MAGNETIC FIELD SENSOR WITH IMMUNITY TO EXTERNAL MAGNETIC INFLUENCES

• US 20140333295A1
• Filed: 05/10/2013
• Published: 11/13/2014
• Est. Priority Date: 05/10/2013
• Status: Active Grant

• Current Assignee: Allegro Microsystems Incorporated (Sanken Electric Company Limited)
• Original Assignee: Allegro Microsystems Incorporated (Sanken Electric Company Limited)
• Inventors: Fernandez, Devon, David, Paul, Taylor, William P.
• Granted Patent: US 9,664,494 B2
• Time in Patent Office: Days
• US Class Current: 324/207.12
• CPC Class Codes: G01B 7/003 for measuring position, not involving coordinate determination coordinate measuring G01B7/004
• CPC Class Codes: G01R 33/0023 Electronic aspects, e.g. circuits for stimulation, evaluation, control; Treating the measured signals; calibration G01R33/0017 takes precedence

Abstract

A magnetic field sensor with enhanced immunity to external magnetic interference is presented. Included is a magnetic field signal generator and a demodulator. The magnetic field signal generator produces a magnetic field signal having a modulated signal portion in a first frequency band based on a sensed modulated AC bias magnetic field. The modulated AC bias magnetic field is produced by movement of ferromagnetic target relative to a bias coil when an AC signal is applied to the bias coil. When the magnetic field signal also includes an unwanted signal portion in a second frequency band based on external magnetic interference, demodulation performed by the demodulator results in the modulated signal portion being shifted from the first frequency band to a third frequency band and the unwanted signal portion being shifted to the first frequency band. The bias coil may be provided as part of the magnetic field sensor.

Claims

1. A magnetic field sensor comprising:

an AC bias coil to generate an AC bias magnetic field capable of being modulated by movement of a ferromagnetic target with respect to the AC bias coil;

a magnetic field signal generator comprising at least one sensing element to sense the modulated AC bias magnetic field and produce a magnetic field signal, the magnetic field signal having a modulated signal portion in a first frequency band based on the sensed modulated AC bias magnetic field;

circuitry including a demodulator to perform a demodulation of the magnetic field signal; and

wherein, when the magnetic field signal further includes an unwanted signal portion in a second frequency band based on a magnetic interference, the demodulation results in the modulated signal portion being shifted from the first frequency band to a third frequency band and the unwanted signal portion being shifted to the first frequency band from the second frequency band.

2. The magnetic field sensor of claim 1, wherein the third frequency band comprises DC.

3. The magnetic field sensor of claim 1, wherein the demodulator further includes a filter to remove the unwanted signal portion from the magnetic field signal after the unwanted signal portion has been shifted to the first frequency band.

4. The magnetic field sensor of claim 3, wherein the circuitry produces a sensor output signal based on the magnetic field signal after demodulation.

5. The magnetic field sensor of claim 1, further comprising a coil driver, coupled between a supply input and the AC bias coil, to provide an AC signal to the AC bias coil.

6. The magnetic field sensor of claim 5, farther comprising a frequency select input, coupled to the coil driver, to allow the frequency of the AC signal to be adjusted by an external application.

7. The magnetic field sensor of claim 5, further comprising a frequency controller, coupled to the coil driver, to provide a frequency select signal based on a frequency selection to the coil driver for adjusting the frequency of the AC signal.

8. The magnetic field sensor of claim 7, wherein the frequency controller is configured to receive the magnetic field signal after demodulation following multiple frequency adjustments, and modify the frequency selection in response thereto.

9. The magnetic field sensor of claim 1, wherein a frequency at which the AC bias magnetic field is generated is set so that the first frequency band excludes frequencies associated with the magnetic interference in the second frequency band.

10. The magnetic field sensor of claim 1, wherein the AC bias coil, magnetic field signal generator and circuitry are provided on a semiconductor die.

