Circuit using capacitor-controlled oscillator:

• Converts capacitance changes into frequency of the oscillation
• FM signal is proportional to the pressure
• Alternatively, a relaxation oscillator using RC to determine the time constant can be used
  for capacitance to time conversion.
• Major problem of these solutions
  • Stability of the frequency or period when no pressure is applied
  • Temperature drift
• Possible scheme to overcome these difficulties uses a pressure insensitive capacitor as the reference
  to null the environmental drift.
• Charge measurement circuits can be also used in liaison with capacitive sensors
• In this case, a fixed voltage, V, is applied to the capacitor, C.
  If the charge stored, Q = CV, is kept constant, a capacitance variations due to pressure
  can be converted into voltage variations.
  A switched capacitor circuit is one of possible solutions of this conversion.

Circuit based on a switched-capacitor charge integrator:

• Two-phase clock to compare the variable sense capacitor, C_x, to a reference capacitor is used
• High sensitivity and immunity from parasitic capacitance