A Compressive-Sensing-Capable CMOS Electrochemical Capacitance Image Sensor with Two-Dimensional Code-Division-Multiplexed Readout

Abstract

Electrochemical capacitance imaging is a technique used to observe biological analyte or processes at the surface of an electrode, immersed in an electrolyte, via small changes in capacitance. This technique has various applications in biosensing such as biomedical diagnostics, neural interfaces and DNA sensors. Complimentary metal-oxide-semiconductor (CMOS) technology is well suited for implementing electrochemical capacitance image sen- sors since high spatial resolution electrode arrays and readout circuitry can be integrated on the same chip.

This thesis presents the design and simulation of a 256 × 256 pixel electrochemical capacitance image sensor fabricated in a 180-nm analog/mixed-signal CMOS process. Our image sensor features a novel two-dimensional code-division-multiplexed (2D CDM) readout architecture that directly outputs analog coefficients of the 2D Walsh transform of the image. To the best of our knowledge, we are the first to implement true 2D CDM readout in the capacitive image sensor space. For passive-pixel sensors, CDM readout yields a signal-to-noise ratio (SNR) increase over traditional time-division-multiplexed (TDM) readout through integrating orthogonal combinations of all pixels for the entire frame time.

Use of the 2D Walsh transform enables compressive sensing at the time of array readout, which is achieved by exploiting the energy compaction property of the Walsh domain. Compressive sensing provides analog lossy image compression that can enable a frame rate increase or power consumption decrease. In addition, our transform domain readout architecture removes the layout requirement for pitch-matched column amplifiers, requiring only one larger column circuit for the full array. Some potential advantages introduced by this include reductions to both amplifier flicker noise and fixed-pattern noise from transistor mismatch.

Our sensor uses two-transistor switched-capacitor pixels with a 3.2 × 3.2 μm² work- ing electrode and 3.88 μm grid pitch to enable charge-based capacitance measurement. On-chip 256-bit parallel Walsh code generators enable power efficient orthogonal code generation. Full-chip post-layout analog simulation with a biological capacitance image demonstrates that we can achieve a structural similarity index (SSIM) of 0.875 versus a reference image. SSIM values range from 0 to 1, where 1 indicates complete image similarity.