A weak magnetic field signal conditioning device

Abstract

This invention discloses a weak magnetic field signal conditioning device, comprising a high-frequency signal generation unit, a magnetoresistive full-bridge induction unit, an instrument differential amplifier unit, a secondary alternating amplifier unit, a phase-sensitive detector unit, and a filtering unit. The high-frequency signal generation unit provides a modulation signal to the magnetoresistive full-bridge induction unit and a reference signal of the same frequency and phase to the phase-sensitive detector unit. The magnetoresistive full-bridge induction unit adopts a spatially orthogonal layout, capturing a three-dimensional magnetic field vector and converting it into a differential electrical signal. The differential signal is amplified by two stages of low-noise amplification by the instrument differential amplifier unit and the secondary alternating amplifier unit, and then sent to the phase-sensitive detector unit for synchronous demodulation. Finally, the filtering unit outputs a DC or low-frequency signal reflecting the magnetic field strength. This invention, through a hardware-level modulation-demodulation architecture combined with the lock-in amplification principle, extracts nanosecond-level weak transient magnetic signals with a high signal-to-noise ratio under strong electromagnetic interference, achieving deep noise stripping and possessing advantages such as high measurement accuracy, strong adaptability, and stable reliability.

Description

Technical Field

[0001] This invention relates to the field of power equipment monitoring technology, specifically to a weak magnetic field signal conditioning device. Background Technology

[0002] High-voltage switchgear, as a critical node in the power grid, has an extremely complex internal environment. The nanosecond-level transient magnetic field signals generated by partial discharge due to insulation aging are extremely weak and easily overwhelmed by strong electromagnetic interference generated within the cabinet by load changes and switching operations. Traditional Hall effect sensors, due to insufficient sensitivity, struggle to capture such rapidly changing weak magnetic fields. Although tunneling magnetoresistive technology boasts ultra-high sensitivity and nanosecond-level response capabilities, capable of capturing signals with a resolution of 10 nT, insufficient anti-interference capability in its front-end signal conditioning circuits can still lead to severe distortion of weak signals during transmission to the processing unit, resulting in inaccurate fault feature extraction. Existing conditioning circuits often struggle to achieve high-fidelity signal extraction and analysis in strong noise environments. Summary of the Invention

[0003] The technical problem to be solved by the embodiments of the present invention is to provide a weak magnetic field signal conditioning device to accurately capture weak magnetic field signals in complex and strong electromagnetic interference environments, so as to provide reliable data support for early fault diagnosis of insulation degradation in switchgear.

[0004] To solve the above-mentioned technical problems, the present invention provides a weak magnetic field signal conditioning device, comprising a high-frequency signal generation unit, a magnetoresistive full-bridge induction unit, an instrument differential amplification unit, a secondary alternating amplification unit, a phase-sensitive detection unit, and a filtering unit, wherein: The output terminal of the high-frequency signal generating unit is connected to the excitation terminal of the magnetoresistive full-bridge induction unit and the reference signal input terminal of the phase-sensitive detector unit, respectively, to provide a modulation signal for the magnetoresistive full-bridge induction unit and a reference signal for the phase-sensitive detector unit. The magnetoresistive full-bridge induction unit is arranged in a spatial orthogonal layout, and its differential signal output terminal is connected to the input terminal of the instrument differential amplifier unit to capture the spatial three-dimensional magnetic field vector and convert it into a differential electrical signal. The output of the instrument differential amplifier unit is connected to the secondary alternating amplifier unit, which is used to pre-amplify the differential electrical signal output by the magnetoresistive full-bridge induction unit. The output of the secondary alternating amplification unit is connected to the signal input of the phase-sensitive detection unit, which is used to further amplify the pre-amplified signal to the amplitude required by the phase-sensitive detection. The output of the phase-sensitive detector unit is connected to the filter unit, which is used to synchronously demodulate the modulated signal and lock the useful signal. The filtering unit is used to output DC signals or low-frequency demodulated signals.

[0005] Preferably, the high-frequency signal generating unit is configured to generate a sinusoidal modulation signal, which also serves as a co-frequency and co-phase reference signal for the phase-sensitive detector unit.

