A High Resolution Capacitance-to-Frequency Converter

This paper describes an integrated sampled-data sinusoidal oscillator with AGC amplitude stabilization, the frequency of which is proportional to an external low value capacitor (0.1 pF to 20 pF). The circuit, based on a switched-capacitor two integrator loop, is insensitive to stray capacitances between sensing nodes and ground and features high resolution (l6 bits) and excellent linearity (better than 0.1%) as well as very low power consumption (less than 0.1 mW). Such a "capacitance-to-frequency converter" is particularly suitable for interfacing variable-capacitance sensors (pressure transducers, accelerometers ...) in applications requiring high accuracy but where signal bandwidth is limited to a few hundred Hz.