Signal Conditioning of Capacitive Accelerometers

Sensing method - depending on the accelerometer design configuration

• Single-ended capacitance
• Differential capacitance

Mode of operation

• Open-loop
• Closed-loop

Measuring circuit

• Bridge circuit
• Switched-capacitor technique
  (possible to measure capacitance changes of 0.05 fF and lower)

Schematic configuration for the capacitive sensing in a single-ended and differential mode:

• Any of the capacitive sensing techniques used in a micromachined accelerometer structure require an ac voltage across the capacitor being measured
• The presence of an ac voltage between the capacitor plates produces an electrostatic field, which generates an attractive force between them
• Other forces can be generated by the sensing-current flow, which will produce an electromagnetic field
• Also the system noise can be a source of an electrostatic force
• Using a single-capacitor sensing, the electrostatic field produced by the sensing waveform can be substantial when small capacitor spacings are used
• This may cause a deflection of the seismic mass
• If the voltage amplitude is too large, the electrostatic forces can even pull and lock the mass down to the substrate, rendering the device inoperable
• Differential sensing, using a movable plate between two fixed plates, can be operated in either an open-loop or a closed-loop signal conditioning configuration
• In both cases, complimentary square waves of a frequency in the MHz range are applied on accelerometer electrodes
• These two waveforms are coupled at the movable plate, where they cancel each other, resulting in zero-amplitude, when the middle plate is in a central position.
• This greatly reduces any tendency for latch up, which exists in the open-loop configuration
• Also, any electrostatic forces created by noise will rend to cancel
• Differential sensing also greatly improves the system linearity and minimizes temperature effects