Low-voltage operation of a CMOS image sensor based on pulse frequency modulation

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.