Image Processing Reference
In-Depth Information
In particular, as shown in Figure 3.4a, a capacitor
C
is connected to and separated from a
voltage source
V
d
through a reset transistor. When the reset pulse in Figure 3.4b is applied
to the gate of the reset transistor, ideally the electric potential of node A of one electrode
of capacitor
C
is expected, as shown in Figure 3.4c. That is, node A is settled at voltage
V
d
excepting only the transition period of the clock pulse. But, in reality, the potential of node
A fluctuates around
V
d
while the reset transistor is on-state, as shown in Figure 3.4d, and
node A is settled at the voltage when the reset transistor changes to off-state. Consequently,
the potential of node A varies at each reset operation. This is kTC noise and the type of
kTC noise caused by the reset operation is called reset noise.
The mechanism is shown schematically in Figure 3.5. The equivalent circuitry is shown
in Figure 3.5a, where resistor
R
means the on-resistance of the reset transistor. Figure 3.5b
shows a schematic diagram of the electric charge (electron) distribution while the reset
transistor is on-state. The electric charges move about randomly based on Brownian
motion by the thermal energy of kT, where
T
is the absolute temperature. When the reset
transistor is on-state, the potential distribution is not uniform because of the nonunifor-
mity of the electric charge distribution by the random motion of the electric charges, since
the channel resistance is finite. Here, the readers are expected to imagine a ruffling sur-
face, like a rippling sea, caused by the Brownian motion of electrons. Thus, the fluctuation
of a nonuniform potential profile is retained while the reset transistor is on-state. When
the transistor changes to off, as shown in Figure 3.5c, the charge quantity existing at node
A is retained. Therefore, the potential of node A is set to the potential at that moment.
φ
R
Reset transistor
(b)
φ
R
V
d
A
Q
(c)
C
0
V
d
(FD)
Potential variation at each reset operation
(a)
Reset noise
V
d
(d)
0
FIGURE 3.4
kTC noise: (a) reset circuitry; (b) reset pulse; (c) potential at node A in an ideal case; (d) potential at node A in
an actual case.
Potential nonuniformity in channel resistance caused by
random motion of electrons while the channel is on-state
R
V
d
V
d
C
V
n
Resistance
C
C
R
Potential at the
moment of change to off
(a)
(b)
(c)
FIGURE 3.5
Generating mechanism of kTC noise: (a) equivalent circuitry; (b) electric charge distribution while reset transis-
tor is on; (c) electric charge distribution at the moment of change to off.
