FLUXGATE MAGNETIC-TO-DIGITAL CONVERTER WITH OVERSAMPLING CLOSED LOOP

• US 20140285189A1
• Filed: 03/21/2014
• Published: 09/25/2014
• Est. Priority Date: 03/21/2013
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Kashmiri, Sayyed Mahdi, Kindt, Willem J.
• Granted Patent: US 9,714,987 B2
• Time in Patent Office: Days
• US Class Current: 324/253
• CPC Class Codes: G01R 33/0041 using feed-back or modulation techniques
• CPC Class Codes: G01R 33/04 using the flux-gate principle

Abstract

A fluxgate sensor including a magnetic-to-digital converter (MDC) can be adapted to measure an external magnetic field B_EXT with a bandwidth f_B. The MDC forward path can include: (a) converting an analog sense signal from the fluxgate sense coil to corresponding oversampled digital data using an oversampling data converter with an oversampling frequency f_S greater than f_B; and (b) loop filtering the oversampled digital data, synchronous with the oversampling frequency f_S, to generate the loop output digital data. The MDC feedback path can include: (a) generating the feedback compensation current I_COMP from the loop output digital data, synchronous with a feedback path frequency f_FB equal to ((M/N)×f_S), where, M and N are integers; and (b) injecting the feedback compensation current I_COMP into the fluxgate compensation coil to induce the compensation field B_COMP, such that the induced compensation field B_COMP nulls the external field B_EXT.

Claims

1. A fluxgate sensor circuit adapted to measure an external magnetic field B_EXT with a bandwidth f_B using at least one fluxgate magnetics element including a fluxgate core with an excitation coil, a compensation coil and a sense coil, and disposed such that the external field B_EXT magnetically couples into the fluxgate core, comprising:

drive circuitry configured to provide to the excitation coil an excitation current I_EXC with an excitation frequency f_EXC; and

a magnetic-to-digital (MDC) control loop, including the fluxgate magnetics element, configured to convert the external field B_EXT into representative sensor data, the MDC control loop including;

a forward path coupled to receive an analog sense signal output of the fluxgate sense coil induced by a sense field in the fluxgate core that corresponds to a difference between the external field B_EXT, and a compensation field B_COMP, and configured to provide MDC loop output digital data as the sensor data representative of the external B_EXT field including,

anti-aliasing circuitry configured to low-pass filter the analog sense signal to provide a band-limited analog sense signal;

oversampling ADC (analog-to-digital conversion) circuitry configured to convert the band-limited analog sense signal to corresponding oversampled digital data based on an oversampling frequency f_S greater than 2f_B; and

digital loop filter circuitry synchronized with f_S, and configured to filter the oversampled digital data to generate MDC loop output digital data corresponding to the sensor data representative of the external field B_EXT; and

a feedback path coupled to receive the MDC loop output digital data, and coupled to the fluxgate compensation coil, and including

feedback compensation circuitry, synchronized with a feedback path frequency f_FB equal to ((M/N)×

f_S), where, M and N are integers, and configured to generate, in response to the MDC loop output digital data, a compensation current I_COMP that is injected into the fluxgate compensation coil, to induce the compensation field B_COMP;

such that the induced compensation field B_COMP nulls the external field B_EXT, so that the compensation current I_COMP corresponds to the sensor data representative of the external field B_EXT.

2. The circuit of claim 1, wherein the fluxgate sensor circuit is adapted to measure differential components B_DM of an external magnetic field B_EXT that also includes a stray field common-mode component B_CM, using two fluxgate magnetics elements including respective magnetic cores with respective excitation, sense and compensation coils, the fluxgate magnetics elements respectively disposed such that differential components B_DM of the external field B_EXT magnetically couple into the respective fluxgate cores, and:

wherein the drive circuitry is configured to provide to the excitation coils respective excitation currents I_EXC each at the excitation frequency f_EXC;

further comprising common mode field compensation circuitry coupled to receive analog sense outputs from respective sense coils, and configured to generate common-mode analog compensation currents I_COMP,CM that are injected into respective compensation coils to induce respective common-mode compensation fields B_COMP,CM to null the common-mode component B_CM of the external field B_EXT;

wherein the MDC control loop is configured to convert the differential components B_DM into representative sensor data, based on differential analog sense signal outputs of respective fluxgate sense coils induced by respective sense fields in respective fluxgate cores, each corresponding to a difference between a differential component B_DM of the external field B_EXT, and a respective compensation field B_COMP,DM;

wherein the feedback compensation circuitry is configured to generate respective differential compensation currents I_COMP,DM that are injected into respective compensation coils to induce respective compensation fields B_COMP,DM to null the corresponding differential component B_DM of the external field B_EXT.