[0006] Preferably, the magnetoresistive full-bridge induction unit includes multiple Wheatstone full-bridge structures composed of tunnel magnetoresistive elements. The multiple Wheatstone full-bridge structures are arranged in spatially orthogonal directions and are used to capture magnetic field components in the X, Y, and Z directions, respectively.

[0007] Preferably, each Wheatstone full-bridge structure includes a pair of symmetrically distributed detection units with opposite magnetic sensitivity directions to suppress common-mode magnetic field interference.

[0008] Preferably, the instrument differential amplifier unit is a preamplifier with high common-mode rejection ratio and low noise characteristics, and its gain is set by adjusting the external resistor to amplify the millivolt-level differential signal output by the magnetoresistive full-bridge induction unit with low noise.

[0009] Preferably, the secondary alternating amplifier unit is a differential proportional amplifier circuit composed of operational amplifiers, used to amplify the signal output by the instrument differential amplifier unit a second time, and suppress common-mode interference through its differential structure.

[0010] Preferably, the phase-sensitive detector unit is a synchronous demodulator, used to multiply the modulation signal output by the secondary alternating amplifier unit with the reference signal from the high-frequency signal generator unit, convert the magnetic signal to be measured to the baseband, and shift the noise to the high-frequency band.

[0011] Preferably, the filtering unit is a second-order Butterworth low-pass active filter, whose cutoff frequency is set to allow the low-frequency envelope component that reflects the change in magnetic field strength to pass through, and to filter out the high-frequency components of second harmonic and above generated after demodulation.

[0012] Preferably, the output of the filtering unit is adapted to be connected to an analog-to-digital converter to realize the analog-to-digital conversion of the demodulated signal.

[0013] Preferably, the working mechanism of the device is modulation-demodulation, specifically including: the high-frequency signal generation unit sends the modulation signal to the magnetoresistive full-bridge induction unit to modulate the weak magnetic field signal onto the high-frequency carrier; the instrument differential amplification unit and the secondary alternating amplification unit perform two-stage low-noise amplification on the carrier-modulated differential signal; the phase-sensitive detection unit uses a reference signal with the same frequency and phase to synchronously demodulate the amplified modulation signal; and the filtering unit performs low-pass filtering on the demodulated signal to extract the low-frequency envelope signal reflecting the magnetic field strength.

[0014] The present invention offers the following advantages: Through a hardware-level modulation-demodulation architecture, combined with phase-sensitive detection and lock-in amplification principles, weak magnetic field signals are modulated to a 10kHz high-frequency band for transmission and amplification. The useful signal is then restored via synchronous demodulation, thereby accurately capturing nanosecond-level weak transient magnetic signals that are difficult for traditional sensors to detect under strong electromagnetic interference conditions, significantly improving the high signal-to-noise ratio extraction capability in strong interference environments. Simultaneously, a second-order Butterworth low-pass filter circuit is employed, utilizing its smooth passband characteristics and precise cutoff frequency design to effectively filter out high-frequency clutter and significantly reduce nonlinear interference noise generated by reflection and diffraction from metal cabinets. Combined with the differential anti-interference structure of the magnetoresistive full-bridge and the high common-mode rejection ratio of the instrumentation amplifier, multiple components work together to achieve noise reduction. Deep stripping; based on this, it fully integrates the high-resolution advantage of the 10nT level of the tunnel magnetoresistive sensor, and through a three-stage precise amplification link consisting of tunnel magnetoresistive full-bridge conversion, 40x pre-amplification, and 20x secondary amplification, combined with precise modulation and demodulation and efficient filtering, ensures minimal distortion in signal transmission and processing, thereby achieving high-fidelity capture of early characteristics of insulation degradation inside the switchgear, improving fault location accuracy and fault feature identification accuracy; in addition, the device adopts a modular design, and each functional unit uses high-performance dedicated chips, which are stable in performance, have high parameter matching degree, support long-term stable operation, and can be seamlessly adapted to the three-axis tunnel magnetoresistive sensing system, accurately capturing and analyzing the three-dimensional spatial magnetic field vector, providing a reliable data foundation for subsequent fault location and identification. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is a schematic diagram of the functional units of a weak magnetic field signal conditioning device according to an embodiment of the present invention.

