A MEMS based electrometer with a low-noise switched reset amplifier for charge measurement

Electrostatic charge measurements are at the base of chemical, physical and biological experiments. In this work the authors present an electrometer based on the vibrating capacitance of a microelectromechanical (MEMS) micromachined resonator. We present improvements on the low-noise readout amplifier by reducing input-referred voltage noise and parasitic capacitances. An amplifier has been designed to have a noise corner frequency well below the device´s operating frequency f_n. The electrometer geometry allows for charge output signal measurements at 2f_n minimizing feedthrough of driving signals. The sensor consists of a set of comb-finger capacitors placed on each side of a moving mass for push-pull driving. Operating at resonance, charge collected on the moving electrode is modulated and the induced voltage is read with a low-leakage very high-input impedance feedback amplifier. Due to the specific readout technique, a switched-reset is used to prevent charge saturation. Reduction of parasitic capacitance and increase in resolution is achieved through the careful selection and placement of discrete electronic components alongside the silicon MEMS chip.