Tech Briefs Magazine - September 2024 - PIT-3
erence level and the pixel TG level is sampled. This level is
compared with a ramp signal ranging from high to low in this
scenario. At the moment the ramp signal and TG level are the
same, a latch pulse is generated.
This pulse is used to latch the counter value, which is the
desired digital code corresponding to the pixel signal. In an
ADC array, there is one ADC per column bus line. The supplies,
grounds, biases and ramp are shared by all the ADCs in
the array. As a result, any disturbances on these shared nets -
which are caused by kickback as sections of the ADC array convert
simultaneously - will be common for all the ADCs when
reading a particular row. Traditionally, optically dark columns
are added on the side of the active pixel array for correcting
any row noise or fixed offsets.
Linearity measures the difference between the measured
output and the ideal output. It is quantified in terms of integrated
nonlinearity (INL). The typical sources of INL in
CMOS image sensors include the pixel output source follower,
VLN current source, ramp, the sampling capacitor at the
ADC front-end, and the preamplifier. The contribution of the
pixel source follower to INL is primarily due to the body effect,
which increases the threshold voltage of the device as the
source voltage increases. This is known as dark signal (Figure
3). VLN current changes as the drain-source voltage of the
VLN device varies (channel length modulation). Variation in
the VLN current affects the transconductance (gm) of the
output source follower of the pixel, making its gain signal dependent
(1) where Rs is output impedance of VLN.
Using a cascode VLN current source helps to reduce the effect
of channel length modulation. However, this has the side
effect of reducing the usable range of the pixel signal as the
VLN device can get out of saturation at bright signal levels.
Ramp is another major source of INL, particularly at dark signal
levels, due to the finite resistance of the current source in
the ramp generator. Finally, in the ADC, the primary sources of
INL are the sampling circuit and the preamp. Sampling circuit
INL occurs because of the signal-dependent resistance of the
switch and the signal-dependent capacitance of the sampling
cap, which affects the settling error of the pixel reset level and
pixel TG level.
Sources of Low-Light INL and Possible
Solutions
As described in Section 2, the ADCs all share the supply/
ground, biases and ramp. If a significant number of ADCs convert
simultaneously, a kickback will occur on the shared nets. Since
CDS is used to cancel random offsets of the pixels and ADCs, conversions
for the same signal level occur in close proximity. As a
dd_az_pre
dd_adc_samp
V IN
VRamp Preamp
CS/H
force_latch
Figure 2: A typical single-slope ADC. The pixel column bus is read using CDS operation. (Image: Forza Silicon)
Photonics & Imaging Technology, September 2024
Comparator
Ccc1
Ccc2
dd_az_comp
adc_en
One Shot
Logic
One Shot
Logic
latch
3
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
Tech Briefs Magazine - September 2024 - 16B
Tech Briefs Magazine - September 2024 - 16C
Tech Briefs Magazine - September 2024 - 16D
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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
Tech Briefs Magazine - September 2024 - PIT-2
Tech Briefs Magazine - September 2024 - PIT-3
Tech Briefs Magazine - September 2024 - PIT-4
Tech Briefs Magazine - September 2024 - PIT-5
Tech Briefs Magazine - September 2024 - PIT-6
Tech Briefs Magazine - September 2024 - PIT-7
Tech Briefs Magazine - September 2024 - PIT-8
Tech Briefs Magazine - September 2024 - PIT-9
Tech Briefs Magazine - September 2024 - PIT-10
Tech Briefs Magazine - September 2024 - PIT-11
Tech Briefs Magazine - September 2024 - PIT-12
Tech Briefs Magazine - September 2024 - PIT-13
Tech Briefs Magazine - September 2024 - PIT-14
Tech Briefs Magazine - September 2024 - PIT-15
Tech Briefs Magazine - September 2024 - PIT-16
Tech Briefs Magazine - September 2024 - PIT-17
Tech Briefs Magazine - September 2024 - PIT-18
Tech Briefs Magazine - September 2024 - PIT-19
Tech Briefs Magazine - September 2024 - PIT-20
Tech Briefs Magazine - September 2024 - PIT-21
Tech Briefs Magazine - September 2024 - PIT-22
Tech Briefs Magazine - September 2024 - PIT-23
Tech Briefs Magazine - September 2024 - PIT-24
Tech Briefs Magazine - September 2024 - PIT-25
Tech Briefs Magazine - September 2024 - PIT-Cov4
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