XRD9818 Datasheet, PDF (19/28 Pages) Exar Corporation – 3-CHANNEL 16-BIT LINEAR CCD/CIS SENSOR SIGNAL PROCESSOR

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XRD9818
3-CHANNEL 16-BIT LINEAR CCD/CIS SENSOR SIGNAL PROCESSOR
REV. 1.0.1
4.2 Programmable Offset Adjust
The offset adjustment circuitry of the XRD9818 is designed to compensate for any offsets present in the CCD
or CIS output signal and/or overall scanner system. The total range of compensation available is -280mV to
+380mV. This is achieved via a 10-bit Offset DAC that applies gain independent offset correction. The 10-bits
of control is broken into 2 ranges, course and fine. There are 2 bits of course control that is designed to
remove offsets in 100mV increments. The remaining 8-bits determine the fine control and has a range of
+/-180mV in 1.4mV increments. Each channel has its own independent offset control.
FIGURE 9. XRD9818 CHANNEL OFFSET BLOCK DIAGRAM
Input
CDS
SH
9-Bit
V_A
PGA
V_B
3:1
MUX
16-Bit
ADC
8-Bit Offset DAC
Fine Adjust
2-Bit Offset
Course Adjust
Offset Block
The offset correction range of both the Course and Fine DAC’s are shown in Figure 10. The Course DAC has
four settings: 0mv, -100mV, 100mV and 200mV. The 2 msb’s, D[9:8], select the desired course offset setting.
The Fine DAC has a range of +/-180mV. The Offset registers 8 lsb’s, D[7:0], select the desired fine setting.
Bits D[6:0] program the magnitude while D[7] selects the polarity of the Fine DAC’s compensation.
FIGURE 10. OFFSET CORRECTION (COURSE & FINE DAC’S)
Course DAC Settings
COx[1:0]
00
01
10
11
Offset (mV)
0
-100
100
200
150mV
Fine DAC
10h
Range _{00h 0Fh}
-150mV
FFh
Code
As can be seen in the Course Offset DAC’s range settings there is more correction range in the positive
direction. This allows the system designer to maximize the usable offset correction range of the XRD9818 for a
variety of imagers. Positive offset is defined as the normal offset direction found in either a CCD or CIS input
signal, as shown in Figure 11.
FIGURE 11. SIGNAL OFFSET POLARITY (CCD AND CIS)
CCD
Negative Offset
Positive Offset
S ig n a l
Ideal Black
CCD Dark Offset
W hite
C IS
S ig n a l
Positive Offset
Negative Offset
W hite
Ideal Black
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