the measurement result, i.e. the value of the resistance, should be expressed in the form of an electric signal, which in many cases should take a digital form.
SUMMARY OF THE INVENTION
It is an object of the invention to meet these requirements by providing a device for measuring the value of a resistance wherein the measurement is carried out automatically within the device, without any external intervention, the result of said measurement being available directly in digital form.
To this end, according to the invention, a magnetic information reading device of the type defined in the opening paragraph is characterized in that it comprises:
a voltage comparator having a first and a second input intended for to receiving, respectively, the bias voltage and a reference voltage, and having an output,
count-up/count-down means having an enable input coupled to the output of the comparator, and having a digital output forming the output of the device, and
a current source for suppling the bias current whose value represents a digital value received by said current source at a control input, said control input being coupled to the digital output of the count-up/count-down means.
The operation of this measurement device is based on the fact that its output signal defines the value of the bias current of the resistance. This current thus varies progressively until the bias voltage, generated by the resistance under the influence of the bias current, is equal to the value of the reference voltage. The count-up/count-down means are then deactivated and their output supplies a digital value representing the value of the resistance.
A variant of the invention provides a device for measuring the value of a resistance to be subjected to a bias voltage and for generating a bias current, which device is characterized in that it comprises:
a current comparator, having a first and a second input, for receiving, respectively, a current representative of the bias current, and a reference signal, and having an output,
count-up/count-down means having an enable input coupled to the output of the comparator, and having a digital output forming the output of the device, and
a voltage source for suppling the bias voltage whose value represents a digital value received by the voltage source at a control input, said control input being coupled to the digital output of the count-up/count-down means.
In this variant of the invention the resistance is biased by means of the bias voltage and, under the influence of this voltage, generates the bias current, this being the object of a comparison with a reference signal. The output signal of the device determines the value of the bias voltage. This voltage thus varies progressively until the value of the bias current is equal to the value of the reference signal. The count-up/count-down means are then deactivated and their output supplies a digital value representing the value of the resistance.
Another variant of the invention provides a device for measuring the value of a resistance through which a bias current is to be passed and across which a bias voltage is to be generated, the values of the bias current and the bias voltage being controllable simultaneously by means of a module for controlling the power dissipated by the resistance, which device is characterized in that it comprises:
a current multiplier for receiving a first current, a second current representative of the bias current, and a third current representative of the bias voltage, which multiplier is intended to generate a fourth current having a value proportional to the quotient of the values of the third current and the second current, multiplied by the value of the first current,
a current comparator having a first input and a second input, respectively, for receiving the fourth current and a reference current, and having an output,
count-up/count-down means having an enable input coupled to the output of the comparator, and having a digital output forming the output of the device, and
a current source for suppling the first current whose value is representative of a digital value received by said current source at a control input, which control input is coupled to the digital output of the count-up/count-down means.
In this other variant of the invention, the values of the bias voltage and bias current are determined by the control module and cannot be influenced individually by the measurement device. Only the ratio between these two values is useful because it remains constant. The function of the current multiplier is thus to generate a signal representing this ratio, which signal is formed by the fourth current and is dependent on the first current, whose value is determined by the output of the device. The first current as well as the fourth current vary progressively until the value of said fourth current has become equal to the value of the reference signal. The count-up/count-down means are then deactivated and their output supplies a digital value representing the value of the resistance. The measurement devices described hereinbefore can be used advantageously in devices for reading magnetic information, having at least one read head comprising at least one magnetoresistive bar intended for generating data pulses which represent information read by the head.
Such reading devices are commonly used for reading information stored on hard disks for computers. These disks generally have a magnetosensitive surface which is to be scanned by the read head. A rotary movement is imparted to the disk, while the read head is coupled to an arm which imparts a radial movement to the head. The disk surface is divided into a multitude of sub-surfaces in which a local magnetic field exists whose sign represents an item of binary information.
The operation of customary reading devices is based on the fact that when a magnetoresistive bar is exposed to a magnetic field its resistance varies.
In the majority of existing magnetic information reading devices the magnetoresistive bar is biased either by means of a d.c. bias current of predetermined value which flows through said bar, in which case the resistance variation generates a voltage pulse, or by means of a d.c. bias voltage of predetermined value applied across said bar, in which case said resistance variation generates a current pulse.
In the two afore-mentioned cases the amplitude of the generated pulses, called data pulses, will be higher as the predetermined value is higher. Therefore, it may seem advantageous to choose the predetermined value as high as possible in order to obtain a signal-to-noise ratio which is as high as possible. However, the predetermined value also dictates the power to be dissipated by the magnetoresistive bar. Thus, a predetermined value which is too large may lead to premature w
De Jong Gerben W.
Pieraerts Eric
Ramaekers Jozef A. M.
Ramalho Joao N. V. L.
Franzblau Bernard
Oda Christine
U.S. Philips Corporation
Zaveri Subhashi
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