[0017] Figure 2 This is a circuit diagram of the high-frequency signal generation unit in an embodiment of the present invention.

[0018] Figure 3 This is a circuit diagram of the instrument differential amplifier unit in an embodiment of the present invention.

[0019] Figure 4 This is a circuit diagram of the secondary alternating amplifier unit in an embodiment of the present invention.

[0020] Figure 5 This is a circuit diagram of the phase-sensitive detection unit in an embodiment of the present invention.

[0021] Figure 6 This is a circuit diagram of the filtering unit in an embodiment of the present invention. Detailed Implementation

[0022] The following descriptions of the embodiments are taken with reference to the accompanying drawings, illustrating specific embodiments in which the present invention can be implemented. These embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention.

[0023] Please refer to Figure 1 As shown, this embodiment of the invention provides a weak magnetic field signal conditioning device, including a high-frequency signal generation unit, a magnetoresistive full-bridge induction unit, an instrument differential amplifier unit, a secondary alternating amplifier unit, a phase-sensitive detector unit, and a filtering unit, wherein: The output terminal of the high-frequency signal generating unit is connected to the excitation terminal of the magnetoresistive full-bridge induction unit and the reference signal input terminal of the phase-sensitive detector unit, respectively, to provide a modulation signal for the magnetoresistive full-bridge induction unit and a reference signal for the phase-sensitive detector unit. The magnetoresistive full-bridge induction unit is arranged in a spatial orthogonal layout, and its differential signal output terminal is connected to the input terminal of the instrument differential amplifier unit to capture the spatial three-dimensional magnetic field vector and convert it into a differential electrical signal. The output of the instrument differential amplifier unit is connected to the secondary alternating amplifier unit, which is used to pre-amplify the differential electrical signal output by the magnetoresistive full-bridge induction unit. The output of the secondary alternating amplification unit is connected to the signal input of the phase-sensitive detection unit, which is used to further amplify the pre-amplified signal to the amplitude required by the phase-sensitive detection. The output of the phase-sensitive detector unit is connected to the filter unit, which is used to synchronously demodulate the modulated signal and lock the useful signal. The filtering unit is used to output DC signals or low-frequency demodulated signals.

[0024] As can be seen from the above settings, the weak magnetic field signal conditioning device provided in this embodiment of the invention adopts a modulation-demodulation mechanism, and achieves high-fidelity extraction and processing of weak magnetic field signals through the coordinated work of multiple functional units.

[0025] As an example, such as Figure 2As shown in this embodiment of the invention, the high-frequency signal generation unit uses the MAX038 signal generation chip. By precisely configuring the 25kΩ resistor between the REF and IIN pins and the 10nF capacitor on the COSC pin, a sinusoidal modulation signal with a frequency of 10kHz, an amplitude of 1V, and a duty cycle of 50% is generated. This frequency avoids the 50Hz frequency band and its harmonic interference commonly found in switch cabinets, providing stable excitation for the magnetoresistive full-bridge induction unit and serving as a synchronous and in-phase reference signal for the phase-sensitive detector unit. The MAX038 chip has advantages such as high frequency accuracy, low output waveform distortion, and adjustable duty cycle. Its output signal frequency can be finely adjusted through the IIN pin current, COSC pin capacitor, and FADJ pin voltage, ensuring the stability and reliability of the modulation signal. Specifically, pin A0 is grounded, and pin A1 is connected to a high voltage to ensure a sine wave output. The output frequency is precisely set to 10kHz using a 25kΩ resistor between pins REF and IIN and a 10nF capacitor on pin COSC. The duty cycle is controlled by the voltage on pin DADJ; in this embodiment, the duty cycle is adjusted to 50%, resulting in a standard sine wave with an amplitude of 1V. In actual testing, the output signal amplitude of this unit can be stabilized at around 1.02V, and the frequency at 10.01kHz.

