A differential charge-transfer readout circuit for multiple output capacitive sensors

Abstract

This paper presents a true differential, low-noise readout scheme for multiple output capacitive sensors. It overcomes the restrictions with multiple sense capacitors connected to a common node that is common in micromachined gyroscopes and multiple axis accelerometers. The architecture is easily integrated into a CMOS process, only requiring single ended operational amplifiers, transmission gates and capacitors. It also supports cancellation of amplifier 1/ f-noise and offset voltage. Theory predicting the resolution of the circuit is presented and compared to noise-simulations. The ultimate noise performance is limited by the fundamental k B T/ C-noise from the sense capacitors. Measurements on a fabricated ASIC verify the feasibility of the concept. The ASIC used as a readout circuit for a triple axis accelerometer shows a resolution of 5.7 aF/√Hz at 20 Hz.