Diego Sierra wrote...

>I have a humidity sensor from Philips (H1
>ftp://ftp.iac.es/out/dsg/H1_Humidity.pdf), it's a device that changes
>its capacity value from 110pF to 140pF. I found its datasheet not on
>Philips web, where I cannot find either an application note about it. On
>its electrical characteristics is talks about a frequency range which
>have no meaning for me.
>
>How could be this device conected into a circuit?

Linear Technology, Inc. has some nice application notes covering
various signal-conditioning tasks.  Here are two that discuss
processing capacitive sensors:

http://www.linear-tech.com/pdf/an3.pdf has a couple of circuits
on pages 7 and 8 that deal with capacitive humidity sensors
similar to the one you have.  These circuits use their LTC1043
switched-capacitor instrumentation building block IC plus some
opamps.

http://www.linear-tech.com/pdf/an87.pdf on pages 87-88 shows
another pair of circuits, also using the LTC1043, for processing
capacitive sensors.  (NOTE: the diagrams show wrong pin numbers
for the opamps in the circuit; the "+" and "-" inputs are labeled
correctly but their pin numbers are reversed.)

>How to measure such a low capacity value?, with an oscilator (555)?, if
>the capacity is so low, could the parasitic capacities easily changes
>the measured value?

Parasitic capacitance is definitely a problem, and the circuits
in AN87 (see above) are designed to minimize the effects of
parasitics.

One thing to remember with capacitive humidity sensors is you
need to keep DC away from the sensor, so it sees only an AC
signal.  The circuits in AN3 show how to do this, and the
technique shown there (putting a large capacitor in series with
the sensor and a large resistor in parallel with it) should be
copied if you use the circuit in AN87 for your humidity sensor.

Hope this helps a bit...

Dave

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