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∼100 kHz; 2) The
CMRR values of the preamplifier of electric field channel at low frequencies
(1 Hz∼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 / [Formula: see text] @10Hz and 8.15V
/ [Formula: see text] @1kHz, while the input noise levels of magnetic field
channel are 8.97nV / [Formula: see text] @10Hz and 6.16V / [Formula: see
text] @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.
A high performance SAR ADC for WLAN analog front end
