Hi,
maybe you've already seen these :

http://www.phanderson.com/picaxe/rh_count.html
http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=1684
http://www.bolivar.udo.edu.ve/microinternet/articulos/Humedad%20Relativa.pdf
http://www.circuitcellar.com/library/print/1101/Wasinger136/wasinger136.pdf
http://www.edn.com/article/CA152878.html

indeed the AD7745 looks very good, unfortunately was not designed for
this sensor.

greetings,
Vasile

On 5/20/06, Brendan Moran <annirack@shaw.ca> wrote:
> 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
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist