CMOS Ring-Oscillator-Based Electrochemical Capacitance Imager with Frequency-Division-Multiplexed Readout

We present a CMOS electrochemical capacitance imager designed to overcome Debye-length-screening effects in integrated electrochemical assays and featuring frequency-division-multiplexed (FDM) column readout. Our imager contains eight pixel arrays where each pixel contains a 5-MHz-180-MHz ring-oscillator-based capacitance-to-frequency converter (CFC) to detect interfacial capacitance changes at an in-pixel working electrode. We implement FDM readout, which reduces column-readout time compared to a time-division-multiplexed approach, by operating each CFC in a column at a unique nominal frequency while summing the pixel output currents. We report experimental results from electronic performance characterization of a $3.0\times 2.5-\mathrm{mm}^{2}$ 420-pixel electrochemical capacitance imager, fabricated in a 1.8-V 0.18-$\mu{\mathrm{m}}$ CMOS process.