Method and apparatus for testing the resistive properties of magneto resistive materials using a time variable magnetic field

An apparatus for characterizing the resistive properties of a magnetoresistive element in response to a varying magnetic field, said apparatus comprising:

• an electromagnet supplied with a time-varying voltage to create a periodically-varying magnetic field encompassing said magnetoresistive element, wherein said periodically-varying magnetic field encompassing said magnetoresistive element has a magnetic field intensity;
• a magnetic field intensity indicator which provides a time dependent magnetic field output signal, V.sub.B (t), which is proportional to said magnetic field intensity of said periodically-varying magnetic field encompassing said magnetoresistive element;
• a bias current source connected to said magnetoresistive element for passing an electrical current through said magnetoresistive element thereby producing a magnetoresistive element voltage signal across said magnetoresistive element which is characteristic of the resistance of said magnetoresistive element, wherein said magnetoresistive element voltage signal has a time-varying component, V.sub.r (t), produced by said periodically-varying magnetic field and a time-invariant component;
• a filter having an input terminal connected to said magnetoresistive element, said filter removing said time-invariant component of said magnetoresistive element voltage signal characteristic of the resistance of said magnetoresistive element and transmitting said time-varying component, V.sub.r (t), of said magnetoresistive element voltage signal characteristic of the resistance of said magnetoresistive element;
• a voltage sensor connected to an output terminal of said filter for detecting said time-varying component, V.sub.r (t), of said magnetoresistive element voltage signal;
• and
• a signal processor connected to said voltage sensor and said magnetic field intensity indicator for receiving said time-varying component of said magnetoresistive element voltage signal, V.sub.r (t), characteristic of the resistance of said magnetoresistive element and said magnetic field output signal, V.sub.B (t), from said magnetic field intensity indicator which is proportional to said magnetic field intensity of said time-varying magnetic field, wherein said signal processor further comprises:
• a first analog-to-digital converter (ADC) which acquires a first time value of said magnetoresistive element voltage signal V.sub.r (t) at a first time t.sub.1 denoted as V.sub.r (t.sub.1) and a second time value of said magnetoresistive element voltage signal V.sub.r (t) at a second time t.sub.2 denoted as V.sub.r (t.sub.2);
• and
• a second analog-to-digital converter (ADC) which acquires a first time value of said magnetic field output signal V.sub.B (t) at said first time t.sub.1 denoted as V.sub.B (t.sub.1) and a second time value of said magnetic field output signal V.sub.B (t) at said second time t.sub.2 denoted as V.sub.B (t.sub.2), wherein said signal processor correlates said first time value of said magnetoresistive element voltage signal V.sub.r (t.sub.1) with said first time value of said magnetic field output signal V.sub.B (t.sub.1) and correlates said second time value of said magnetoresistive element voltage signal V.sub.r (t.sub.2) with said second time value of said magnetic field output signal V.sub.B (t.sub.2) thereby determining the resistive characteristics of said magnetoresistive element as a function of said magnetic field intensity of said periodically-varying magnetic field.

A method for measuring the resistance of a magnetoresistive device as a function of the magnetic field applied thereto, said method comprising the steps of:

• applying a periodically-varying magnetic field to said magnetoresistive device, wherein said periodically-varying magnetic field includes multiple cycles of substantially equal time duration;
• providing a magnetic field output signal which is proportional to said periodically-varying magnetic field;
• passing an electrical current through said magnetoresistive device thereby producing a magnetoresistive device voltage signal across said magnetoresistive device which is characteristic of the resistance of said magnetoresistive device, wherein said magnetoresistive device voltage signal has a time-varying component produced by said periodically-varying magnetic field and a time-invariant component;
• providing a filter at a first terminal of said magnetoresistive device;
• providing said magnetoresistive device voltage signal which is proportional to the resistance of said magnetoresistive device at said filter;
• removing said time-invariant component of said magnetoresistive device voltage signal with said filter;
• acquiring a plural integral number, N, of measurements of said time-varying component of said magnetoresistive device voltage signal during one or more of said cycles of said periodically-varying magnetic field;
• acquiring said plural integral number, N, of measurements of said magnetic field output signal during one or more of said cycles of said periodically-varying magnetic field, wherein for each integral number comprising said N measurements, said measurement of said time-varying component of said magnetoresistive device voltage signal and said measurement of said magnetic field output signal are acquired substantially simultaneously, thus providing N pairs of data wherein each pair of data includes a measurement of said time-varying component of said magnetoresistive device voltage signal and said magnetic field output signal which occurred at substantially the same time;
• and
• analyzing said pairs of data of said time-varying component of said magnetoresistive device voltage signal and said time-varying component of said magnetic field output signal which is proportional to said periodically-varying magnetic field to determine the resistance of said magnetoresistive device as a function of the magnetic field applied to said magnetoresistive device.