[0026] The magnetoresistive full-bridge induction unit adopts a spatial orthogonal layout design. Its differential signal output is connected to the input of the instrument's differential amplifier unit, enabling the capture and conversion of the three-dimensional spatial magnetic field vector into a differential electrical signal. The magnetoresistive full-bridge induction unit consists of a Wheatstone full-bridge structure composed of four TMR (Tunnel Magneto Resistance) elements. Based on the magnetoresistive effect, an external magnetic field alters the magnetization direction of the free layer of the TMR elements, thereby adjusting the electron tunneling probability and causing the resistance of paired elements to change in opposite directions. This results in a differential voltage signal with a good linear relationship to the magnetic field strength. To achieve three-dimensional spatial magnetic field measurement, TMR elements with different sensitive directions are arranged in a spatial orthogonal layout to capture the magnetic field components in the X, Y, and Z directions respectively. The spatial magnetic field distribution characteristics are accurately reconstructed through the collaborative work of multiple units. Simultaneously, a differential anti-interference design is employed, with a pair of symmetrically distributed detection units with opposite magnetic sensitive directions, effectively suppressing common-mode magnetic field interference and improving the signal-to-noise ratio.

[0027] As an example, such as Figure 3As shown in this embodiment of the invention, the instrument differential amplifier unit uses the AD623 low-noise instrumentation amplifier manufactured by Analog Devices (ADI) as the preamplifier. This chip features low noise, small size, low power consumption, and high common-mode rejection ratio. Furthermore, its gain can be adjusted using only an external variable resistor, with a gain range of 1 to 1000 times and a maximum error of ±0.35%. Its input voltage noise is 19nV / Hz at 1kHz and ≤130nV / Hz in the 0.1Hz~10Hz frequency band, effectively suppressing the common-mode noise of the TMR bridge output. In this embodiment, an external resistor RP3 (10KΩ) is connected to R... g 3 (4.5KΩ) in parallel, according to the gain formula G=1+100KΩ / R g The calculated amplification factor is approximately 40 times. RP6 (10KΩ) is the zero-adjustment resistor. Adjusting this resistor when there is no signal input will make the output signal zero, avoiding zero-point drift that could affect measurement accuracy. With this setting, the weak millivolt-level differential signal output by the TMR element can be amplified to the hundreds of millivolt level without signal distortion.

[0028] As an example, such as Figure 4 As shown in this embodiment of the invention, the secondary alternating amplifier unit uses an OPA27 operational amplifier to form a differential proportional amplifier circuit. This operational amplifier has characteristics such as low noise, high gain, and high linearity, making it suitable for further amplifying weak signals. The secondary alternating amplifier unit is powered by ±12V. R9 (2KΩ) is the feedback resistor, R13 (100Ω) is the input resistor, and R16 is the matching resistor (100Ω). According to the differential proportional amplification formula, the amplification factor A = -R9 / R13. The amplification factor is set to 20 times, further boosting the pre-amplified signal to a range sufficient to drive the demodulator and meet the signal amplitude requirements of subsequent phase-sensitive detection. Through the differential proportional amplification structure, common-mode interference is effectively suppressed, ensuring the stability and accuracy of signal amplification.

[0029] As an example, such as Figure 5As shown in the embodiment of the invention, the phase-sensitive detection unit uses the AD630 precision equalizer / demodulator from Analog Devices (ADI). This chip operates on ±5V power and achieves synchronous demodulation through multiplication operations, outputting a signal containing a DC component and a second harmonic component, enabling precise synchronous demodulation of the modulated signal. Its working principle involves multiplying the amplified modulated signal with a reference signal from the MAX038. Only the magnetic signal to be measured, which has the same frequency and phase as the reference signal, is converted to baseband, while random noise is shifted to a higher frequency band, effectively suppressing common-mode interference and locking onto the useful signal. The high linearity and stability of the AD630 chip ensure the accuracy of the demodulation process, laying a solid foundation for subsequent signal processing. It can be understood that the locked useful signal refers to the in-phase modulated signal formed by modulating the weak magnetic field generated by partial discharge within the high-voltage switchgear, captured by the magnetoresistive full-bridge induction unit. This signal contains characteristic information of the magnetic field strength to be detected, providing an effective feature source for fault diagnosis.

