Sub-femtofarad capacitive sensing for microfabricated transducers using correlated double sampling and delta modulation

The theory, design, and measured performance of a circuit which measures very small capacitances, particularly suitable for microfabricated sensors relegated to a high parasitic environment, is presented. The circuit uses an existing capacitive sensing ASIC configured as a delta modulator to achieve a sense resolution of 0.53 fF at a 6-kHz sample rate in the presence of board-level parasitics. The applicability of the circuit to microfabricated sensors has been successfully demonstrated using a bulk-micromachined vertical transducer that is similar to many pressure sensors and accelerometers.