An apparatus for determining the resistance of a magnetoresistive element in response to a time-varying magnetic field, comprising:

• an electromagnet located adjacent said magnetoresistive element wherein a time-varying voltage is applied to said electromagnet, thereby generating a time-varying magnetic field having a time-varying magnetic field intensity;
• a magnetic field intensity indicator which provides a magnetic field output signal, V.sub.B (t), which is proportional to said time-varying magnetic field intensity of said time-varying magnetic field at said magnetoresistive element;
• a current source connected to said magnetoresistive element for passing an electrical current through said magnetoresistive element thereby producing a magnetoresistive element voltage signal across said magnetoresistive element which is characteristic of the resistance of said magnetoresistive element, wherein said magnetoresistive element voltage signal has a time-varying component, V.sub.r (t), produced by said time-varying magnetic field and a time-invariant component;
• a filter connected to said magnetoresistive element, wherein said filter substantially blocks said time-invariant component of said magnetoresistive element voltage signal and transmits said time-varying component, V.sub.r (t), of said magnetoresistive element voltage signal;
• a voltage sensor connected to said filter for detecting said time-varying component V.sub.r (t) of said magnetoresistive element voltage signal;
• a signal sampler connected to said voltage sensor and said magnetic field intensity indicator, wherein said signal sampler acquires a plurality of real-time values of said magnetoresistive element voltage signal V.sub.r (t) including first and second real-time magnetoresistive element voltage signal values, V.sub.r (t.sub.1) and V.sub.r (t.sub.2) at first and second real-times t.sub.1 and t.sub.2, respectively, and a plurality of real-time values of said magnetic field output signal V.sub.B (t) including first and second real-time magnetic field output signal values, V.sub.B (t.sub.1) and V.sub.B (t.sub.2) at said first and second real-times t.sub.1 and t.sub.2, respectively;
• and
• a signal processor connected to said signal sampler wherein said signal processor receives and correlates said plurality of real-time values of said magnetoresistive element voltage signal V.sub.r (t) and said plurality of real-time values of said magnetic field output signal V.sub.B (t) as a function of time, including correlating said first real-time value of said magnetoresistive element voltage signal V.sub.r (t.sub.1) with said first real-time value of said magnetic field output signal V.sub.B (t.sub.1) and correlating said second real-time value of said magnetoresistive element voltage signal V.sub.r (t.sub.2) with said second real-time value of said magnetic field output signal V.sub.B (t.sub.2), thereby determining the resistance of said magnetoresistive element as a function of said time-varying magnetic field intensity.

An apparatus for rapidly determining the resistance of a magnetoresistive element at a plurality of different magnetic field values comprising:

• an electromagnet located adjacent the magnetoresistive element wherein an alternating current (AC) having an AC frequency is applied to said electromagnet, thereby subjecting the magnetoresistive element to a periodic time-varying magnetic field;
• a magnetic field intensity indicator which provides a magnetic field output signal, V.sub.B (t), which is proportional to said periodic time-varying magnetic field at the magnetoresistive element;
• a current source connected to the magnetoresistive element for passing an electrical current through the magnetoresistive element thereby producing a magnetoresistive element voltage signal across the magnetoresistive element which is characteristic of the resistance of the magnetoresistive element, wherein said magnetoresistive element voltage signal has a periodic time-varying component, V.sub.r (t), produced by said periodic time-varying magnetic field and a time-invariant component;
• a capacitor connected to the magnetoresistive element, wherein said capacitor substantially blocks said time-invariant component of said magnetoresistive element voltage signal and transmits said periodic time-varying component, V.sub.r (t), of said magnetoresistive element voltage signal;
• a voltage sensor connected to said capacitor for detecting said periodic time-varying component V.sub.r (t) of said magnetoresistive element voltage signal;
• a multiplexed analog-to-digital converter (ADC) which operates at an ADC frequency which is greater than said AC frequency, said ADC having a first ADC input connected to said magnetic field intensity indicator for acquiring n values of said magnetic field output signals V.sub.B (t.sub.n), during each of a plurality of cycles of said AC frequency, and a second ADC input connected to said voltage sensor for acquiring n values of said magnetoresistive element voltage signals V.sub.r (t.sub.n) during each of a plurality of cycles of said AC frequency;
• and
• a signal processor connected to said multiplexed analog-to-digital converter wherein said signal processor receives and correlates said n values of said magnetoresistive element voltage signals V.sub.r (t.sub.n) and said n values of said magnetic field output signals V.sub.B (t.sub.n) such that for each value of n, V.sub.r (t.sub.n) corresponds to said magnetoresistive element voltage signal measured when the magnetic field at the magnetoresistive element is substantially equal to V.sub.B (t.sub.n), thereby determining the resistance of the magnetoresistive element at a plurality of different values of said time-varying magnetic field intensity.