In ultra-low light conditions the presence of dark current becomes a major source of noise for a CMOS sensor. Standard
dark current compensation techniques, such as using a dark reference frame, bring significant improvements to dark
noise in typical applications. However, applications requiring long integration times mean that such techniques cannot
always be used. This paper presents a differential dark current compensating pixel. The pixel is made up of a differential
amplifier and two photodiodes: one light shielded photodiode connected to the non-inverting input of the opamp and a
light detecting photodiode connected to the inverting input of the opamp. An integrating capacitor is used in the feedback
loop to convert photocurrent to voltage, and a switched capacitor network is present in parallel with the light shielded
pixel, which is used to satisfy the output equation to compensate the dark current. The pixel uses 150 μm x 150 μm
photodiodes and is fabricated in a standard 0.18 μm, 6M1P, CMOS process. The results show that the pixel is light
sensitive and has a linear output as expected. However, the dark current is not predictably controlled. Further work will
be carried out on the pixel design, and particularly the switched capacitor circuit, to determine the cause of the non-predictability
of the pixel output.
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