High performance amplifier topologies implemented with a micro-machined vibrating capacitor

Abstract

In this work, the design of a MEMS based differential amplifier is investigated. The goal of this investigation is to design, fabricate and characterize a differential amplifier whose performance is based on a physically coupled, but electrically isolated fully differential mechanical transconductor input stage that is fabricated using SOI-MEMS technology. The MEMS sensor will act as a vibrating capacitor input stage. It will provide galvanic isolation and up-modulation of the input signal as it vibrates. The galvanic isolation facilitates low-leakage inputs and a very wide input common mode voltage range. The up-modulation provides a means for achieving a low input referred offset voltage and low-noise via the use of correlated double sampling or chopper stabilization. At the system level, this amplifier consists of two major loops: the drive loop and a sense loop. The drive loop includes half of the MEMS structure along with some electronics and provides a means of moving the beam at a constant frequency. The drive loop's design was facilitated by describing function analysis. The drive loop vibrated the beam at its mechanical resonance because at that frequency, the displacement of the beam is maximized for a given electrostatic force and consequently, the sensitivity of the amplifier is maximized. The sense loop includes the other half of the beam and some electronics whose role is to process the differential input signal applied at the MEMS structure's inputs. Common-mode rejection is performed by the mechanical transconductor, while the sense loop's crossover frequency sets the signal bandwidth.

(cont.) The performance of the amplifier agreed very well with hand calculations and simulations. The noise performance was dominated by the total noise at the preamplifier's input. The noise performance achieved in this design was 55 ... Hz , which is higher than that of other high performance amplifiers. Based on the analytical model created for the amplifier, a noise level of 450 ... Hz can be achieved when the circuitry is fully integrated with the sensor.