CMOS analog and mixed-Signal circuit design for modern integrated sensor front-end
Integrating modern sensors in an on-chip system by using CMOS analog and mixed-signal circuit design has been gaining significant attention in the recent few years. As CMOS process technologies scale down and the demand for battery-operated portable equipment increases, low power and low supply voltage becomes an important design criterion. This dissertation includes two aspects of the CMOS analog and mixed-signal circuit design for modern integrated sensor front-end research: a second-order inverter-based sigma-delta modulator as an analog-to-digital converter in the modern integrated sensor applications to realize the low power and low voltage application and a digital DC-tuning variable gain amplifier as an analog signal conditioning circuit in a Doppler radar motion sensor application to solve the DC offset issues. In this dissertation, one of the research works focused on the design of the low voltage and low power sigma-delta modulator and its implementation in the smart sensor applications. The presented sigma-delta modulator uses a simple inverter to replace the operational transconductance amplifier in the traditional integrator block in the modulator. The supply voltage can be less than the summation of the threshold voltage of both PMOS and NMOS transistors in the inverter. Hence, the power consumption is reduced. Low-voltage-low-power dissipation of the circuit in each block is desired to avoid circuit self-heating in the temperature senor application in order to maintain the accuracy of the temperature sensor. Two second-order inverter-based sigma-delta modulators with feedback topology and with feed-forward topology were designed, fabricated and tested respectively. The second-order inverter-based sigma-delta modulator with feed-forward topology was implemented with a scattered relative temperature sensor system in a single chip. The experimental results demonstrate the low voltage operation of the inverter-based sigma-delta modulator and the sensing accuracy of the temperature sensor system by using the inverter-based sigma-delta-sigma modulator. In most of the traditional miniature Doppler radar systems, ac coupling is adopted between the radio frequency (RF) front-end and baseband amplifier to block the dc offset due to clutter reflection. However, ac coupling causes distortion to low frequency signals and loses the dc information, which represent slowly changing or stationary moment of the target. A DC-coupled radar sensor system using a variable gain amplifier with software-configured digital tuning architecture is presented in this dissertation to realize dc tuning and dc offset calibration. Furthermore, to process the output signals from the variable gain amplifier, a low voltage-low power inverter-based second-order sigma-delta modulator has been implemented in this sensor application. The frequency of mechanical motion can be detected from the spectrum of the output digital bit stream from the modulator presented here without a decimation filter, which can significantly reduce cost and power consumption. Experiments show that the presented digital dc–tuning architecture using variable gain amplifiers can completely relieve the radar sensor from circuit saturation and the inverter-based sigma-delta modulator can perform under low voltage and low power to accurately detect the movement frequency of the target. Both the presented VGA and modulator show great potential for integrating the whole Doppler radar sensor system into a low-power high-accuracy single chip in the future.