ACS-I1024-CPGA Photon Vision Systems, Inc., ACS-I1024-CPGA Datasheet - Page 17

• Manufacturer Part Number: ACS-I1024-CPGA
• Description: High Performance Area CMOS Image Sensor
• Manufacturer: Photon Vision Systems, Inc.

Active Column Sensor Series Pixel Operations
This is a review of pixel operations as allowed by ACS sensor configuration. Figure # 1, is representative of the pixel
schematic of the ACS sensor. Normal pixel operation consists of a sequence of steps that involves resetting the pixel
and the hold site, an integration period to accumulate the video signal, a sampling of the video signal and back ground
information, and then starting the process again for the next frame of video. The order of steps may change and some
steps may be repeated within a cycle, but the fundamental steps are included. There are three allowed operations for
sensing the video signal. The first method, named “Rolling Shutter”, the background is sampled on the Hold
capacitor and then the pixel video value is transferred and re-sampled for KTC reduction. The second method named,
“Electronic Shutter” is useful for freeze frame operations. In this method the video signal is sampled on the Hold
capacitor to end exposure and store the video value and the pixel is reset and re-sampled for video background
information. The third method, named “Adaptive Exposure Control” for nondestructive read operations, allowing the
video to be sampled and re-sampled until the desired signal has accumulated, the pixel is then reset and re-read for
video background information. Which method is utilized will depend on the application. The detailed discussion that
follows will assist.
Before we begin the detailed discussion of the pixel operations, the pixel structure and its components will be
described. All pixel operations are accomplished on a row-by-row or global basis. A row operation is defined as all
pixels in a row as having the same signal or operation performed at the same time. A global operation is defined as the
signal or operation is applied to all pixels on the array at the same time. All descriptions herein involve row operations
unless otherwise noted. Referring to Figure # 1, at the top of the drawing is the dual gate Select FET of the Active
Column Sensor (ACS). This dual gate FET has the dual purpose of selecting a row of pixels as well as sensing the
described above allows for KTC noise reduction. This read-out method starts with the Hold FET and the Bias FET
selected. This allows for the pixel and the capacitor to be reset to the initial reset bias. The Bias FET and the Hold
FET are subsequently released or deselected. The pixel is allowed to integrate for one frame time and the row is
selected for reading. The value in the Hold capacitor that was set to the initial reset values is read first. The Hold FET
is selected, allowing the Pixel value to be read next. The reset and pixel values are subtracted from each other for
virtually pattern noise free video. The process is repeated from frame to frame in normal rolling shutter video and the
same can be accomplished in a global frame sequence. Reading or sensing are used interchangeably for the ACS
sensors as the accumulated pixel signal is only sensed when read. The signal isn’t destroyed as in Charge Coupled
Devices (CCDs) and in effect is only sensed and left undisturbed for further reading allowing noise elimination or
Video S.
Figure # 1 ACS Pixel
Hold
Capacitor
N Well or N+
Product DATASHEET ACS-I XXX Rev. 10/31/00 Subject to change without notice. Page 17 of 26
Bias FET
Select FET
Pixel
Hold FET

Video D.
Photon Vision Systems, Inc. 1999, 2000
Row Select
Reset Select
Hold Select
Reset Bias
PRELIMINARY
accumulated charge within the pixel. The Row
Select line turns each row of Select FETs “on”.
The select FET is part of a Unity Gain
Amplifier, with the remainder of the UGA
located at each column and is the reason that
the source and drain of the Select FET run the
length of the column of pixels. The UGA
configuration per pixel eliminates the pixel to
pixel variations known as Fixed Pattern Noise
that plague other image sensors. The next FET
below the Select FET is the Hold FET that is
controlled by the Hold Select line. When the
Hold FET is selected or turned “on” and turned
back “off”, or deselected, the value in the pixel
photodiode is sampled and stored on the
capacitor. The value stored could either be the
reset value the pixel is reset to or the
accumulated pixel value after the pixel has
integrated light for a given time period. When
the row is selected and the Hold FET is
deselected the value stored in the capacitor is
the value sensed by the UGA. The next FET is
the Bias FET, when selected biases the Pixel to
the reset value via the Rest Bias line.
In more detail the “Rolling Shutter” method