[0030] The filtering unit, as the final output, outputs a clean DC signal or a low-frequency demodulated signal, providing a high-quality signal for subsequent analog-to-digital conversion and data processing. It should be noted that a clean DC signal or low-frequency demodulated signal refers to a DC level signal or low-frequency envelope signal that, after synchronous demodulation by the phase-sensitive detector unit and filtering out high-frequency noise of two harmonics and above by the filtering unit, retains only the DC level signal or low-frequency envelope signal reflecting changes in magnetic field strength and free from high-frequency noise and spurious interference. As an example, such as... Figure 6 As shown, the filtering unit is specifically a second-order Butterworth low-pass filter circuit, using an LM833 operational amplifier to form an active filter. The LM833 has advantages such as low noise, high gain-bandwidth product, and large output swing, making it suitable for high-precision filtering circuits. This filter has a smooth passband and a moderate transition band drop-off speed, effectively filtering out high-frequency noise while ensuring good response characteristics of the output signal. The LM833 uses a ±12V power supply, and the passband voltage amplification factor A = R34 / R31. In this embodiment, R34 = R31 = 113kΩ, and R32 is 56kΩ, so the passband amplification factor is 1. Based on the cutoff frequency formula, C32 = 0.05uF and C1 = 0.2uF are selected to ensure a cutoff frequency of 20Hz, effectively filtering out high-frequency components of the second harmonic and above generated by demodulation, extracting the low-frequency envelope reflecting changes in magnetic field strength, and achieving effective background noise removal.

[0031] Furthermore, the output of the high-frequency signal generation unit is connected in two ways: one to the excitation terminal of the magnetoresistive full-bridge induction unit, providing a 10kHz sinusoidal excitation signal; the other is connected to the reference signal input terminal (SIN+ pin) of the phase-sensitive detector unit, providing a demodulation reference signal with the same frequency and phase. In the magnetoresistive full-bridge induction unit, the differential signal output of each full bridge is connected to the input terminal (IN+, IN- pins) of an instrument differential amplifier unit, realizing the conversion and output of the magnetic field signal to an electrical signal. The input terminal of the instrument differential amplifier unit receives the differential signal output from the magnetoresistive full-bridge induction unit, amplifies it by 40 times with low noise, and then connects it to the input terminal (+IN, -IN pins) of the secondary alternating amplifier unit through its output terminal (OUT pin). The secondary alternating amplifier unit receives the signal amplified by the instrument differential amplifier unit, amplifies it by 20 times, and then connects it to the signal input terminal of the phase-sensitive detector unit through its output terminal (OUT pin). The phase-sensitive detector unit receives the modulated signal amplified by the secondary alternating amplifier unit at its signal input terminal, and the reference signal input terminal receives the reference signal output by the high-frequency signal generator unit. After synchronous demodulation, the output terminal is connected to the input terminals (+IN, -IN pins) of the second-order Butterworth low-pass filter circuit. The second-order Butterworth low-pass filter circuit outputs a clean DC signal or a low-frequency demodulated signal, which can be directly connected to an ADC chip (such as AD7608) for analog-to-digital conversion, providing signal support for subsequent data processing and fault analysis.

[0032] The working mechanism of this invention is modulation-demodulation, and the specific workflow is as follows: Signal Carrierization: The high-frequency signal generation unit (MAX038) generates a 10kHz high-frequency sinusoidal modulated signal, which is sent to the TMR magnetoresistive full-bridge induction unit as an excitation signal. Under this excitation, the TMR full-bridge modulates the static or low-frequency weak magnetic field signal (generated by partial discharge) in the environment onto the 10kHz carrier, utilizing the low noise characteristic of the high-frequency band for transmission, avoiding the signal being overwhelmed by strong low-frequency interference during transmission. Simultaneously, the TMR full-bridge captures the three-dimensional spatial magnetic field components through three spatially orthogonally arranged axial units, and outputs the corresponding differential modulated signals.

