Image Processing Reference
In-Depth Information
V
dd
V
dd
V
in
V
in
Drive
transistor
Drive
transistor 1
Drive
transistor 2
V
out
V
out
V
LG
V
LG
Load
transistor
Load
transistor 2
Load
transistor 1
(a)
(b)
FIGURE 2.25
Configuration of SFA: (a) single-stage configuration; (b) dual-stage configuration.
According to the necessary frequency characteristic and the necessary ability to drive
the following stage circuit, SFA is sometimes used with single-stage and other times with
dual- or triple-stage cascade arrangements, as Figure 2.25 indicates.
2.3.3 Correlated Double Sampling Circuit
When signal charges
Q
are introduced in FD, the essential information is
V
Q
in Figure 2.23c,
because it has the direct information about charge quantity
Q
. Therefore, the difference
between the potentials of the capacitor
C
before and after reception of signal charge is
(ϕ
S
− ϕ
B
), as shown in Figure 2.23b and c. A circuit to get the difference between the two is
a correlated double sampling (CDS) circuit. The reset levels in Figure 2.23b and c are the
same, because the level is set by a single reset operation, that is, they are correlated to each
other.
In actuality however, a small charge amount
Q
cannot drive a CDS circuit. So, after
multiplication of the amount of signal charge
Q
by SFA, the CDS operation is achieved.
Figure 2.26 shows that the output of the FDA, which uses an SFA as a buffer, is followed
by a CDS circuit.
In this way, the difference of output of SFA between input ϕ
S
and ϕ
B
is expressed as fol-
lows, referring to Equation 2.5:
(
)
−
(
)
(
)
-
φ
−
VG VG VG
V
φ
=−
φ
(2.6)
S
th
B
th
S
B
V
V
that is, the input potential difference (ϕ
S
− ϕ
B
) multiplied by SFA voltage gain
G
V
; and
the threshold voltage of the drive transistor,
V
th
, has vanished. As will be discussed in
Chapter 5, an SFA in a CCD has little influence on the value of
V
th
because only one SFA is
commonly utilized. However, in cases when SFAs are arranged in each pixel like CMOS
sensors, the variation of
V
th
, which can be as low as a few millivolts, will have a serious
influence as a fixed-pattern noise on the image quality so that the disappearance of the
V
th
influence is very important.
