Vasile Surducan wrote: > On 5/20/06, Brendan Moran 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. > > 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. The following technique was presented in Circuit Cellar several years ago, first in the Connect Time column in issue #61, and later I used it as an EQ question in issue #124. Since it's no longer available online, I'll reproduce the original diagram here as ASCII art. You can use this circuit with non-differential sensors if you simply replace one of C1 or C2 with a fixed capacitor set to the middle of the range you're interested in. With larger capacitors, you could probably also reduce the clock frequency. -- Dave Tweed Question The following circuit is used to read a tilt sensor that is implemented as a differential capacitor -- when the sensor is tilted, C1 increases while C2 decreases (or vice-versa). Both capacitors vary between 4 and 30 pF. 1000 pF +---||---+-----------------+----o A | | | | A K | D1 K A D3 | | C1 C2 | 10 VAC o-----+ +---||---+---||---+ 1 MHz | | | | o-+ | A V K | | D2 K Gnd A D4 V | | | Gnd +---||---+-----------------+----o B 1000 pF The voltage differential between points A and B varies with the position of the sensor. How would you analyze the operation of this circuit? Answer The 10V, 1MHz source causes the diodes to conduct in pairs -- D1 and D4 conduct on the positive peaks, and D2 and D3 conduct on the negative peaks. Since D1 and D2 never conduct at the same time, the net current through them is directly porportional to the value of C1. Similarly, the current through D3 and D4 is proportional to the value of C2. If C1 and C2 have the same value, the D1/D2 current equals the D3/D4 current, and the average voltage difference between points A and B is zero (although both are swinging up and down at 1 MHz). On the other hand, if the sensor is unbalanced, say C1 increases and C2 decreases, more current will flow in D1/D2 than in D3/D4, causing the average voltage at B to rise relative to A. Note that the difference between A and B can't exceed two forward diode drops (about 1.5 V) in either direction, and in fact, the voltage between them will be related to the net current flow by the diode equation. For values less than 1 V, the voltage varies nearly linearly with the capacitance difference. Additional information on this topic can be found in INK #61 ConnectTime. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist