A 2erms− Temporal Noise CMOS Image Sensor With In-Pixel 1/f Noise Reduction and Conversion Gain Modulation for Low Light Imaging

This work presents a low noise active pixel sensor. It uses one additional transistor compared to standard 4T pixel to obtain 1/f noise reduction and high conversion gain. 1/f noise is reduced by periodically switching the in-pixel source follower between depletion and inversion. The depletion state is achieved by re-configuring the source follower into a MOS capacitor and depleting the channel by controlling the source-drain terminal. The state change of the in-pixel source follower also enhances the conversion gain. The test chip has a 64 × 64 pixel array with a 10 μm pixel pitch fabricated in 350 nm AMS process. The measured integrated noise of a pixel in the frequency band 128 Hz to 50 KHz is 728 μV
_rms
without switching. At a switching frequency of 4 MHz, the integrated noise reduces to 306 μV
_rms
after switching resulting in a 7.5 dB reduction. The input referred noise is reduced from 26e
_rms
^-
to 7.3e
_rms
^-
after applying switching. When both, switching and conversion gain enhancement are applied, the input referred noise is further reduced to 2.3e
_rms
^-
with an enhanced conversion gain of 398 μV/e
^-
.