Question
In the AD7747 datasheet it is mentioned that the capacitance input range is ±8
pF (changing), and it can accept up to 17 pF common-mode capacitance (not
changing). Can this range be extended?
Answer
There is a method of extending the capacitance range for the AD7745 and AD7746.
The range extension circuit is provided in circuit note CN-0129:
http://www.analog.com/static/imported-files/circuit_notes/CN0129.pdf
This circuit has to ensure that the charge transfer within the sensing
capacitance CSENS remains within the input range of the AD7745/AD7746. To
achieve this, the excitation voltage needs to be decreased by a factor of F, so
that the sensing capacitance connected to the input can be increased by a
factor F.
The method used of scaling the AD7745/46 excitation voltage does not work for
the AD7747, unfortunately. The excitation signal is not externally available on
the AD7747, it is connected internally with the modulator to one CIN pin.
The only possible method is to scale the CIN capacitance by connecting a small
capacitor in series with the capacitive sensor. See suggestion in the paragraph
attached below. Unfortunately I don't have any data to show how successful or
stable this suggested solution might be.
"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 could be corrected in the host uC software.
Also, the AD774x input range might not be fully used with implication of lower
signal/noise ratio, but I believe the AD774x 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."