A successive approximation register analog-to-digital converter for low-cost microbolometers

Mahsereci, Yiğit Uygar
Commercialization of infrared (IR) vision is of vital importance for many applications, such as automobile and health care. The main obstacle in front of the further spread of this technology is the high price. The cost reduction is achieved by placing on-chip electronics and diminishing the camera size, where one of the important components is the analog-to-digital converter (ADC). This thesis reports the design of a successive approximation register (SAR) ADC for low-cost microbolometers and its test electronics. Imaging ADCs are optimized only for the specific application in order to achieve the lowest power, yet the highest performance. The successive approximation architecture is chosen, due to its low-power, small-area nature, high resolution potential, and the achievable speed, as the ADC needs to support a 160x120 imager at a frame rate of 25 frames/sec (fps). The resolution of the ADC is 14 bit at a sampling rate of 700 Ksample/sec (Ksps). The noise level is at the order of 1.3 LSBs. The true resolution of the ADC is set to be higher than the need of the current low-cost microbolometers, so that it is not the limiting factor for the overall noise specifications. The design is made using a 0.18µm CMOS process, for easy porting of design to the next generation low-cost microbolometers. An optional dual buffer approach is used for improved linearity, a modified, resistive digital-to-analog converter (DAC) is used for enhanced digital correction, and a highly configurable digital controller is designed for on-silicon modification of the device. Also, a secondary 16-bit high performance ADC with the same topology is designed in this thesis. The target of the high resolution ADC is low speed sensors, such as temperature sensors or very small array sizes of infrared sensors. Both of the SAR ADCs are designed without switched capacitor circuits, the operation speed can be minimized as low as DC if an extremely low power operation is required. A compact test setup is designed and implemented for the ADC. It consists of a custom designed proximity card, an FPGA card, and a PC. The proximity card is designed for high resolution ADC testing and includes all analog utilities such as voltage references, voltage regulators, digital buffers, high resolution DACs for reference generation, voltage buffers, and a very high resolution Δ-Σ DAC for input voltage generation. The proximity card is fabricated and supports automated tests, because many components surrounding the ADC are digitally controllable. The FPGA card is selected as a commercially available card with USB control. The full chip functionalities and performances of both ADCs are simulated. The complete layouts of both versions are finished and submitted to the foundry. The ADC prototypes consist of more than 7500 transistors including the digital circuitry. The power dissipation of the 16-bit ADC is around 10mW, where the 14-bit device consumes 30mW. Each of the dies is 1mm x 5mm, whereas the active circuits occupy around 0.5mm x 1.5mm silicon area. These chips are the first steps in METU for the realization of the digital-in digital-out low cost microbolometers and low cost sensors.


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Citation Formats
Y. U. Mahsereci, “A successive approximation register analog-to-digital converter for low-cost microbolometers,” M.S. - Master of Science, Middle East Technical University, 2012.