Capacitive sensor system for sub-nanometer displacement measurement

We report an integrated front-end circuit for capacitive displacement sensors, suitable for measuring small displacements superimposed on a much larger stand-off distance between the sensing and the target electrodes. In order to cancel the effect of the resulting offset capacitance, a circuit technique called electrical zoom-in is applied in the interfacing circuit. After removing the offset capacitance of a sensor with 10 pF nominal capacitance, the output-swing of the front-end corresponds to 100 fF of capacitance variation only, relaxing the specification of the following analog-to-digital converter (ADC) stage by a factor of 40.The zoom-in front-end can achieve an input referred capacitance resolution of 30 aF (RMS) from a nominal sensor capacitance of 10 pF, with a measurement latency of only 5 us. This result is achieved with 2.4 mW power consumption in the front-end.