Large area X-ray CMOS digital pixel sensor based on pulse width modulation for high frame rate applications

In wafer-scale CMOS image sensors, the 3-transistor structure suffers from its low speed. To overcome this limitation, we propose a digital pixel sensor which has a digital-pixel output instead of analog-pixel output as in a conventional 3-transistor pixel. The digital pixel sensor can provide a high frame because it eliminates analog-to-digital conversion time. In addition, it removes the noise from long analog-signal paths. As a prototype, we designed a 4.5mm × 4.0mm chip having 24 × 16 pixels of 100μm × 100μm using standard 0.18μm CMOS technology. With 2MHz clock, the readout time for each pixel was 6micro seconds. When 3000pixels are in each column of a full-wafer-size CMOS image sensor, 50fps can be achieved with the digital pixels based on pulse width modulation.