Differential Ring Oscillator Based Capacitance Sensor for Microfluidic Applications

A simple high frequency capacitance sensor with 180 aF sensitivity is designed for a wide range of microfluidic applications. The sensor is implemented utilizing differential ring oscillators operating at similar to 240 MHz with a differential signal at similar to 20 MHz. The sensor occupies similar to 2 cm x 2 cm on a printed circuit board. The sensor is tuned using two precision variable capacitors and has a full scale range of similar to 1.5 pF. The sensor was able to detect less than 1% Isopropyl Alcohol in DI water and to detect 15 mu m polystyrene spheres flowing over 25 mu m lines and spaces coplanar electrodes in a microfluidic channel. The compact differential ring oscillator based architecture of the design makes it suitable to be integrated into microprocessor based systems for detection in Lab on Chip or Lab on Board applications.