Development and evaluation of an ultralow-noise sensor system for marine electric field measurements

In this paper, we describe the development of an ultralow-noise sensor system consisting of non-polarizable Ag/AgCl electrodes and a chopper amplifier for detecting marine electric field signals. Ag/AgCl electrodes were fabricated using a constant current density to clean the electrode cores and were then electrolytically chloridized by applying a constant potential. An amplifier was developed using the chopper stabilization (CHS) technique to reduce the 1/ƒ noise and the initial offset. To achieve ultralow-noise performance, we took advantage of newly developed materials in the fabrication of Ag/AgCl electrodes and proposed a residual ripple reduction loop to decrease the modulated noise and the offset of the chopper amplifier. Typical measured noise levels are 0.6 nV/rt(Hz) for the Ag/AgCl electrodes and 0.55 nV/rt(Hz) for the chopper amplifier at 1 Hz. The source resistance between pairs of Ag/AgCl electrodes is approximately 5 Ω. The offset potential is typically ±0.1 mV with a drift of less than 5 μV/day. The chopper amplifier had an offset voltage below 0.2 μV with a −3 dB bandwidth from 0.01 Hz to 40 Hz and a variable gain from 80 dB to 120 dB to achieve a large dynamic range. This sensor system is used for marine electric field measurements with high precision.