  In this paper, an in-pixel chopping technique to reduce the low-frequency or
1/f noise of the source follower (SF) transistor in an active pixel sensor
(APS) is presented. The SF low-frequency noise is modulated at higher
frequencies through chopping, implemented inside the pixel, and in later stage
eliminated using low-pass filtering. To implement the chopping, the
conventional 3T APS architecture is modified, with only one additional
transistor of minimum size per pixel. Reduction in the noise also enhances the
dynamic range (DR) of the image sensor. The test circuit is fabricated in UMC
0.18 um standard CMOS technology. The measured results show a reduction of 1/f
noise by approximately 22 dB for 50 MHz chopping frequency.
