C2.3 - MEMS Capacitive Sensor Interfaces: Can we Solve the Challenge?

SENSOR+TEST Conferences 2009
2009-05-26 - 2009-05-28
Congress Center Nürnberg
Proceedings SENSOR 2009, Volume I
C2 - Sensor Electronics II (Capacitive Sensors)
R. Puers - Katholieke Universiteit Leuven, Leuven, Belgium
287 - 292


It is well known that integrated capacitive mechanical sensors offer much higher potentials than their piezo-resistive counterparts as far as sensitivity, temperature behaviour, stability and power consumption are concerned [1]. However, their adoption in regular applications is still moderate, despite these excellent characteristics. One reason is clear : inherently, it is not wise to miniaturize such sensors. The concept is in fact not very compatible with the basics of MEMS, i.e. : the parasitics become dominant when reducing the dimensions ! The exploitation of miniaturized capacitive sensors is often inhibited by the presence of parasitic effects, such as environmental noise, parasitic capacitances and leakage resistors. These effects are much less pronounced for piezoresistive devices. The mentioned parasitic effects are of a fundamental nature and are inherently related to miniaturization in the case of a capacitive device. Down scaling of the sensor dimensions implies down scaling of the active capacitance values (to the second power !). Thus, full integration implies active sensing capacitances that get as low as a few fF only. Hence, high output impedances and noise sensitivity cannot be avoided and the effects of stray capacitances become more dominating. The only way to cope with these difficulties is to incorporate a dedicated readout circuit in close proximity of the sensor, i.e. in the sensor package itself. This paper has a dual approach to this problem : at first, it will highlight techniques to cancel out such parasitic effects, and secondly, it will focus on a impoved low power offset cancellation solution.