A Novel Calibration-Free Fully Integrated CMOS Capacitive Sensor for Life Science Applications

Abstract

CMOS capacitive sensors for label-free monitoring of biological/chemical reactions are typically prone to inaccuracies due to the parasitic elements and mismatch rooted in the CMOS fabrication process as well as real-time changes inside the sample solution. The former can usually be compensated by employing differential circuits and static calibration of the sensor before the experiment. On the other hand, changes in the sample solution such as sedimentation of non-target molecules or change in the conductivity of solution can significantly alter the operating point and result in inaccurate sensor readings that might require recalibration of the sensor during the experiment. In this paper, we present a CMOS capacitive sensor that is calibration-free via the creation of time-resolved three-dimensional (3D) surface electrochemical profiles. These 3D profiles uncover the variations of both target and unwanted parasitic capacitances. The sensor includes on-chip interdigitated electrodes (IDEs), a wide input dynamic range (IDR) differential capacitance-to-current converter, a digitally programable reference capacitor, and an oscillator-based analog-to-digital converter (ADC), and is implemented using 0.35 µm AMS CMOS process. The IDR covers a change in capacitance as small as 1 fF up to 1.27 pF with a minimum detectable change of 0.416 fF.