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

This article presents the implementation of a capacitance-to-time converter in standard 0.18-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text {m}$ </tex-math></inline-formula> CMOS technology. It is based on a square-wave relaxation oscillator that incorporates both pulsewidth 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}_{{0}}$ </tex-math></inline-formula> resistor values. Characterization tests proved that the fabricated chips exhibit the expected performance across the full range of use, achieving a sensitivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${207}~\mu $ </tex-math></inline-formula>s/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta $ </tex-math></inline-formula>pF in the differential mode (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${110}~ \mu $ </tex-math></inline-formula>s/pF in the single mode) and a resolution of 0.5 fF in the differential mode ((0.9 fF in the single mode). The circuit occupies a silicon area of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${0.0044}~ \text {mm}^{2}$ </tex-math></inline-formula>, with a maximum power consumption of 1.5 mW. 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${100}~ \mu $ </tex-math></inline-formula>L. Measuring the dielectric constant in liquids was also demonstrated by using specifically designed 3-D-printed fluidic capacitance sensors.