DR-Pix_WhitePaper.pdf
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a different approach has emerged to improve inter-
scene DR and sensor performance through the addition
of a high-sensitivity mode to the sensor’s operation.
Two modes are combined in one pixel design – low
conversion gain (LCG) for large charge handling capacity
in bright scenes and a high conversion gain (HCG) mode
with increased sensitivity and low read noise for low-
light scenes, providing tremendous benefit for DSLR,
surveillance, and notebook/PC cameras, as well as
automotive imaging systems, where image sensors are
expected to capture images/video in extreme low-light
conditions without sacrificing the performance in high-
light conditions to do so.
Relationship between Conversion Gain and
Full-Well Capacity
To understand the benefit of adding an HCG mode, it
is important to discuss the trade-off between CG, the
measure of the sensitivity of the charge detection at the
FD node in a CMOS image sensor pixel, and FW in a pixel
design.
The CG is actually an inverse way of expressing the
capacitance of the FD node. Capacitance can be calculated
as the ratio of the amount of charge required to change
the potential of a node by one volt (see Equation 1),
where Q is charge in Coulombs, and V is the potential in
Volts. This means that, as the capacitance of the FD node
increases, the conversion gain and hence, sensitivity of the
FD node decreases.
C= Q/V
Equation 1: Basic formula for capacitance
Introduction
Real-world cameras often encounter both very bright
scenes and very dark scenes. To capture these scenes,
image sensors are typically optimized for one extreme
at the price of degraded performance for the other. The
challenge is how to design an image sensor to work
optimally in all scene conditions.
For the image sensor within these cameras, this translates
into requirements for larger full-well (FW) capacities
as well as increased sensitivity. This is true for both the
larger pixels heavily used in PCs, gaming, automotive,
surveillance, and digital single-lens reflex (DSLR) cameras
and the ever-shrinking pixel sizes used in mobile phone
applications and digital still cameras (DSCs). However,
higher sensitivity could limit FW capacity, the maximum
achievable signal-to-noise ratio (SNR), and the total
dynamic range (DR) of the sensor.
Pixels greater than 2 microns often have their FW capacity
defined by the photodiode’s charge holding capacity,
rather than the pixel’s voltage swing, due to its larger
photosensitive area. To increase the charge handling
capacity of this pixel, it is common to connect a physical
capacitor to the floating diffusion (FD) node. However, this
typically results in lower conversion gain (CG). This, in turn,
means reduced sensitivity and increased input-referred
read noise, thereby compromising low-light sensitivity
and reducing the sensor’s DR even though the sensor is
capable of measuring larger signals.
Many approaches to increasing DR focus on achieving
high intra-scene dynamic range. However, these HDR
techniques do not improve low-light sensitivity or reduce
noise to improve low-light image captures. As a result,
Leveraging Dynamic Response Pixel Technology
to Optimize Inter-scene Dynamic Range
An Aptina
TM
Technology White Paper
aptina.com
©2010 Aptina Imaging Corporation. All rights reserved. 9/10 EN.L
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