XRD98L56 Datasheet, PDF (23/35 Pages) Exar Corporation – CCD Image Digitizers with CDS, PGA and 10-Bit A/D

XRD9855/9856
XRD98L55/98L56
Frame calibration uses the OB lines available at the
start and end of the array, see the dark shaded regions
at the top and bottom of Figure 15, to perform its auto-
calibration.
The dark shaded regions of Figure 15 are the OB lines
at the start and end of the CCD array. Typically, these
OB lines are the largest blocks of OB pixels available
on the array. Using these areas will allow the XRD9855
and XRD9856 to achieve calibration before any active
pixels are processed. This means that the XRD9855
and XRD9856 can achieve calibration for the very first
frame if OB lines are used for calibration at the start of
the array.
Active Pixels
The timing needed for Frame Calibration Mode is
shown in Figure 16. In Frame Calibration Mode,
Enable_Cal needs to be active during the OB line
output from the CCD. Enable_Cal gates the XRD9855
and XRD9856’s auto-calibration logic and must never
be high when CLAMP is active. Clamp still needs to be
active once a line, either during start of line or end of
line OB pixels.
Frame calibration is useful for applications where fast
calibration is needed. With frame calibration, the
XRD9855 and XRD9856 can achieve calibration be-
fore the first frame is started.
Frame Calibration
(OB) Pixels
Optically Black
(OB) Pixels
N+1
N
Frame Calibration
(OB) Pixels
Figure 15. OB Lines Used For Frame Calibration on a Typical CCD Array
Line N
Active Video
pixels on
OB LINES
OB*pixels
CCD Signal
EnableCal
Clamp
RSTCCD
SHP
SHD
Vertical Shift
Line N+1
Dummy &
OB*pixels
Active Video
pixels on
OB LINES
Rev. 1.01
* Note: OB = Optically Black or Shielded pixels.
Figure 16. Frame Calibration Mode Timing, CLK_POL= High
23