[0033] Differential Extraction and Secondary Amplification: The weak differentially modulated signal (millivolt level) output from the TMR magnetoresistive full-bridge induction unit first enters the instrument differential amplifier unit (AD623). This unit, with its high common-mode rejection ratio and low noise characteristics, suppresses common-mode interference while amplifying the signal 40 times, converting it to a hundred-millivolt level signal. The signal then enters the secondary alternating amplifier unit (OPA27), where it is further amplified 20 times, increasing the signal amplitude to the 10V level, meeting the driving requirements of the phase-sensitive detector unit. Both secondary amplification processes employ low-noise, high-stability amplifier chips and circuit structures to ensure undistorted signals while preserving fault characteristic information to the maximum extent possible.

[0034] Phase-sensitive demodulation: The phase-sensitive detector unit (AD630) receives the modulated signal after secondary amplification, and simultaneously receives a reference signal (from MAX038) that is in phase and at the same frequency as the modulated signal. Internally, the chip uses the reference signal as a reference for synchronous switching and performs multiplication demodulation on the modulated signal. Since the magnetic signal under test is in phase and at the same frequency as the reference signal, the demodulated signal is converted into a signal containing a DC component (reflecting the magnetic field strength) and a second harmonic component. Random noise and interference signals of different frequencies are shifted to higher frequencies, achieving initial separation of the useful signal from the interference signal.

[0035] Low-pass extraction: The demodulated signal enters a second-order Butterworth low-pass filter circuit (LM833). This circuit, with a cutoff frequency set to 20Hz, completely filters out all high-frequency interference components at twice the frequency (20kHz) and above, retaining only the low-frequency envelope and DC component reflecting changes in magnetic field strength. The final output DC level shows a good linear relationship with the magnetic field strength sensed by the TMR sensor, achieving the goal of retrieving extremely weak magnetic signals from a noisy environment. This provides a high-quality signal source for subsequent analog-to-digital conversion, data analysis, and fault diagnosis.

[0036] As can be seen from the above description, compared with the prior art, the beneficial effects of the present invention are as follows: The present invention uses a hardware-level modulation-demodulation architecture, combined with phase-sensitive detection and lock-in amplification principles, to modulate weak magnetic field signals to a 10kHz high-frequency band for transmission and amplification, and then restores the useful signal through synchronous demodulation. This allows for the accurate capture of nanosecond-level weak transient magnetic signals that are difficult for traditional sensors to identify under strong electromagnetic interference, significantly improving the high signal-to-noise ratio extraction capability under strong interference environments. Simultaneously, the use of a second-order Butterworth low-pass filter circuit, utilizing its smooth passband characteristics and precise cutoff frequency design, not only effectively filters out high-frequency noise but also significantly reduces nonlinear interference noise generated by reflection and diffraction from the metal cabinet. Combined with the differential anti-interference structure of the magnetoresistive full-bridge and the high common-mode rejection ratio of the instrumentation amplifier, multiple stages work together... It achieves deep noise stripping; based on this, it fully integrates the high-resolution advantage of the 10nT level tunnel magnetoresistive sensor, and through a three-stage precise amplification link consisting of tunnel magnetoresistive full-bridge conversion, 40x pre-amplification, and 20x secondary amplification, combined with precise modulation and demodulation and efficient filtering, it ensures minimal distortion during signal transmission and processing, thereby achieving high-fidelity capture of early characteristics of insulation degradation inside the switchgear, improving fault location accuracy and fault feature identification accuracy; in addition, the device adopts a modular design, and each functional unit uses high-performance dedicated chips, which are stable in performance, have high parameter matching degree, support long-term stable operation, and can be seamlessly adapted to the three-axis tunnel magnetoresistive sensing system, accurately capturing and analyzing three-dimensional spatial magnetic field vectors, providing a reliable data foundation for subsequent fault location and identification.