11. The magnetic field sensor of claim 10, wherein the AC bias coil comprises a planar spiral coil.

12. The magnetic field sensor of claim 11, wherein the sensing element comprises at least one spin-valve, AMR, GMR, TMR, or MTJ type sensing element.

13. The magnetic field sensor of claim 11, further comprising a coil driver, coupled between a supply input and the AC bias coil, to provide an AC signal to the AC bias coil.

14. The magnetic field sensor of claim 1, wherein the magnetic field signal generator and circuitry are provided on a semiconductor die, further comprising a package that includes a lead frame coupled to the semiconductor die and located between the semiconductor die and the AC bias coil, and further includes a package body to enclose the semiconductor die, at least a portion of the lead frame and the AC bias coil together.

15. The magnetic field sensor of claim 14, wherein the AC bias coil comprises a solenoid type coil.

16. The magnetic field sensor of claim 15, wherein the solenoid type coil comprises a soft ferromagnetic core to concentrate the AC bias magnetic field.

17. The magnetic field sensor of claim 14, wherein the package body comprises a mold portion to enclose the die, at least a portion of the lead frame and the AC bias coil.

18. The magnetic field sensor of claim 14, wherein the package body comprises a first mold portion to enclose the die and at least a portion of the lead frame and a second mold portion to enclose the AC bias coil.

19. The magnetic field sensor of claim 18, wherein the second mold portion comprises a ferromagnetic mold material.

20. The magnetic field sensor of claim 1, wherein the at least one sensing element is a selected one of a Hall-effect element, magnetoresistance (MR) element or other type of magnetic field sensitive element.

21. The magnetic field sensor of claim 20, wherein material used to implement the at least one sensing element is a selected one of type IV semiconductor material or a type III-V semiconductor material.

22. A magnetic field sensor comprising:

a magnetic field signal generator comprising at least one sensing element to sense a modulated AC bias magnetic field and produce a magnetic field signal, the magnetic field signal having a modulated signal portion in a first frequency band based on the sensed modulated AC bias magnetic field;

circuit including a demodulator to perform a demodulation of the magnetic field signal; and

wherein, when the magnetic field signal further includes an unwanted signal portion in a second frequency band based on a magnetic interference, the demodulation results in the in modulated signal portion being shifted from the first frequency band to a third frequency band and the unwanted signal portion being shifted to the first frequency band from the second frequency band.

23. The magnetic field sensor of claim 22, wherein the third frequency band comprises DC.

24. The magnetic field sensor of claim 22, wherein the demodulator further includes a filter to remove the unwanted signal portion from the magnetic field signal after the unwanted signal portion has been shifted to the first frequency band.

25. The magnetic field sensor of claim 24, wherein the circuitry produces a sensor output signal based on the magnetic field signal after demodulation.

26. The magnetic field sensor of claim 22, wherein the AC bias magnetic field is generated by an external coil driven by an AC signal.

27. A method, comprising:

producing a magnetic field signal comprising a modulated signal portion in a first frequency band based on sensing a modulated AC bias magnetic field and an unwanted signal portion in a second frequency hand based on sensing a magnetic interference; and

performing a demodulation of the magnetic field signal to shift the modulated signal portion from the first frequency band to a third frequency band and the unwanted signal portion to the first frequency band from the second frequency band.

28. The method of claim 27, wherein the third frequency band comprises DC.

29. The method of claim 27, wherein performing the demodulation further comprises filtering to remove the unwanted signal portion from the magnetic field signal after the unwanted signal portion has been shifted to the first frequency band.

30. The method of claim 29, further comprising:

producing an output signal based on the magnetic field signal after demodulation.

31. The method of claim 30, further comprising:

providing an AC signal to an AC bias coil to generate an AC bias magnetic field which, when modulated by movement of a ferromagnetic target with respect to the AC bias coil, produces the modulated AC bias magnetic field.

32. The method of claim 31, wherein the AC signal has a frequency that is an adjustable frequency.

33. The method of claim 32, further comprising:

setting the frequency of the AC signal so that the first frequency band excludes frequencies associated with the magnetic interference in the second frequency band.