Towards fully digital magnetometer

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  • Sensors and Actuators A 106 (2003) 3437

    Towards fully digital magnetometerA. Cerman, P. Ripka

    Department of Measurement, Faculty of Electrical Engineering, Czech Technical University, Technicka 2, 166 27 Prague 6, Czech Republic

    Abstract

    New topology of digital fluxgate magnetometer is proposed. The main target is to achieve high dynamic range by using fast analog-to-digital converter (ADC) for the detection and precise deltasigma () digital-to-analog converter (DAC) in the feedback. First resultsshow that the electronics can match 20-bit (120 dB) precisions of the sensors. 2003 Elsevier B.V. All rights reserved.

    Keywords: Fluxgate magnetometer; Digital signal processing; Intelligent sensor systems

    1. Introduction

    Fluxgate sensors are known as the most sensitivevectorial magnetic field sensors working at room tem-perature [1]. Only the SQUID magnetometers are moresensitive, but the demand of liquid helium or nitrogen (forhigh-temperature SQUID) and limited dynamic range makethem unusable for satellite and portable applications. A newgeneration of anisotropic magnetoresistive sensors (AMRs)can obtain comparable sensitivity with fluxgates, but theirtemperature and long-time instabilities degrades them forlow-performance systems only [2].

    Progress in the electronic technologies allows theminiaturization of the fluxgate sensors to the size properfor on-chip integration [3,4]. The sensitivity of themicro-fluxgate sensors is reduced compared to standard-sizefluxgate sensors and their dynamic range is also smaller,but high-quality interface is still necessary to match thesensor performance. Furthermore, the sensitivity of themicro-fluxgate sensors can be increased by the increasingof excitation frequency of the sensor, which, moreover,brings the demand of higher speed of the interface signalprocessing. A digital signal processing of the sensor outputsignal seems to be better candidate for on-chip integratedinterface of the micro-sensors than standardly used analogsignal processing for its relatively simple implementationand possibility of the change of system parameters, whichallows simple system adapting for different measuringranges or different conditions of the measurement.

    Corresponding author. Tel.: +420-2-2435-3945;fax: +420-2-3333-9929.E-mail address: ripka@feld.cvut.cz (P. Ripka).

    2. Digitalization of the fluxgate magnetometers

    The digital signal processing of the fluxgate sensor out-put directly comes out from the standardly used analogsignal processing. The block diagram of a one-channel ana-log fluxgate magnetometer is shown in Fig. 1. The sensoris excited by the excitation current generated by the exci-tation circuitry. Output signal of the sensor is filtered andpre-amplified. The high-pass filter suppresses frequencieslower than second-order harmonic and also the dc compo-nent (in case that the sensing coil is used also as a feed-back coil) to prevent overloading of the pre-amplifier. It isgenerally known, that the signal, which is proportional tothe measured magnetic field, is modulated on all even-orderharmonics and its largest amplitude is usually held on thesecond-order harmonic. Therefore, the phase-sensitive de-tector is mostly tuned to the second-order harmonic. Thefunction of the integrator is an amplification to assure enoughlarge feedback loop gain and a stabilization of the negativefeedback system. The output voltage of the integrator is con-verted into a feedback current, which is fed to the separatedfeedback coil or to the sensing coil. The measured mag-netic field is in frequency range up to several tens of Hz andthe excitation frequency of the fluxgate sensors is several ofkHz for standard-size sensors or even hundreds of kHz formicro-fluxgate sensors. Therefore, the sensing coil can beused for both mentioned functions. The using of feedbacktechnique dramatically suppresses the sensor non-linearityand hysteresisin fact, sensor works as a zero indicator.

    At the present time, we can recognize two basic waysin an attempt to digitize the fluxgate magnetometers. Thefirst of them is using a deltasigma () modulation inthe negative feedback loop and the second is a fully digital

    0924-4247/$ see front matter 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0924-4247(03)00128-6

  • A. Cerman, P. Ripka / Sensors and Actuators A 106 (2003) 3437 35

    Fig. 1. Analog fluxgate magnetometer.

    signal processing of the sensor output signal. Both of themwill be described in Sections 3 and 4.

