A 555 is a pretty good, cheap oscillator for this. It also has the advantage that one side of the cap is grounded. This allows a neat trick :=20 Connect the low side of the sensor and a reference cap to PIC pins. By enabling and grounding each pin in turn, you can measure the frequency for each of the caps and calculate the ratio. This eliminates almost all potential errors - stray capacitance, voltage & temp fluctuations. The accuracy is just dependent on the stability of the reference capacitor. On Wed, 3 Apr 2002 12:58:55 -0500, you wrote: >Build an oscillator using the sensor as the capacitance. >Use a PIC to measure the period. Convert period to Humidity using >math. For greater accuracy, measure multiple periods (say 10). > >My CMETER PIC controlled capacitance meter is overkill for what >you need, but you might find some useful ideas contained therein >that you can use. CMETER will measure from .01 pf to over >16,000 microfarads and has auto-zeroing and the ability to >cancel out the effects of stray capacitance. > >With a differential of 40 pf, CMETER will display 40.00 > >Fr. Thomas McGahee > > >----- Original Message ----- >From: "Diego Sierra" >To: >Sent: Tuesday, April 02, 2002 5:07 PM >Subject: [EE]: How to measure low cap (120pF) > > >> Hi! >> >> 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? >> >> 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? >> >> Thanks in advance, >> Diego. >> >> -- >> http://www.piclist.com hint: PICList Posts must start with ONE topic: >> [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads >> >> -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics