This thesis focuses on the design of a high input impedance sensor readout system based on a continuous-time sigma-delta modulator with finite-impulse-response digital-to-analog converter feedback.

Both the system-level and circuit-level design techniques of this system are investigated. The concept and design methods of finite-impulse-response digital-to-analog converters are introduced. The first integrator with source degeneration resistors and an input Gm-boosting cell is designed to achieve high input impedance and linearity. Both simulation and post-layout simulation results confirm the expected effective number of bits of 15 bits in the readout performance. The design is fabricated in a standard 180nm CMOS technology.

A photoplethysmogram (PPG) is an important optically-obtained bio-signal that enables convenient daily monitoring of heart rate and oxygen saturation (SpO2). Most state-of-the-art PPG sensing systems require an off-chip photodiode, and the photodiode's output parasitic capacitance limits the power-noise trade offs. The objective of this project is to implement a monolithic low-power PPG sensor for heart-rate detection. A novel sensing system has been developed, which has arrays of photogate imagers as front-ends and a compact successive approximation register analog-to-digital converter serving as the readout circuit. The readout power consumption is 0.64 µW at a sampling rate of 40 Hz. The minimum required LED power consumption is predicted to be 3 µW according to the post-layout simulation. The die area is 6.05 mm² including the pads. Compared with the prior art, the readout power consumption is reduced by four times and the die area is reduced by 3 times. The chip was submitted for fabrication in June in TSMC180nm.

Optical sensor has been widely used in daily life. One of the important applications of optical sensors is the ambient light sensor installed in portable devices. The screen brightness of portable devices like smartphones is designed to be automatically adjusted based on the ambient environment. The ambient light detection is realized using optical devices like Photodiode (PD), where the incident light is converted into an analog current. The induced PD current is measured by a readout circuit so that the output of the readout circuit controls the screen brightness accordingly.
This thesis presents an energy-efficient current-mode optical sensor readout circuit that achieves a wide dynamic range. Hourglass ADC is used as the core structure of the readout circuit, applying current-to-frequency conversion. The output of the readout is the digital count within 100ms conversion time. The dark counts of the designed PD readout circuit are less than three counts. The noise count is less than one when output counts are smaller than 100 and less than 1% when output counts are more than 100. Fabricated in a 0.18μm CMOS process, the readout circuit achieves a 160dB dynamic measurement range comparable to the prior-of-art. The entire circuit consumes power of0.113mW and occupies a chip area of 0.056mm2.