Differential capacitive transducer “Twin-T” measurement circuit

Discussion in 'General Electronics Chat' started by b5ee, Nov 4, 2008.

  1. b5ee

    Thread Starter New Member

    Nov 4, 2008
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    I tried the "The Differential capacitive transducer “Twin-T” measurement circuit" located at http://www.allaboutcircuits.com/vol_2/chpt_12/6.html without success. The article suggested that this was useful at "Capacitive transducers provide relatively small capacitances for a measurement circuit to operate with, typically in the picofarad range". Is there a suggestion for calculating resistance values based on specific capacitance value/ranges? Thanks in advance for any help you can provide.
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    The twin T is actually a form of bridge circuit. With any bridge circuit, you want the source impedance (generator) to have a very low impedance, and your voltage detector to have a very high impedance (infinite, if possible) You also have to drive this particular circuit hard enough to keep the diodes in full conductance. Mismatch between the two diodes can affect the accuracy of this bridge....which is why it's not my favorite configuration. A more conventional Wheatstone with a sensitive detector (such as a receiver) is much preferable.


    eric
     
  3. b5ee

    Thread Starter New Member

    Nov 4, 2008
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    When you say a more convential Wheatstone bridge, I assume that you are recommending a 4 capacitor bridge with the unknown capacitor being 1 of the 4. I have managed to get that working but the mentioned article discussed an easy to use alternative, that's why I tried it. Is the sensitive detector you refer to a diode?
     
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    For measuring such small values of capacitance, you're going to want to use a relatively high frequency radio generator....otherwise, the reactance of the capacitors is going to be HUGE...along with comparative errors.

    When you do this, you have the option of using a radio receiver for a null detector...whick will not load the DUT at all.

    The twin T works great for LARGE values of capacitance...but not for picofarads! You're much better with a genuine R.F. bridge.

    Eric
     
  5. b5ee

    Thread Starter New Member

    Nov 4, 2008
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    Thanks for the help
     
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