Design of Analog Front-end Circuit for Audio-frequency Magnetotelluric Instrument

High-performance audio-frequency magnetotelluric (AMT) instrument is one means of obtaining high-quality electromagnetic (EM) data. To improve the ability of AMT system to obtain high-quality data, this paper presents a design for a high-performance analog front-end circuit for AMT instrument. It mainly consists of the input protection, preamplifier, passive high pass filter, power frequency notch filter, programmable amplifier, and active low pass filter. In addition, this paper proposes a design of low-noise, high-performance preamplifier, which improves the common-mode rejection ratio (CMRR) of analog front-end circuit and effectively enhances the signal-to-noise ratio (SNR) of the circuit. The front-end circuit utilized two-stage twin-T notch filter to effectively suppress the strong interference of fundamental component of power frequency. Also, it used signal relays to control circuit gain and selection of cutoff frequency of anti-aliasing filter, resulting in the improvement of the capability of the analog-to-digital Converter (ADC) to distinguish weak EM signal. The measured results of the electric field and magnetic field channel showed that: 1) The circuit works in frequency range of 1 Hz similar to 100 kHz; 2) The CMRR values of the preamplifier of electric field channel at low frequencies (1 Hz similar to 1 kHz) are 111 dB and 97 dB when the gains are 20 dB and 6 dB respectively; 3) The maximum attenuation fundamental power frequency can reach -39.46 dB and -39.04 dB respectively; 4) The total harmonic distortion rate at 1 kHz is 0.022% and 0.029% respectively; 5) The input noise levels of electric field channel are 12.67nV / root Hz @10Hz and 8.15V / root Hz @1kHz, while the input noise levels of magnetic field channel are 8.97nV / root Hz @10Hz and 6.16V / root Hz @1kHz; and 6) In conclusion, the analog front-end circuit is superior to meet the requirements of the AMT methods, and provides a useful reference for the development of AMT instrument.