Constant charge operation of capacitor sensors based on switched-current circuits

A new method to perform the readout of capacitive sensors is presented in this paper. The sensitivity of integrated capacitive sensors is investigated and compared considering two different approaches. The traditional approach, in which a voltage source is connected via a resistor to the capacitor, and a completely different approach, in which the charge residing in the capacitor is kept constant. With this second technique, the charge injected to define the dc operating point is maintained constant and a linear relationship between the external excitation and the voltage monitored is obtained. As pointed out by the authors in previous works, the use of current pulses allows injecting and controlling the charge stored in an electrostatic microactuator. In order to avoid the effect of the leakage currents, the charge is periodically refreshed. In this way, the charge is maintained constant inside the refresh cycle. In this paper, it is shown that this technique can be applied to capacitive sensors instead of electrostatic microactuators. The effects derived of using the temporal window, inside the refresh cycle to sense an external excitation, are further investigated. It is shown that the sensitivity of the sensor is increased in front of the obtained one with the voltage drive mode because the plates of the sensor can be fixed after the voltage pull-in distance without instability. Moreover, no voltage penalty is produced because the voltage developed across the transducer decreases after the pull-in distance. This method is introduced and studied through simulations, confirming the theoretical predicted behavior.