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14 May 2007 Pixelwise readout integrated circuits with pixel-level ADC for microbolometers
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Pixelwise integrated circuits involving a pixel-level analog-to-digital converter (ADC) are studied for 320 × 240 microbolometer focal plane arrays (FPAs). It is necessary to use the pixelwise readout architecture for decreasing the thermal noise. However, it is hard to locate a sufficiently large integration capacitor in a unit pixel of FPAs because of the area limitation. To effectively overcome this problem, a two step integration method is proposed. First, after integrating the current of the microbolometer for 32&mgr;s, upper 5bits of the 13bit digital signal are output through a pixel-level ADC. Then, the current of the microbolometer is integrated during 1ms after the skimming current correction using upper 5bits in a field-programmable gate array (FPGA), and then lower 8bits are obtained through a pixel-level ADC. Finally, upper 5bits and lower 8bits are combined into the digital image signal after the gain and offset correction in digital signal processor (DSP) Each 2×2 pixel shares an readout circuit, including a current-mode background skimming circuit, an operational amplifier(op-Amp), an integration capacitor and a single slope ADC. When the current of a microbolometer is integrated, the integration capacitor is connected between a negative input and an output of the op-Amp. Therefore a capacitive transimpedance amplifier (CTIA) has been employed as the input circuit of the microbolometer. When the output of a microbolometer is converted to digital signal, the Op-Amp is used as a comparator of the single slope ADC. This readout circuit is designed to achieve 35×35&mgr;m2 pixel size in 0.35&mgr;m 2-poly 3-metal CMOS technology.
© (2007) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
C. H. Hwang, C. B. Kim, Y. S. Lee, B. G. Yu, and H. C. Lee "Pixelwise readout integrated circuits with pixel-level ADC for microbolometers", Proc. SPIE 6542, Infrared Technology and Applications XXXIII, 654221 (14 May 2007);


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