[0037] The above description is merely a preferred embodiment of the present invention and should not be construed as limiting the scope of the invention. Therefore, any equivalent variations made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A weak magnetic field signal conditioning device, characterized in that, It includes a high-frequency signal generation unit, a magnetoresistive full-bridge induction unit, an instrument differential amplifier unit, a secondary alternating amplifier unit, a phase-sensitive detector unit, and a filter unit, wherein: The output terminal of the high-frequency signal generating unit is connected to the excitation terminal of the magnetoresistive full-bridge induction unit and the reference signal input terminal of the phase-sensitive detector unit, respectively, to provide a modulation signal for the magnetoresistive full-bridge induction unit and a reference signal for the phase-sensitive detector unit. The magnetoresistive full-bridge induction unit is arranged in a spatial orthogonal layout, and its differential signal output terminal is connected to the input terminal of the instrument differential amplifier unit to capture the spatial three-dimensional magnetic field vector and convert it into a differential electrical signal. The output of the instrument differential amplifier unit is connected to the secondary alternating amplifier unit, which is used to pre-amplify the differential electrical signal output by the magnetoresistive full-bridge induction unit. The output of the secondary alternating amplification unit is connected to the signal input of the phase-sensitive detection unit, which is used to further amplify the pre-amplified signal to the amplitude required by the phase-sensitive detection. The output of the phase-sensitive detector unit is connected to the filter unit, which is used to synchronously demodulate the modulated signal and lock the useful signal. The filtering unit is used to output DC signals or low-frequency demodulated signals.

2. The apparatus according to claim 1, characterized in that, The high-frequency signal generating unit is configured to generate a sinusoidal modulation signal, which also serves as a co-frequency and co-phase reference signal for the phase-sensitive detector unit.

3. The apparatus according to claim 1, characterized in that, The magnetoresistive full-bridge induction unit includes multiple Wheatstone full-bridge structures composed of tunnel magnetoresistive elements. These multiple Wheatstone full-bridge structures are arranged in spatially orthogonal directions and are used to capture magnetic field components in the X, Y, and Z directions, respectively.

4. The apparatus according to claim 3, characterized in that, Each Wheatstone full-bridge structure contains a pair of symmetrically distributed detectors with opposite magnetic sensitivity directions, used to suppress common-mode magnetic field interference.

5. The apparatus according to claim 1, characterized in that, The instrument differential amplifier unit is a preamplifier with high common-mode rejection ratio and low noise characteristics. Its gain is set by adjusting the external resistor and is used to amplify the millivolt-level differential signal output by the magnetoresistive full-bridge induction unit with low noise.

6. The apparatus according to claim 1, characterized in that, The secondary alternating amplifier unit is a differential proportional amplifier circuit composed of operational amplifiers, used to amplify the signal output by the instrument differential amplifier unit a second time, and to suppress common-mode interference through its differential structure.

7. The apparatus according to claim 1, characterized in that, The phase-sensitive detector unit is a synchronous demodulator, used to multiply the modulation signal output by the secondary alternating amplifier unit with the reference signal from the high-frequency signal generator unit, convert the magnetic signal under test to the baseband, and shift the noise to the high-frequency band.

8. The apparatus according to claim 1, characterized in that, The filtering unit is a second-order Butterworth low-pass active filter, whose cutoff frequency is set to allow the low-frequency envelope component that reflects the change in magnetic field strength to pass through, and to filter out the high-frequency components of the second harmonic and above generated after demodulation.

9. The apparatus according to claim 1, characterized in that, The output of the filtering unit is adapted to connect to the analog-to-digital converter unit to realize the analog-to-digital conversion of the demodulated signal.

10. The apparatus according to any one of claims 1-9, characterized in that, Its working mechanism is modulation-demodulation, specifically including: the high-frequency signal generation unit sends the modulation signal to the magnetoresistive full-bridge induction unit, modulating the weak magnetic field signal onto the high-frequency carrier; the instrument differential amplifier unit and the secondary alternating amplifier unit perform two-stage low-noise amplification on the carrier-modulated differential signal; the phase-sensitive detector unit uses a reference signal with the same frequency and phase to synchronously demodulate the amplified modulation signal; the filtering unit performs low-pass filtering on the demodulated signal to extract the low-frequency envelope signal reflecting the magnetic field strength.

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