KAF−1603
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4
Output Structure
Charge presented to the floating diffusion is converted
into a voltage and current amplified in order to drive off-chip
loads. The resulting voltage change seen at the output is
linearly related to the amount of charge placed on the
floating diffusion. Once the signal has been sampled by the
system electronics, the reset gate (fR) is clocked to remove
the signal, and the floating diffusion is reset to the potential
applied by Vrd (see Figure 4). More signal at the floating
diffusion reduces the voltage seen at the output pin. In order
to activate the output structure, an off-chip load must be
added to the Vout pin of the device such as shown in
Figure 5.
Dark Reference Pixels
There are 4 light shielded pixels at the beginning of each
line, and 12 at the end. There are 4 dark lines at the start of
every frame and 4 dark lines at the end of each frame. Under
normal circumstances, these pixels do not respond to light.
However, dark reference pixels in close proximity to an
active pixel can scavenge signal depending on light intensity
and wavelength and therefore will not represent the true dark
signal.
Dummy Pixels
Within the horizontal shift register are 10 leading
additional pixels that are not associated with a column of
pixels within the vertical register. These pixels contain only
horizontal shift register dark current signal and do not
respond to light. A few leading dummy pixels may scavenge
false signal depending on operating conditions. There are
two more dummy pixels at the end of each line.
Image Acquisition
An electronic representation of an image is formed when
incident photons falling on the sensor plane create
electron-hole pairs within the sensor. These photon-induced
electrons are collected locally by the formation of potential
wells at each photogate or pixel site. The number of
electrons collected is linearly dependent on light level and
exposure time and non-linearly dependent on wavelength.
When the pixel’s capacity is reached, excess electrons will
leak into the adjacent pixels within the same column. This
is termed blooming. During the integration period, the fV1
and fV2 register clocks are held at a constant (low) level,
and the sensor is illuminated. See Figure 9. The sensor must
be illuminated only during the integration period. Light
must not reach the sensor during the time the image is read
out. This is usually accomplished with the use of
a mechanical shutter or a pulsed light source.
Charge Transport
Referring to Figure 10, the integrated charge from each
photogate is transported to the output using a two-step
process. During this readout time, the sensor needs to be
protected from all light through the use of a shutter or pulsed
light source. Each line (row) of charge is first moved from
the vertical CCD to the horizontal CCD register using the
fV1 and fV2 register clocks. The horizontal CCD is
presented a new line on the falling edge of fV2 while fH1
is held high. The horizontal CCD then transports each line,
pixel by pixel, to the output structure by alternately clocking
the fH1 and fH2 pins in a complementary fashion. On each
falling edge of fH2 a new charge packet is transferred onto
a floating diffusion and sensed by the output amplifier.
