A CMOS Indirect Time of Flight Sensor with Active Reset Technique for Reset Noise Suppression

Abstract

Image sensors especially 3-transistor image sensor and I-ToF image sensor have been suffered from reset noise which is represented as kTC noise. Due to their operational properties, reset noise is always remained in photodiode or FD nodes. The noise from reset operation can affect the quality of image. In 3-transitor image sensor, the output image from reset operation of the sensor has noise on the image itself and that is getting worsen in the low light intensity where the light signal is not that much larger than the noise from image sensor. Likewise, reset noise affects the depth accuracy of I-ToF sensor. Because the signal from reflected light is not accurate due to the reset noise. To overcome those kind of problems, active reset technique is proposed. The active reset technique is circuit technique for reset noise suppression. Active reset requires additional components such as an amplifier and loop connection switches. And the noise contribution from those additional components can be neglect. First, conventional hard reset is performed. Since the conventional hard reset can make the same initial state within few nano seconds, there is no residual electrons generated by the previous frame. Then the active reset is activated. The negative feedback helps to suppress the sampled hard reset noise. No matter how much noise is sampled on the photodiode or FD node in I-ToF sensor Thanks to the active reset technique, the estimated reset noise can be inversely proportional to the square root of the gain of amplifier. It means that the reset noise can be suppressed up to 96 %. And the actual measurement results show that the reset noise is suppressed as 92% compared with the reset noise from conventional reset operation.