3. The circuit of claim 1, configured to sense current, wherein the magnetic field B_EXT is induced in the at least one fluxgate magnetics element by a current.

4. The circuit of claim 1, wherein the feedback compensation circuitry is configured to generate the I_COMP compensation current synchronized with one of f_EXC and 2×

f_EXC, such that transitions of the I_COMP compensation current are synchronized with fluxgate core saturation cycles.

5. The circuit of claim 4, wherein the feedback compensation circuitry comprises:

digital-to-analog conversion circuitry, synchronized with f_FB equal to ((M/N)×

f_S), and configured to convert the MDC loop output digital data to an analog I_COMP signal corresponding to the compensation current I_COMP; and

sample/hold circuitry synchronized with one of f_EXC or 2×

f_EXC, and configured to sample-and-hold the analog I_COMP signal such that transitions of the I_COMP compensation current are synchronized with fluxgate core saturation cycles.

6. The circuit of claim 4, wherein the feedback compensation circuitry comprises >

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a sigma delta DAC including;

noise shaping circuitry synchronized with f_FB equal to ((M/N)×

f_S), and configured to noise-shape the MDC loop output digital data to provide noise-shaped digital data;

DAC circuitry configured to convert the noise-shaped digital data to an analog DAC signal; and

a reconstruction filter synchronized with one of f_EXC and 2×

f_EXC, and configured to filter the analog DAC signal, and provide the compensation current I_COMP, such that transitions of the I_COMP compensation current are synchronized with fluxgate core saturation cycles.

7. The circuit of claim 4, wherein the feedback compensation circuitry comprises:

noise shaping circuitry synchronized with f_FB equal to ((M/N)×

f_S), and configured to noise-shape the MDC loop output digital data to output noise-shaped digital data;

FIR DAC circuitry, including

time delay circuitry configured to delay the noise-shaped digital data by a predetermined delay, and provide time-delayed digital data;

digital sample/hold circuitry synchronized with one of f_EXC and 2×

f_EXC, and configured to latch the time-delayed digital data as latched digital data; and

a predetermined number of DAC current sources, each gain-weighted by a predetermined FIR filter impulse response coefficient, and configured to convert, synchronous with one of f_EXC and 2×

f_EXC, the latched digital data into the I_COMP compensation current, such that transitions of the I_COMP compensation current are synchronized with fluxgate core saturation cycles.

8. The circuit of claim 7, wherein the oversampling frequency f_S is an integer multiple of f_EXC, and

wherein the FIR DAC circuitry is configured to FIR filter the noise-shaped digital data including notch filtering to suppress frequency components of f_EXC and at least one even harmonic of f_EXC.

9. The circuit of claim 7,

wherein the FIR DAC circuitry is folded according to the symmetrical impulse response to FIR filtering the noise-shaped digital data.

10. The circuit of claim 1, wherein the anti-aliasing circuitry and the oversampling data converter circuitry comprise a continuous time sigma-delta converter.

11. The circuit of claim 1, wherein the oversampling frequency is an integer multiple of f_EXC, and

wherein the digital loop filter is configured to filter the oversampled digital data including notch filtering to suppress frequency components of f_EXC and at least one even harmonic f_EXC.

12. The circuit of claim 1, further comprising

amplifier circuitry coupled to the input to the oversampling data converter circuitry, and configured to introduce a predetermined gain to reduce input referred quantization noise;

wherein, the oversampling data converter circuitry is configured to convert an analog sense signal that is band-limited by the anti-aliasing circuitry and amplified by the amplifier circuitry.

