FET parasitic capacitance.

Discussion in 'General Electronics Chat' started by MiG, Mar 13, 2009.

  1. MiG

    Thread Starter New Member

    Mar 13, 2009
    Hi guys, looking for one or two suggestions here.

    I'm working on a postional sensing system, based on the change in capacitance found between two plates. Now, the change in position between the plates will be very subtle over time and therefore I need a sensitive instrument for measuring this.

    So I've chosen an Analog devices Capacitive to Digital converter(AD7747), a nice little device that does the hard bit for me.

    My difficulty is this, the AD7747 is intended for single ended operation over no more than 17pF, with the positive plate of the cap connected directly to the 7747 input pin (where the AC impedence is measured via an internal oscillator souce and digitized via a sigma delta modulator).

    In my system however, there are of necessity, several not so perfect FET and MOSFET devices as more than one device has to be switched onto the sensed capacitance at any one time (including quite a heavy duty voltage source for charging it).

    So, perhaps long winded but I was wondering if anybody had good suggestions for eliminating the substrate capacitances Cds, Cdg etc from my switches, as these have a tendency to be in the same range of the capacitor I am trying to sense then they obviously corrupt the information I am trying to record.

    The braindead obvious answer is to bypass these with other capacitors, however, this may cause problems in several areas of my system. So, I was wondering if there were other solutions available.
  2. KL7AJ

    Senior Member

    Nov 4, 2008
    Yes...if you know exactly WHERE the undesirable capacitance is, you can NEUTRALIZE it by incorporating it in a bridge network. Look up references to amplifier neutralization.