An Improved Switched Capacitor CFC for Capacitive Sensors Representable Using π-Model With Automatic Offset Elimination

In this article, a novel
$\pi $
-model capacitive readout circuit that measures changes in sensor capacitance independent of offset capacitance is presented. The
$\pi $
-model capacitor consists of three capacitances (
$C_{x2}$
: between electrodes, and
$C_{x1}$
and
$C_{x3}$
: between each electrode and ground), effective for sensing both the measurand detection and its relative position. The final output of the switched-capacitor converter is insensitive to the parasitic capacitance from shielding cables or sensor structures, and its use of a dc source further enhances the measurement accuracy. The measurement update rate is increased by a settling time enhancement unit (STEU). The proposed feedback mechanism reduces the worst case linearity error from 3.5% (during open-loop configuration) to 0.24%. The developed prototype automatically eliminates offset capacitance (from 20 to 240 pF) and has a negligible cross-sensitivity error (<0.12%) between capacitance measurements in the
$\pi $
-model. This CFC can provide the best update rate (typically 150 Hz), in comparison to the existing
$\pi $
-model capacitive sensing architectures.