ACS-1024-LG Photon Vision Systems, Inc., ACS-1024-LG Datasheet - Page 5

• Manufacturer Part Number: ACS-1024-LG
• Description: Image Sensor, CMOS|CCD, 1026x1026 Pixels, 132pin LCC
• Manufacturer: Photon Vision Systems, Inc.

This is a review of the ACS pixel operations. 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 a separate sampling of the background 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 four allowed operations for sensing the
video signal. In the first method, named “Rolling Shutter”
with normal CDS, the background is sampled on the Hold
capacitor and then the pixel video value is transferred and re-
sampled. The background signal is automatically subtracted
from the pixel video value to provide clean video with a
minimum level of noise. In the second method, “Rolling
Shutter” with reverse CDS, the video signal is read out first,
and then the pixel is reset and the background read out. The
third method named, “Snapshot Shutter” is useful for freeze
frame operations. In this method, the video signal is sampled
on the Hold capacitor and the pixel is reset and re-sampled
for background information. Once again the background is
subtracted from the video value to provide clean video. The
fourth method, named “Adaptive Exposure Control”, allows
the pixel to be read without being rest. This allows the video
to be sampled and re-sampled until the desired signal has
accumulated. Then the pixel is reset and re-read for video
background information.
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.
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 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.
The “Rolling Shutter” CDS method described above reduces KTC noise. 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 (CCD’s) and in effect is only sensed and left
Active Column Sensor Series Pixel Operations
Product DATASHEET ACS PDS0003 Subject to change without notice. Page 5 of 26
A row operation is defined as all pixels in a row as having the same signal or operation performed at
Photon Vision Systems, Inc.
Copyright© 2002 Rev A
Video S.
Figure # 1 ACS Pixel
Hold
Capacitor
N Well or N+
Bias FET
Select FET
Pixel
Hold FET
Video D.
Row Select
Reset Select
Hold Select
Reset Bias