Noise performance versus hardware implementation in high sensitivity digital GMI sensors

International audienceIn the Giant Magneto-Impedance (GMI) sensors, one of the important features that limit thedetection performance is the equivalent magnetic noise level. In the white noise region, theperformance is mainly limited by the noise from the electronic conditioning not by the intrinsicnoise of the sensing element [1].Digital approaches for the electronic conditioning of these sensors were introduced in recentworks [1]-[2]. These promising approaches and the related software are commonly known asSoftware Defined Radio (SDR). They are based on a direct sampling of the voltage across thesensitive element using an Analog-to-Digital Converter (ADC). The amplitude-demodulation isthen digitally achieved using a Direct-Down-Conversion (DDC) to obtain the final sensor output.By far, this DDC generally includes a local Numerically-Controlled Oscillator (NCO), a set ofdigital filters, multipliers and digital gain as well asrounding circuits(necessary for the real-timeprocessing since only a limited number of bits can be processed by the hardware). All of theseinvolved digital functions are sources of noise [3].In this paper, thekey rules for the real-time implementation of these digital functions arepresented. The idea is to clearly show that the optimization of the noise performance requiresstringent implementation rules. These rules include, but are not limited to, the choice of theparameters of the NCO, the type andthe number of coefficients of the digital filters, the size ofaccumulators of the multipliers and of the rounding circuits. By respecting the proposed rules,the noise of the DDC can be less than the Least Significant Bit (LSB) of the ADC. In this case,the noise of the sensor in the white noise region is mainly dominated by the ADC.To validate the concept, a prototype of an on off-diagonal GMI sensor was developed using a 12-bit ADC having a sampling frequency of 125 MHz. The DDC was implemented using a Field-Programmable-Gate-Array (FPGA). With this realization, performance of 1.8 pT/√Hz isobtained in the white noise region (for sensitive element of 25 mm long). Improving the noiseperformance requires the use of an ADC with high resolution and/or higher sampling frequency.We show that with a technological jump of the ADC technology, the intrinsic noise of the sensitive element could be achieved with the digital concept