XRD98L63 Datasheet, PDF (30/41 Pages) Exar Corporation – CCD Image Digitizers with CDS, PGA and 12-Bit A/D

XRD98L63
MULTIPLE GAIN MODE
The Benefits of Multiple Gain Mode
The Multiple Gain mode switches the gain of the
Programmable Gain Amplifier (PGA) at the pixel rate.
The Multiple Gain logic will switch the PGA gain
according to two user defined patterns. Each pattern
can be from one to four pixels long.
This allows a color digital camera system to set
different PGA (analog) gains for the different color
pixels. Most CCDs with RGB Color Filter Arrays (CFA)
have weaker signal response for the Blue pixels than for
Red or Green pixels. Using the Multiple Gain mode, the
Blue pixels can be amplified with higher gain than the
Red or Green pixels before being digitized by the ADC.
This allows all colors to take advantage of the full ADC
resolution.
Enable the Multiple Gain mode by writing a "1" to the
"MultGain" bit in the Control register.
Even Line Pattern
PRE[1:0]
01
Even Pixel Repeat
Counter
0 to PRE[1:0]
SBLK
Odd Pixel Repeat
Counter
0 to PRO[1:0]
01
PRO[1:0]
Odd Line Pattern
R Gr
10 11
ELP1[1:0]
ELP2[1:0]
ELP3[1:0]
ELP4[1:0]
00 01 10 11
0
1
00 01 10 11
01 00
Gb B
OLP4[1:0]
OLP3[1:0]
OLP2[1:0]
OLP1[1:0]
Overview of Multiple Gain Mode
The Multiple Gain mode is designed assuming the color
filter array is made up of lines (rows) which alternate
between two different pixel patterns. We will refer to the
two patterns as the Even pattern and the Odd pattern.
In a typical camera design using an RGB CFA, the even
lines will have Red & Green alternating pixels, while the
Odd lines will have Green & Blue alternating pixels. The
XRD98L63 allows the patterns to be defined with a
single Green gain used on both Even and Odd lines, or
with two different Green gains for Even and Odd lines.
There are three main steps to setting up and using the
Multiple Gain mode:
1) Select the appropriate Interlaced or Progressive
scan clocking mode.
2) Program the Even and Odd Line Pattern registers
to match the color filter array used on the CCD.
3) Program the Gain Registers.
CDS
PGA
ADC
CAL
PGA mux
00
01
10
11
+Σ
−
+Σ
−
+Σ
−
0
1
+Σ
−
+Σ
−
Line
Pattern
Select
even pattern R Gr
odd pattern Gb B
Example pattern
B
Gb
R
Gr
Rev.1.01
Figure 24. Block Diagram of the Multiple Gain Logic
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