XRD98L63 Datasheet, PDF (19/41 Pages) Exar Corporation – CCD Image Digitizers with CDS, PGA and 12-Bit A/D

XRD98L63
BLACK LEVEL OFFSET CALIBRATION
To get the maximum color resolution and dynamic
range, the XRD98L63 uses a digitally controlled cali-
bration circuit to correct for offset in the CCD signal as
well as offset in the CDS, PGA & ADC signal path. This
calibration is done while the CCD outputs Optical Black
(OB) pixels.
CCD
signal
CDS
PGA
12-bit
Reg
ADC
DB[11:0]
Black Level
Offset Calibration
Loop
Offset Calibration
Logic
+
OB[7:0]
-
Figure 10. Simplified Block Diagram of
Black Level Offset Calibration Loop
Offset Register
D9
D8
D7
D6
D5
D4
D3
X
X
0
0
0
0
0
X
X
0
0
0
0
0
.
.
.
X
X
1
1
1
1
1
X
X
1
1
1
1
1
In the default "Line" timing mode, OB pixels are
sampled when CAL is active at the start, or end, of
each CCD scan line. CAL can be programmed to be
active high or active low; please see the Timing section
for more details about clock polarity. Averaging will
span as many lines as needed to get the number of OB
pixels programmed by Avg[1:0]. Updates to the offset
DACs occur during the Optical Black pixel time after a
complete iteration. A complete iteration includes the
pixel clipping, averaging, calculation of the offset
difference, and calculation of the DAC update values.
After a complete iteration, the averager is reset, and
the logic waits for the number of lines programmed in
the “Wait A” & “Wait B” registers, WL[11:0], before
starting the next iteration.
D2
D1
D0
0
0
0
0
0
1
1
1
0
1
1
1
ADC Output
Black Level (LSB)
0
1
.
.
.
254
255
Table 3. Black Level Output Control
Hot Pixel Clipper
CCD’s occasionally have hot pixels. These are defec-
tive pixels, which always output a bright level. To
ensure the Black Level is not affected by hot pixels in
the OB area, the Hot Pixel Clipper limits pixel data from
the ADC to a maximum value of 511 (1FFh). This
clipping only affects the data used by the internal
calibration logic. Data on the ADC output bus,
DB[11:0], is not clipped.
4096
Pixel Averager
After the clipper, the logic takes an average of the
Optical Black pixels. The number of pixels to be
averaged can be selected as one of the following: 32,
64, 128, or 256. The Avg[1:0] bits in the Calibration
register are used to program the number of pixels to
average. This averaging function filters out noise and
prevents image artifacts. The calibration logic will
average OB pixels over as many lines as required to get
the programmed number of pixels to average.
511
0
0
511
2048
A D C D a ta
4096
Figure 11. Hot Pixel Clipper
Rev.1.01
In the Multiple Gain Mode, the logic keeps separate
avarages for even and odd lines.
Avg[1]
0
0
1
1
Avg[0]
0
1
0
1
# of Pixels
to Average
32
64
128 (default)
256
Table 4. Programming the Pixel Averager
19