QIs it possible to extend the range of measured capacitances above 4 pF by
decreasing excitation signal e.g. to a value of VDD/4 ?
A1) Connect a known fixed capacitor in series with the sensor. That would work
by equation C = C1 x C2 / (C1 + C2). This method results in a non-linear
relation between the sensor capacitance and the output data, but this can be
corrected in the host uC software. Also, the AD7746 input range might not be
fully used with implication of lower signal/noise ratio, but I believe the
AD7746 itself has enough resolution to keep the system resolution on required
level. My main concern would be how stable is the serial capacitor over
temperature and temperature compensation for that.
2) Reduce the excitation voltage. The AD7746 basically measures charge and
reducing the excitation voltage, proportionally higher capacitance can be
measured. See the attached pdf file for more details.