A 180-nm CMOS Integrated Capacitance-to-Time Converter for Capacitive Sensing Applications

This paper presents the implementation of a capacitance-to-time converter in standard 0.18 μm CMOS technology. It is based on a square wave relaxation oscillator that incorporates both pulse width modulation (PWM) and proportional to magnitude (PM) dependencies. The circuit is designed to interface with differential capacitance sensors, but can also interface with single capacitance sensors using a reference capacitor. It works with various capacitive values, ranging from a few nanofarads to less than one picofarad, considering different external R0 resistor values. Characterization tests proved that the fabricated chips exhibit the expected performance across the full range of use, achieving a sensitivity of 207 μs/ΔpF in differential mode (110 μs/pF in single mode), and a resolution of 0.5 fF in differential mode (0.9 fF in single mode). The circuit occupies a silicon area of 0.0044mm2, with a maximum power consumption of 1.5mW. The circuit has also been successfully tested in real applications, such as measuring liquid levels in microfluidic applications. By using COTS level-meters, it achieved a sensitivity of 0.4 pF/mL with a resolution of 100 μL. Measuring the dielectric constant in liquids was also demonstrated by using specifically designed 3D-printed fluidic capacitance sensors.