XRD98L63 Datasheet, PDF (20/41 Pages) Exar Corporation – CCD Image Digitizers with CDS, PGA and 12-Bit A/D

XRD98L63
CCD
CDS
+
signal
Gain[9:0]
PGA
+
12-bit ADC
Reg
DB[11:0]
ManCAL
CDE, CDO
From Serial
Interface
Registers
FDE, FDO
W L[11:0]
OBL[11:0]
Even & Odd
Coarse
Accumulators
Even & Odd
Fine
Accumulators
Offset Calibration Logic
Mode
Hold
DNS[1:0]
OB[7:0]
From Gain Logic Gain[9:0]
Black Level
Offset Calibration
Loop
Hot Pixel
Clipper
DNS
Filter
+
+
-
Pixel
Averager
Avg[1:0] OBdel[2:0]
Figure 12. Detailed Block Diagram of the Black Level Offset Calibration Logic
Offset Difference
Next, the Offset register value, OB[7:0], is subtracted
from the OB pixel average. If the difference is positive,
the offset DACs are adjusted to reduce the effective
ADC output code. If the difference is negative, the
offset DACs are adjusted to increase the effective ADC
output code. The DNS option will affect how the DAC
adjustments are made.
Coarse & Fine Accumulators
The Coarse and Fine Accumulators are the registers
which hold the digital codes for the Coarse and Fine
Offset DACs. The Offset DAC adjustments are made
by adding to or subtracting from the value in the Fine
accumulator. If there is an overflow or underflow in the
Fine Accumulator, the Fine Accumulator is reset to it’s
mid-scale value, and the Coarse Accumulator is
incremented or decremented accordingly.
In the Multiple Gain Mode, there are separate accumu-
lators for even and odd lines.
Calibration Options
Digital Noise Suppression (DNS) Filter
The purpose of this option is to eliminate small
changes in the Black Level offset by making the
calibration system less sensitive to small changes in
the measured offset. In this mode, the user has the
option of selecting from three filter settings, see Table 5.
DNS[1]
0
0
1
1
DNS[0]
0
1
0
1
DNS Filter Width
OFF
Narrow (default)
Medium
Wide
Table 5. DNS Threshold Programming
To activate the Digital Noise Suppression mode, write
to the DNS[1:0] bits in the Calibration register.
By default, the Digital Noise Suppression is ON and set
to the narrow filter width.
Rev.1.01
20