13. A fluxgate sensor system adapted to measure an external magnetic field B_EXT with a bandwidth f_B, comprising

at least one fluxgate magnetics element including a fluxgate core with an excitation coil, a compensation coil and a sense coil, and disposed such that the external field B_EXT magnetically couples into the fluxgate core;

drive circuitry configured to provide to the excitation coil an excitation current I_EXC with an excitation frequency f_EXC; and

a magnetic-to-digital (MDC) control loop, including the fluxgate magnetics element, configured to convert the external field B_EXT into representative sensor data, the MDC control loop including;

a forward path coupled to receive an analog sense signal output of the fluxgate sense coil induced by a sense field in the fluxgate core that corresponds to a difference between the external field B_EXT, and a compensation field B_COMP, and configured to provide MDC loop output digital data as the sensor data representative of the external B_EXT field including,

anti-aliasing circuitry configured to low-pass filter the analog sense signal to provide a band-limited analog sense signal;

oversampling ADC (analog-to-digital conversion) circuitry configured to convert the band-limited analog sense signal to corresponding oversampled digital data based on an oversampling frequency f_S greater than 2f_B; and

digital loop filter circuitry synchronized with f_S, and configured to filter the oversampled digital data to generate MDC loop output digital data corresponding to the sensor data representative of the external field B_EXT; and

a feedback path coupled to receive the MDC loop output digital data, and coupled to the fluxgate compensation coil, and including

feedback compensation circuitry, synchronized with a feedback path frequency f_FB equal to ((M/N)×

f_S), where, M and N are integers, and configured to generate, in response to the MDC loop output digital data, a compensation current I_COMP that is injected into the fluxgate compensation coil, to induce the compensation field B_COMP;

such that the induced compensation field B_COMP nulls the external field B_EXT;

so that the compensation current I_COMP corresponds to the sensor data representative of the external field B_EXT.

14. The system of claim 13, wherein the fluxgate sensor circuit is adapted to measure differential components B_DM of an external magnetic field B_EXT that also includes a stray field common-mode component B_CM, using two fluxgate magnetics elements including respective magnetic cores with respective excitation, sense and compensation coils, the fluxgate magnetics elements respectively disposed such that differential components B_DM of the external field B_EXT magnetically couple into the respective fluxgate cores, and:

wherein the drive circuitry is configured to provide to the excitation coils respective excitation currents I_EXC each at the excitation frequency f_EXC;

further comprising common mode field compensation circuitry coupled to receive analog sense outputs from respective sense coils, and configured to generate common-mode analog compensation currents I_COMP,CM that are injected into respective compensation coils to induce respective common-mode compensation fields B_COMP,CM to null the common-mode component B_CM of the external field B_EXT;

wherein the MDC control loop is configured to convert the differential components B_DM into representative sensor data, based on differential analog sense signal outputs of respective fluxgate sense coils induced by respective sense fields in respective fluxgate cores, each corresponding to a difference between a differential component B_DM of the external field B_EXT, and a respective compensation field B_COMP,DM;

wherein the feedback compensation circuitry is configured to generate respective differential compensation currents I_COMP,DM that are injected into respective compensation coils to induce respective compensation fields B_COMP,DM to null the corresponding differential component B_DM of the external field B_EXT.

15. The system of claim 13, configured to sense current, wherein the magnetic field B_EXT is induced in the at least one fluxgate magnetics element by a current.

16. The system of claim 13, wherein the feedback compensation circuitry is configured to generate the I_COMP compensation current synchronized with one of f_EXC and 2×

fEXC, such that transitions of the I_COMP compensation current are synchronized with fluxgate core saturation cycles.

17. The system of claim 16, wherein the feedback compensation circuitry comprises:

digital-to-analog conversion circuitry, synchronized with f_FB equal to ((M/N)×

f_S), and configured to convert the MDC loop output digital data to an analog I_COMP signal corresponding to the compensation current I_COMP; and

sample/hold circuitry synchronized with one of f_EXC or 2×

f_EXC, and configured to sample-and-hold the analog I_COMP signal such that transitions of the I_COMP compensation current are synchronized with fluxgate core saturation cycles.

18. The system of claim 16, wherein the feedback compensation circuitry comprises a sigma delta DAC including:

noise shaping circuitry synchronized with f_FB equal to ((M/N)×

f_S), and configured to noise-shape the MDC loop output digital data to provide noise-shaped digital data;

DAC circuitry configured to convert the noise-shaped digital data to an analog DAC signal; and

a reconstruction filter synchronized with one of f_EXC and 2×

f_EXC, and configured to filter the analog DAC signal, and provide the compensation current I_COMP, such that transitions of the I_COMP compensation current are synchronized with fluxgate core saturation cycles.

