An ultra-low drift DC chopper amplifier using MOSFET for large value of signal-source resistance

Describes an ultra-low drift, MOSFET, and DC chopper amplifier which can be used for signal-source resistance of 0.1 to 10 MΩ. The sensitivity of this chopper amplifier decreases with the increase in signal-source resistance. The authors found that the decrease in the sensitivity is caused mainly by the drain-substrate and source-substrate capacitances in the MOSFET, and corrected the problem by the bootstrapping method. When using bootstrapping, these capacitances are reduced, and the temperature drift due to high signal-source resistance is improved. For an input signal source of 10 MΩ, the drift is reduced to one-tenth of its original value. Thus a DC amplifier with a temperature coefficient of within ±10 nV/°C for a signal-source resistance of 0.1~10 MΩ was successively built.