DROP: A CMOS Differential Ring-Oscillator Sensing Platform for Nano-Liter Droplet Detection

In this article, a differential ring-oscillator sensing platform for nanodroplet detection is presented. The system targets applications with volume sample requirements as low as 20 nL. The sensing mechanism is based on the dielectric constant variations of an interdigitated capacitor, embedded in a ring-oscillator structure, due to interactions with the liquid sample. Sensing is performed differentially using two oscillators operating at 692 ± 1 and 591 ± 1 MHz resulting in a frequency difference of 101 ± 1 MHz. The prototype of the sensor is fabricated using 180-nm CMOS process and occupies an area of 0.54 × 0.49 mm2. The experimental results validate the performance of the fabricated sensor to detect different concentrations of ethanol and methanol in water with sensitivity of 0.54 MHz/[L/L]% and 0.22 MHz/[L/L]%, respectively. The on-chip sensor is supplied with a 3.2-V voltage source and the average current for nominal operation is 10 mA.