19. The system of claim 16, wherein the feedback compensation circuitry comprises:

noise shaping circuitry synchronized with f_FB equal to ((M/N)×

f_S), and configured to noise-shape the MDC loop output digital data to output noise-shaped digital data;

FIR DAC circuitry, including

time delay circuitry configured to delay the noise-shaped digital data by a predetermined delay, and provide time-delayed digital data;

digital sample/hold circuitry synchronized with one of f_EXC and 2×

f_EXC, and configured to latch the time-delayed digital data as latched digital data; and

a predetermined number of DAC current sources, each gain-weighted by a predetermined FIR filter impulse response coefficient, and configured to convert, synchronous with one of f_EXC and 2×

f_EXC, the latched digital data into the I_COMP compensation current, such that transitions of the I_COMP compensation current are synchronized with fluxgate core saturation cycles.

20. The system of claim 19, wherein the oversampling frequency f_S is an integer multiple of f_EXC, and

wherein the FIR DAC circuitry is configured to FIR filter the noise-shaped digital data including notch filtering to suppress frequency components of f_EXC and at least one even harmonic of f_EXC.

21. The system of claim 19,

wherein the FIR DAC circuitry is folded according to the symmetrical impulse response to FIR filtering the noise-shaped digital data.

22. The system of claim 13, wherein the anti-aliasing circuitry and the oversampling data converter circuitry comprise a continuous time sigma-delta converter.

23. The system of claim 13, wherein the oversampling frequency is an integer multiple of f_EXC, and

wherein the digital loop filter is configured to filter the oversampled digital data including notch filtering to suppress frequency components of f_EXC and at least one even harmonic f_EXC.

24. The system of claim 13, further comprising

amplifier circuitry coupled to the input to the oversampling data converter circuitry, and configured to introduce a predetermined gain to reduce input referred quantization noise;

wherein, the oversampling data converter circuitry is configured to convert an analog sense signal that is band-limited by the anti-aliasing circuitry and amplified by the amplifier circuitry.

25. A method of measuring an external magnetic field B_EXT with a bandwidth f_B, useable with a fluxgate sensor with at least one fluxgate magnetics element including a fluxgate core with an excitation coil, a compensation coil and a sense coil, where the fluxgate sensor is disposed such that the external field B_EXT magnetically couples into the fluxgate core, comprising:

driving the fluxgate excitation coil with an excitation current I_EXC with an excitation frequency f_EXC;

receiving from the fluxgate sense coil an analog sense signal induced by a sense field in the fluxgate core that corresponds to a difference between the external field B_EXT, and a compensation field B_COMP;

converting the analog sense signal into loop output digital data corresponding to sensor data representative of the external field B_EXT, including;

converting the analog sense signal to corresponding oversampled digital data using an oversampling data converter with an oversampling frequency f_S greater than 2f_B; and

loop filtering the oversampled digital data, synchronous with the oversampling frequency f_S, to generate the loop output digital data; and

converting the loop output digital data into a feedback compensation current I_COMP corresponding to the sensor data representative of the external field B_EXT, including

generating the feedback compensation current I_COMP from the loop output digital data, synchronous with a feedback path frequency f_FB equal to ((M/N)×

f_S), where, M and N are integers; and

injecting the feedback compensation current I_COMP into the fluxgate compensation coil to induce the compensation field B_COMP, such that the induced compensation field B_COMP nulls the external field B_EXT.

26. The method of claim 25,

further comprising band limiting the analog sense signal to prevent aliasing;

and

wherein the band-limited analog sense signal is converted to corresponding oversampled digital data.

27. The method of claim 25, adapted to measure differential components B_DM of an external magnetic field B_EXT that also includes a stray field common-mode component B_CM, using two fluxgate magnetics elements including respective magnetic cores with respective excitation, sense and compensation coils, wherein respective excitation coils are driven with respective excitation currents I_EXC each at the excitation frequency f_EXC, and wherein the fluxgate magnetics elements are respectively disposed such that differential components B_DM of the external field B_EXT magnetically couple into the respective fluxgate cores, and:

further comprising generating, from the analog sense outputs from respective sense coils, common-mode analog compensation currents I_COMP,CM that are injected into respective compensation coils to induce respective common-mode compensation fields B_COMP,CM to null the common-mode component B_CM of the external field B_EXT;

wherein the differential components B_DM are converted into representative sensor data, based on differential analog sense signal outputs of respective fluxgate sense coils induced by respective sense fields in respective fluxgate cores, each corresponding to a difference between a differential component B_DM of the external field B_EXT, and a respective compensation field B_COMP,DM; and

wherein the feedback compensation circuitry is configured to generate respective differential feedback compensation currents I_COMP,DM that are injected into respective compensation coils to induce respective compensation fields B_COMP,DM to null the corresponding differential component B_DM of the external field B_EXT.

