I'm trying to measure a varying capacitance.  The device in particular
is the HS1101 humidity sensor.

The specs are:
180pF at 55%RH
0.34pF/%RH

It would be simpler if it were either a lot bigger or a lot smaller.

I'm looking for any suggestions of how to build a capacitance
measurement device with a full swing measurement input of 160pF-200pF.

I've looked at four approaches so far:

1) RC inverter oscillator
2) Current mirror feeding HS1101 for fixed time, measured with
sample/hold circuit
3) Feed signal through RC filter, where C is the HS1101, and read the
amplitude
4) Measure with the Analog Devices AD7745 Capacitance to Digital Converter

1), while simple, is non-linear and requires a large callibration table
or a floating point calculation.
2) Again, non-linear, but easier to calculate and easier to measure.
More complex circuitry
3) Haven't looked too hard at this one.
4) Sadly the device only has a +/- 4.096pF range.

I like 4) the best, as it's the easiest to interface, and the easiest to
design.  If there's a way to add negative capacitance (I know this can
be done as the AD7745 uses CAPDACs to subtract capacitance from its
inputs) and divide the capacitance (linearly) by 2, then this is the way
to go IMO.

Thanks,
Brendan

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