EPC901 Datasheet, PDF (13/37 Pages) Espros Photonics corp – CCD line sensor 1024x1 pixel

5. Imager operation
5.1. General remarks
The epc901 line imager chip is based on a backside illumination technology (BSI). The image is taken from the backside of the chip,
whereas the electrical circuits and the pins are on the frontside. Thus, the chip must be flip-chip mounted to the PCB in order to expose the
backside to the incoming light.
It is not possible with BSI to shield the photosensitive area with an integrated, electrically controlled shutter when there no light shall be
detected. In other words, the pixel CCD is continuously photo-sensitive and collects charge generated by the detected photons (unwanted
exposure). Thus, the CCD must be flushed or erased by the unwanted charge before an image can be acquired.
READ Pulse - The pulse applied to pin READ_CLK to initiate the read-out of a frame.
READ Clock - The clock applied to pin READ_CLK to read out the frame (after the Read Pulse).
5.2. Image acquisition
After the exposure time defined by the user, the charge collected in the CCD has to be shifted into an area which is not photo-sensitive.
This area is called frame store. The shift process samples the charge collected by the CCD in the frame store. The following diagram
shows this operation.
CCD operation
(unwanted)
exposure
flush CCD
T_FLUSH
automatic
CCD flush
exposure to
acquire an
image
automatic
shift to frame
store (FIFO)
(unwanted)
exposure
T_FLUSH
exposure time
T_SHIFT
CLR_PIX
SHUTTER
T
PULSE_CLR_PIX
DATA_RDY
Figure 7: Image acquisition timing
The acquisition of a frame is controlled by a pulse on pin SHUTTER (see Figure 7). The rising edge of a pulse on SHUTTER triggers the
internal flush operation to erase the CCD from any unwanted electrons prior to the exposure. The exposure starts automatically after the
flush operation is completed. The CCD collects electrons thereafter as long as the SHUTTER pin is high. Upon the falling edge of
SHUTTER, the charge collected in the CCD pixel field is automatically shifted to the CCD frame store which is a photo-insensitive area
(see (1) in Figure 8). As soon as the frame is ready for read-out, the signal DATA_RDY goes high. The image is ready to be readout as
described in section 5.4..
SHUTTER
DATA_RDY
READ_CLK
T
SHIFT
T
CDS
(1)
(2)
(3)
Figure 8: Illustrative example of single-frame acquisition and read-out
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Characteristics subject to change without notice
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Datasheet_epc901-V5.6
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