    3. Using of the deltasigma modulation

    The block diagram of the digital magnetometer using modulation is shown in Fig. 2. Two additional blocksare added to the basic diagram modulator and de-modulator of the feedback signal.

    The output signal of the integrator is modulated by the modulator. Its output bit-stream is the output signalfrom the interface. The modulated signal is demodulatedback to the feedback signal by using 1-bit digital-to-analogconverter (DAC). The demodulated signal can be used di-rectly as a feedback signal (pulse feedback) or can be con-sequentially filtered by the analog low-pass filter (analogfeedback). The main feature of this principle is effectivesuppressing of the quantization noise [5], which allows toachieve high dynamic range. digital magnetometer using pulse feedback was pre-

    sented in [6]. Unfortunately, the pulse negative feedbacksignal cannot meet the zero input condition of the fluxgatesensor. Thus, the resulting parameters of this system are lim-ited by the insufficient balance of the feedback loop.

    The magnetometer using analog compensation inthe feedback is published in [7]. Although the second-ordercustom-made modulator is used, the third-order noise

    Fig. 2. Magnetometer with modulation.

    Fig. 3. PSD of the magnetometer from [7].

    shaping of the quantization noise is obtained. This effectis caused by the analog integrator in the negative feedbackloop. The resulting parameters of this system are limited bythe parameters of the sensor, not by the parameters of theinterface. The output power spectrum density is shown inFig. 3 [7]. A sharp peak presents the measured magneticfield with frequency 25.4 Hz and amplitude of100T. Thelevel of noise up to 1 kHz is 1/f (Barkhausen) noise, which istypical for all types of fluxgate sensors. The resulting param-eters of this magnetometer are following: measuring range,100T; noise level, 120 nT pp; noise PSD1/2, 125 nT/Hzat the rate of 1 Hz; non-linearity,

  • 36 A. Cerman, P. Ripka / Sensors and Actuators A 106 (2003) 3437

    Fig. 4. Fully digital fluxgate magnetometer.

    5. Our fully digital fluxgate magnetometer

    Design of our fully digital fluxgate magnetometer directlycomes out from the basic architecture and its block dia-gram is shown in Fig. 5. The magnetometer consists of aninput analog pre-processing unit (high-pass filter and ana-log pre-amplifier), a 16-bit high-speed precise A/D con-verter, a 16-bit digital signal processor, a high-linear precise

    Fig. 5. Our design of the fully digital magnetometer.

    20-bit D/A converter, an output analog filter/current driver,a host-processor and an excitation circuitry.

    The output signal of the sensor is pre-amplified andits amplitude is adapted to the input range of the A/Dconverter. The converter is high-speed precise SAR con-verter AD7664 with maximal nominal sampling frequencyof 570 kHz. The sampling frequency of the converter isdriven from the DSP and it can be set in the range from32 to 480 kSPS with step of 32 kSPS. Output data from theanalog-to-digital converter (ADC) are given to the DSP.DSP is 16-bit fix-point ADSP2181. Program of the DSP isstored in the external boot memory and after switching-onthe system is down-loaded to the DSP. All signal process-ing necessary for the extraction of the signal proportionalto the measured magnetic field is executed in the DSP (fil-tering, phase-sensitive detection, integration). Furthermore,additional function, as a, for example, sensor non-idealitiescompensation, can be implemented. Resulting data from thedigital detection are fed to the host-processor (AT89C4051).This co-processor has two functionssending data to theD/A converter and also communication with personal com-puter (PC). Program of the host-processor is stored in itsinternal FLASH memory. Because frequency range of the

  • A. Cerman, P. Ripka / Sensors and Actuators A 106 (2003) 3437 37

    measured magnetic field is in range of several tens of Hz,the delay caused by double-sending of the data (from DSPto host-processor and from host-processor to DAC) is neg-ligible. The D/A converter is based on modulationprinciple (DAC1220). Its nominal resolution is 20 bits. Anover-sampling frequency of the converter is derived fromthe DSP main frequency. Output analog signal from theconverter is filtered by an analog second-order low-passfilter and converted into a feedback current.

