Magnetic field sensor circuit
A magnetic field sensor circuit comprises a first magneto-resistive sensor (Rx) which senses a first magnetic field component in a first direction to supply a first sense signal (Vx). A first flipping coil (FC 1 ) applies a first flipping magnetic field with a periodically changing polarity to the first magneto-resistive sensor (Rx) to cause the first sense signal (Vx) to have alternating different levels synchronized with the first flipping magnetic field. A second magneto -resistive sensor (Ry) senses a second magnetic field component in a second direction different than the first direction to supply a second sense signal (Vy). A second flipping coil (FC 2 ) applies a second flipping magnetic field with a periodically changing polarity to the second magneto -resistive sensor (Ry) to cause the second sense signal (Vy) to have an alternating different levels synchronized with the second flipping magnetic field. The first flipping magnetic field and the second flipping magnetic field have a phase shift. A differential amplifier (AMP 1 ) receives the first sense signal (Vx) and the second sense signal (Vy) to obtain a difference signal (Vd). A first synchronous demodulator (DEM 1 ) receives the difference signal (Vd) and a first switching signal (Q 1 ) being phase locked to the alternating different levels of the first sense signal (Vx) to supply a first output signal (Vox) indicating the first magnetic field component. A second synchronous demodulator (DEM 2 ) receives the difference signal (Vd) and a second switching signal (Q 2 ) being phase locked to the alternating different levels of the second sense signal (Vy) to supply a second output signal (Voy) indicating the second magnetic field component.
1. A magnetic field sensor circuit comprising:
a first magneto-resistive sensor for sensing a first magnetic field component in a first direction to supply a first sense signal,
a first flipping coil for applying a first flipping magnetic field with a periodically changing polarity to the first magneto-resistive sensor for causing the first sense signal to have alternating different levels synchronized with the first flipping magnetic field,
a second magneto-resistive sensor for sensing a second magnetic field component in a second direction different than the first direction to supply a second sense signal,
a second flipping coil for applying a second flipping magnetic field with a periodically changing polarity to the second magneto-resistive sensor for causing the second sense signal to have an alternating different levels synchronized with the second flipping magnetic field, a phase of the first flipping magnetic field and the second flipping magnetic field being different,
a differential amplifier configured to receive the first sense signal and the second sense signal to obtain a difference signal,
a first synchronous demodulator configured to receive the difference signal and a first switching signal being phase locked to the alternating different levels of the first sense signal to supply a first output signal indicating the first magnetic field component, and
a second synchronous demodulator configured to receive the difference signal and a second switching signal being phase locked to the alternating different levels of the second sense signal to supply a second output signal indicating the second magnetic field component.
2. A magnetic field sensor circuit as claimed in claim 1 , wherein the first magneto-resistive sensor is a first magneto-resistive element, and wherein the second magneto-resistive sensor is a second magneto-resistive element, the magnetic field sensor circuit further comprises:
a first current source for supplying a first current to the first magneto-resistive element to obtain the first sense signal, and
a second current source for supplying a second current to the second magneto-resistive element to obtain the second sense signal.
3. A magnetic field sensor circuit as claimed in claim 2 , wherein the first synchronous demodulator is operable to be temporarily kept on hold for obtaining the first output signal being a measure for an offset, wherein the circuit further comprises and a DC-level controller for receiving the first output signal for controlling a DC level of the second current source to compensate for the offset.
4. magnetic field sensor circuit as claimed in claim 3 , further comprising an analog to digital converter for converting the difference signal into a digital difference signal, and wherein the DC-level controller comprises a digital to analog converter for controlling the second current source in response to a value of the first output signal.
5. A magnetic field sensor circuit as claimed in claim 1 , further comprising a first driver for supplying a current having a first sequence of pulses with alternating polarity to the first flipping coil and a second driver for supplying a current having a second sequence of pulses with alternating polarity to the second flipping coil, and wherein the pulses of the first sequence and the second sequence have a non-zero phase shift.
6. A magnetic field sensor circuit as claimed in claim 1 , further comprising a first low-pass filter or a first integrator for low-pass filtering the first output signal, and a second low-pass filter or second integrator for low-pass filtering the second output signal.
7. A magnetic field sensor circuit as claimed in claim 6 , wherein the first low pass filter and the second low-pass filter have a bandwidth in a range from 500 to 1000 Hz.
8. An electronic compass comprising the magnetic field sensor circuit of claim 1 and an orientation determining circuit coupled to the magnetic field sensor circuit for determining the orientation with respect to the earth.
9. A hand-held apparatus comprising the electronic compass of claim 8 .
10. A hand-held apparatus as claimed in claim 9 , further comprising a hard disk and a free fall detector coupled to the magnetic field sensor circuit and the accelerometer for detecting whether the hand-held apparatus is falling and if so: for parking a head of the hard disk in a parking zone.
11. A vehicle comprising the magnetic field sensor circuit (MFSC) of claim 1 .