Low-noise chopper amplifier design for low-frequency induction magnetometers

Induction magnetometers (IM) are commonly used in the magnetotelluric method for exploration. 1/f noise in the low-frequency band of the preamplifier reduces the signal-to-noise ratio of IM, thus affecting the exploration results. We designed a low-noise chopper amplifier with equivalent input voltage noise of 9.8 nV/root Hz at 1 mHz, 2.1 nV/root Hz at 0.01 Hz, and 1.9 nV/root Hz at 0.1 Hz; 1/f noise corner frequency of 8 mHz, and equivalent input current noise of 200 fA/root Hz at 1 mHz, 100 fA/root Hz at 0.01 Hz, and 60 fA/root Hz at 0.1 Hz. The IM using this chopper amplifier is 85.5 cm long, 6.0 cm in diameter, and has a total weight of 4.8 kg. Its sensitivity is 300 mV/nT during 20 Hz and 1 kHz, with a theoretical equivalent input magnetic field noise of 0.3 pT/root Hz at 1 Hz and a measurement frequency range of 1 mHz to 1 kHz.