ABSTRACT Inspired by biological information scheme, pulse frequency modulation (PFM) technique is robust for noise sources due to its digital encode of analog signals. In a viewpoint of image sensors, PFM is also useful for a wide dynamic range and has already been demonstrated over 60 dB. We have designed a pixel circuit of a CMOS image sensor using PFM for the next generation architecture of vision chips. The chip is fabricated using a standard 0.35 µm double poly, triple metal CMOS technology. The photodiode is a parasitic pn diode between p-well and n-diffusion with the size of 2 µm squares. The top of the photodiode is covered with third metal and 1 µm square hole is open for aperture. Feedback circuits consist of a Schmitt trigger and two inverters. We have demonstrated by introducing PFM the chip works well under the power supply voltage of 0.55V with 50 dB. Such a low voltage operation suggests deep sub-micron technologies, for example, 0.18 µm technologies could be applied to the sensor. The other important point in our chip is that the photodiode is very small in size of 2 µm x 2 µm with the aperture size of 1 µm x 1 µm. This enables us to realize an image sensor with a small fill factor, which is very useful for vision chips where functional circuits are integrated in each pixel
