Highly Sensitive Capacitive Sensor Based on Injection Locked Oscillators with ppm Sensing Resolution

This paper presents the design and the implementation of an ultra-sensitive capacitive sensor based on an injection-locked oscillator architecture able to achieve ppm sensing resolution. The proposed RF IC chip, implemented on a BiCMOS SiGe 0.25 µm technology, is able to detect capacitance changes in the range of few attoF, induced by dielectric disturbance occurring on on-chip integrated sensing capacitors. Indeed, this paper demonstrates that, once locked on a 4.693 GHz and -50 dBm injection signal, the proposed sensing system can efficiently detect differences of free running frequencies as low as 10 kHz between two oscillators. Considering a differential measurement approach with two injection locked oscillators (one used as a sensor, the other as a reference), a 3 attoF difference between both LC tank sensing capacitors might be detectable. This results, in the present case, in a 5 ppm sensing sensitivity.