Capacitive interfacing for MEMS humidity and accelerometer sensors

This paper presents a study on the design issues of the electronic interfacing circuit for MEMS humidity and accelerometer sensors. The study focuses on capacitive sensing method as its one of the most important and widely used techniques in interfacing the micro sensors. Some of capacitive sensing techniques are reviewed and appropriate read out interfacing circuit for humidity and accelerometer sensors were chosen depends on their sensing capacitance magnitude. The Hartley oscillator circuit is proposed to interface the MEMS humidity sensor which is its capacitance is in the range of 180-500pF (G.Nagy, 2007) depending on the relative humidity values. The feedback circuit of Hartley Oscillator is designed at 998.473 kHz of resonant frequency with 167.09 degree of phase shift. At this particular frequency, the maximum gain achieved at about 11dB. To ensure the oscillation begins, the amplifier with 13dB of gain is connected to the feedback circuit. The result of the oscillator circuit, which is based on capacitance to frequency converter are generally 5-15% of calculated results. Chopper stabilization sensing circuit is chosen for interfacing MEMS accelerometer sensor. This sensor was based on an acceleration sensing capacitor which is the displacement of the moving electrode results in a change in the capacitance. The chopper stabilization results the proportional output voltage to capacitance changes for a very small sensing capacitance magnitude which is less than 1pF. (G.Nagy, 2007).These two interfacing read out electronics circuit was simulated by using National Instrument Multisim 10.1 software. The results from both simulation shows that the capacitive interfacing circuits are suitable for each sensor application.