Image Processing Reference
In-Depth Information
(
V
dd
-
V
th
)
G
V
Gradient: -
G
V
/
C
in
0
Signal charge quantity
Q
sig
FIGURE 5.40
Signal charge quantity dependence of output voltage of a pixel in the case of an electron.
voltages are obtained by a differential circuit. Figure 5.42b shows a correlated double
sampling (CDS) circuit,
31
discussed in Section 2.3.3.
In recent small pixel image sensors, it is hard to arrange a differential circuit as an indepen-
dent circuit in each column because of the larger circuit size and the narrow column pitch.
Therefore, a differential circuit is used as a common circuit at the sensor output or off-chip
differential circuit, although high-frequency driving is necessary. In this case, two signal out-
puts with and without signal charges, having a band frequency around 10-30 MHz, have to
be independently transmitted, so twice the original pixel signal transmitting rate is necessary.
On the other hand, CDS circuits are so much smaller that it is possible to arrange one
at each column; therefore, canceled signals can be obtained at each column. A canceling
operation is carried out at each column by parallel processing within a narrow bandwidth
<1 MHz. This is a big advantage from the viewpoint of noise characteristic. So CDS circuits
are employed more than differential circuits for offsetting cancelation. The operational
method used for offset cancelation in the CMOS sensor,
32
which was mass-produced for
Offset variation canceled
output voltage of pixel A
Offset variation canceled
output voltage of pixel B
=
Output of pixel A
Output of pixel B
0
Signal charge quantity
Q
sig
FIGURE 5.41
Signal charge quantity dependence of output voltage of two pixels in the case of electron.
