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

Abstract

Induction Magnetometers (IM) are commonly used in Magnetotelluric Method for exploration. 1/f noise in the low frequency band of the preamplifier reduce 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/√Hz at 1 mHz, 2.1 nV/√Hz at 0.01 Hz, 1.9 nV/√Hz at 0.1 Hz, 1/f noise corner frequency of 8 mHz, and equivalent input current noise of 200 fA/√Hz at 1 mHz, 100 fA/√Hz at 0.01 Hz, 60 fA/√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/√Hz at 1 Hz and a measurement frequency range of 1 mHz to 1 kHz.