28. The method of claim 25, adapted to sense current, wherein the magnetic field B_EXT is induced in the at least one fluxgate magnetics element by a current.

29. The method of claim 25, further comprising synchronizing the feedback I_COMP compensation current with one of f_EXC and 2×

fEXC, such that transitions of the feedback I_COMP compensation current are synchronized with fluxgate core saturation cycles.

30. The method of claim 25, wherein converting the loop output digital data into the feedback compensation current I_COMP is accomplished by:

noise-shaping the MDC loop output digital data, synchronized with f_FB equal to ((M/N)×

f_S), to provide noise-shaped digital data;

delaying the noise-shaped digital data by a predetermined delay, and providing time-delayed digital data;

performing a sample-and-hold operation, synchronized with one of f_EXC and 2×

f_EXC, to latch the time-delayed digital data as latched digital data; and

converting the latched digital data into the feedback I_COMP compensation current, synchronous with one of f_EXC and 2×

f_EXC, using a predetermined number of DAC current sources, each gain-weighted by a predetermined FIR filter impulse response coefficient, such that transitions of the I_COMP compensation current are synchronized with fluxgate core saturation cycles.

31. The method of claim 30, wherein the oversampling frequency f_S is an integer multiple of f_EXC, and further comprising at least one of:

(a) notch filtering the oversampled digital data to suppress frequency components of f_EXC and at least one even harmonic f_EXC, and (b) notch filtering the noise-shaped digital data to suppress frequency components of f_EXC and at least one even harmonic of f_EXC.

32. A fluxgate sensor circuit adapted to measure an external magnetic field B_EXT with a bandwidth f_B using at least one fluxgate magnetics element including a fluxgate core with an excitation coil, a compensation coil and a sense coil, and disposed such that the external field B_EXT magnetically couples into the fluxgate core, comprising:

drive circuitry configured to provide to the excitation coil an excitation current I_EXC with an excitation frequency f_EXC; and

a magnetic-to-digital (MDC) control loop, including the fluxgate magnetics element, configured to convert the external field B_EXT into representative sensor data, the MDC control loop including,

a forward path coupled to receive an analog sense signal output of the fluxgate sense coil induced by a sense field in the fluxgate core that corresponds to a difference between the external field B_EXT, and a compensation field B_COMP, and configured to provide MDC loop output digital data as the sensor data representative of the external B_EXT field, including;

ADC (analog-to-digital conversion) circuitry configured to convert, based on a sampling frequency f_S, one of the analog sense signal and an integrated analog sense signal, to corresponding digital ADC data; and

a forward path integration function, synchronized with fS, comprising one of;

(i) analog integration circuitry coupled to an input of the ADC circuitry, and configured to integrate the analog sense signal to provide the integrated analog sense signal, such that the ADC data corresponds to the MDC loop output digital data, and (ii) digital loop filter circuitry coupled to an output of the ADC circuitry and configured to integrate the ADC data, and provide the MDC loop output digital data; and

a feedback path coupled to receive the MDC loop output digital data, and coupled to the fluxgate compensation coil, including

feedback compensation circuitry configured to generate, in response to the MDC loop output digital data, a compensation current I_COMP that is injected into the fluxgate compensation coil to induce a compensation field B_COMP that nulls the external field B_EXT, so that the compensation current I_COMP corresponds to the sensor data representative of the external field B_EXT, the feedback compensation circuitry including;

noise shaping circuitry, synchronized with a feedback frequency f_FB equal to (M/N)×

f_S (M and N are integers), and configured to noise-shape the MDC output sensor digital data to provide noise-shaped digital data; and

FIR DAC circuitry, including

 

time delay circuitry configured to delay the noise-shaped digital data by a predetermined delay, and provide time-delayed digital data;

 

digital sample/hold circuitry synchronized with one of f_EXC and 2×

f_EXC, and configured to latch the time-delayed digital data as latched digital data; and

 

a predetermined number of DAC current sources, each gain-weighted by a predetermined FIR filter impulse response coefficient, and configured to convert, synchronous with one of f_EXC and 2×

f_EXC, the latched digital data into the I_COMP compensation current;

 

such that transitions of the I_COMP compensation current are synchronized with fluxgate core saturation cycles.