A sensitive differential capacitance to voltage converter for sensor applications

There is a need for capacitance to voltage converters (CVC's) for different ial capacitive sensors like pressure sensors and accelerometers which can m easure both statically and dynamically. A suitable CVC is described in this paper. The CVC proposed is based on a symmetrical structure containing two half ac bridges, is intrinsically immune to parasitic capacitances and res istances, is capable of detecting capacitance changes from de up to at leas t 10 kHz, is able to handle both single and differential capacitances, and can easily be realized with discrete components. Its sensitivity is very hi gh: detectable capacitance changes of the order of 2 ppm of the nominal val ue (24 aF with respect to a nominal capacitance of 12 pF) result in a measu red output voltage of 1.5 mV. However, due to drift the absolute accuracy a nd resolution of the CVC is limited to 3.5 ppm. A differential acceleromete r for biomedical purposes was connected to the CVC and showed a sensitivity of 4 V/g. The measured rms output voltage noise in the frequency range of 2-50 Hz is 750 mu V, resulting in a signal to noise ratio of 75 dB at an ac celeration of 1 g in the frequency range of 2-50 Hz.