    The magnetometer also includes an excitation circuitry.The excitation frequency of the sensor is 1 kHz and it isderived from the sampling frequency of the ADC by usingof a programmable frequency divider. It means that if thesampling of the ADC is changed, the DSP also changes thedividing ratio and therefore the excitation frequency is still1 kHz. This function is used for preserving of an integersampling frequency to excitation frequency ratio. It is im-portant for suppressing of the spectrum leakage of the dig-ital detected signal. The picture of digital magnetometer isshown in Fig. 6.

    Fig. 6. Fully digital fluxgate magnetometer.

    6. Conclusion

    The effort of digitalization of the fluxgate magnetome-ter is summarized in this paper. The presented fully digitalfluxgate magnetometer has been designed and tested. At thepresent time, the hardware of the magnetometer was fin-ished and work is continuing on the implementation of thesoftware for DSP and host-processor and on finding of theproper algorithms necessary for the magnetic field detec-tion.

    As described earlier, one from the most serious prob-lems of the fully digital fluxgate magnetometer is theDAC non-linearity. Thus, in parallel with the presentedwork, we have also designed the custom-made high-linear18-bit D/A converter based on pulse-width modulation(PWM) principle. The results from both projects will becompared.

    References

    [1] P. Ripka, et al., Magnetic Sensors and Magnetometers, Artech,Boston, 2001.

    [2] P. Ripka, M. Vopalensky, A. Platil, M. Doscher, K.-M.H. Lenssen,AMR magnetometer, in: Book of Abstracts, vol. 15, Proceedingsof the Soft Magnetic Materials Conference, Bilbao, 254255, 2003,pp. 639641.

    [3] P. Ripka, S.O. Choi, A. Tipek, S. Kawahito, M. Ishida, Symmet-rical core improves micro-fluxgate sensors, in: Book of Abstracts,Proceedings of the Eurosensors 2000, Copenhagen, pp. 499500,899903.

    [4] L. Chiesi, P. Kejik, B. Janossy, R.S. Popovic, CMOS planar 2Dmicro-fluxgate sensor, Sens. Actuators A: Phys. 82 (130) (2000)174180.

    [5] S.R. Norswofthy, R. Schreier, G.C. Temes (Eds.), DeltaSigma DataConverters, IEEE Press, Piscataway, NJ, 1997.

    [6] S. Kawahito, Ch. Maier, M. Schneider, M. Zimmermann, H. Baltes,A 2-D CMOS micro-fluxgate sensor system for digital detection ofweak magnetic fields, IEEE J. Solid-State Circuits 34 (12) (1999)18431851.

    [7] S. Kawahito, A. Cerman, K. Aramaki, Y. Tadokoro, A weak magneticfield measurement system with micro-fluxgate sensor and deltasigmainterface, IEEE Trans. Instrum. Meas. 52 (February (1)) (2003) 103110.

    [8] A. Cerman, A. Tipek, P. Ripka, Magnetometer for new Czech satelliteMIMOSA, in: Proceedings of the Conference on Applied Electronics,Pilsen, 2000, pp. 4246.

    [9] H. Auster, et al., Concept and first results of a digital fluxgatemagnetometer, Meas. Sci. Technol. 6 (1995) 477481.

    [10] E.B. Pedersen, et al., Digital fluxgate magnetometer for the Astrid-2satellite, Meas. Sci. Technol. 10 (1999) N124N129.

    Towards fully digital magnetometerIntroductionDigitalization of the fluxgate magnetometersUsing of the delta-sigma modulationFully digital fluxgate magnetometerOur fully digital fluxgate magnetometerConclusionReferences