Tech Briefs Magazine - September 2024 - PIT-2

Analysis and
Simulation of
Low-Light INL
in CMOS Image
Sensors
W
ith the increase in demand for higher resolution
image sensors, pixel pitch has been reduced
to fit a larger number of pixels within
the same sensor size. In order to read at the
same frame rate, multiple rows must be read concurrently.
This requires multiple analog to data converters (ADCs) per
pixel pitch. ADC pitch has been reduced even further as a result,
requiring a tighter layout. The likelihood of parasitic
coupling increases, which manifests as electrical crosstalk. In
single-slope ADC architecture, ADCs have some shared common
nets, including supplies, grounds, biases and ramp. A
larger number of ADCs converting at the same time can cause
kickback on these shared nets. This can lead to non-idealities,
one of which is nonlinearity.
In addition to traditional sources of nonlinearity, array-level
effects become more visible due to the larger and denser array
size of the ADCs. These can cause dips in the linearity plot
which are tedious to correct and require multiple correction
curves. Traditionally, optically black columns are used for row
noise correction and offset cancellation. Since they all have
similar signal levels, they convert at the same time and introduce
a disturbance on shared nets. This can create nonlinearity
in the darker regions of an image, which is more apparent
to the human eye. In addition, the number of ADC conversions
in the dark can change depending on the scene. As a
result, the magnitude of the disturbance varies and makes it
more difficult to correct. It is therefore better to eliminate the
problem at the source.
Single-Slope ADC and Linearity
Column-parallel ADC architecture and a single-slope ADC
schematic are shown in Figure 1 and Figure 2 respectively.
The ADCs use correlated double sampling (CDS) to read
the pixel voltage. Auto-zeroing (AZ) is completed with respect
to the pixel reset level. The ramp is held at a fixed ref2
Figure
1: Column-parallel readout architecture. (Image: Forza Silicon)
Photonics & Imaging Technology, September 2024
Lead Image: Forza Silicon

Tech Briefs Magazine - September 2024

Table of Contents for the Digital Edition of Tech Briefs Magazine - September 2024

Tech Briefs Magazine - September 2024 - Intro
Tech Briefs Magazine - September 2024 - Sponsor
Tech Briefs Magazine - September 2024 - Cov1
Tech Briefs Magazine - September 2024 - Cov2
Tech Briefs Magazine - September 2024 - 1
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Tech Briefs Magazine - September 2024 - 16A
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Tech Briefs Magazine - September 2024 - Cov3
Tech Briefs Magazine - September 2024 - Cov4
Tech Briefs Magazine - September 2024 - PIT-Cov1
Tech Briefs Magazine - September 2024 - PIT-Cov2
Tech Briefs Magazine - September 2024 - PIT-1
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