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 (2-20 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 (16 bits) and excellent linearity (better than 0.1 percent) 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 hertz.