Image Processing Reference
In-Depth Information
Figure 5.31a is a schematic diagram of the image area of an MOS sensor. As shown in
the figure, the image area is made up of two dimensionally arrayed pixels, each of which
has one PD, a vertical switch MOSFET, and parts of a row select line and vertical signal
line. For vertical scanning, the output part of the vertical shift register, which selects a line
to read out, called
accessing
, is connected to the row select line to transmit an output pulse
to the vertical switch MOSFET to read out signal charges in the PD. Horizontal scanning,
horizontal shift register, column select line, horizontal switch MOSFET, and horizontal
signal line are arranged the same as in vertical operation. The horizontal signal line is
connected to the video voltage source by way of output resistor and off-chip pre-amplifier.
Operation of the scanning circuit (shift register or decoder), which selects a row or a col-
umn to access, is shown in Figure 5.31b. This is an example of a shift register. The shift
register is activated by a start pulse, which is not shown in the figure at first. Then the
first pulse of the driving clock pulse ϕ
cl
is applied to the input part, and a pulse to select a
row or column appears at Output 1, although no pulse is generated at other outputs. The
next clock input pulse causes a pulse at Output 2. Thus, the output part at which a pulse
appears shifts one by one at every pulse of driving clock pulse ϕ
cl
. As the output position
shifts in series in the shift-register circuit, all outputs need to be scanned. On the other
hand, any output can be accessed in the decoder circuitry. Although the circuit is larger
and more input clock numbers are necessary, this type of circuitry is employed for applica-
tions that need flexible access function such as partial readout.
Operation of MOS sensors is described in Figure 5.32, starting with Figure 5.32a. First, as
shown in Figure 5.32b, a row select pulse generated by the vertical shift register is applied
to vertical switches in the first row by way of the first row select line. Since the vertical
switches turn to on-state, signal charges integrated in PDs on the first lines are moved to
each corresponding vertical signal line in the mode shown in Figure 5.30b. The potential of
the PD just before readout has been reduced from video voltage
V
V
set at the previous read-
out by an amount of the quantity of integrated signal charges. Since the ranges of capaci-
tance of the PD
C
PD
and vertical signal line
C
VSL
are generally around some femtofarads (fF)
Row select line
Vertical switch
MOSFET
Vertical signal line
Pixel
Photodiode
Input
φ
cl
φ
cl
Time
Image
area
Output
Output 1
Output2
Output 3
Horizontal switch
MOSFET
Transversal signal line
Output (
n
-1)
Output
n
Preamplifier
Column
select line
Output resistor
Horizontal shift register
Video voltage
V
V
(a)
(b)
FIGURE 5.31
MOS image sensor: (a) basic configuration; (b) operation of shift register.
