A fully differential rail-to-rail CMOS capacitance sensor with floating-gate trimming for mismatch compensation

This paper presents a fully differential capacitance sensor employing the CBCM technique to map differential input capacitances into rail-to-rail differential output voltages. The circuit has been designed for measuring capacitances in the ±25-fF range, appropriate for sensing live cells using on-chip microelectrodes. An array architecture based on a shielded current routing bus has been developed for incorporating the capacitance measurement circuit into sensor arrays, with each pixel comprising four minimum-size digital transistors, enabling high-density integration. In addition to improving spatial resolution, the shielded current bus also eliminates the need for individual pixel calibration, conserves sensor evaluation speed, and provides protection from junction leakage. The sensor employs a 3-phase clocking scheme that enables on-chip gain tuning. The paper also presents a modified version of the sensor circuit incorporating floating-gate transistors for mismatch compensation and output offset cancellation, performed using a combination of impact-ionized channel hot electron injection and Fowler-Nordheim tunneling mechanisms. Chips comprising both versions of the sensor circuits in test arrays employing the shielded current routing bus were fabricated in a commercially available 2-poly, 3-metal, 0.5-µm CMOS process. The sensor operation was demonstrated by measuring on-chip test capacitances comprising single and interdigitated metal electrodes, configured using different capacitance compensation schemes. The differential sensor in combination with the shielded current bus exhibits a maximum sensitivity of 200 mV/fF, a resolution of 15 aF, and an output dynamic range of 65 dB.