3D Time-of-flight distance measurement with custom - Universität ...

SOLID-STATE IMAGE SENSING 69
The standard deviation of the photon shot noise ∆Nshot is equal to the square root of
the number of photons or photogenerated charge carriers N:
∆ Nshot = N
Equation 3.14
Photocharge conversion noise includes all noise sources that disturb the “optical“
information in the process chain of converting the optically generated electron hole
pairs into an analogous output signal. Figure 3.15 illustrates a source follower
stage as a typical example of a detection stage in semiconductor imagers.
Photocharge conversion noise is composed of several contributions, viz. reset
noise, 1/f noise (flicker noise), amplifier Johnson noise (thermal noise) and shot
noise of dark current. All these contributions increase with temperature.
reset
V reset
C
output
diffusion
Figure 3.15 Source follower photo detection stage. The output diffusion is directly
connected to the gate of a source follower transistor. The lightproportional
amount of charge collected on the capacitance C of the
output diffusion node controls the current in the first amplifier stage.
By activating the reset switch, the charge on the output node can be
dumped to the reset source. [PS2].
Reset noise, also known as kTC noise, describes the Johnson noise (resistor
noise) influence of the transistor channel of the reset transistor on the charge at the
output diffusion. Every time the reset transistor is activated, the output diffusion is
charged to the reset voltage Vreset. This corresponds to a certain amount of reset
charge Qreset on the conversion capacitance C (capacitance of the output diffusion
node) varying by the reset charge noise ∆Qreset.
∆ Qreset = k ⋅ T ⋅ C
Equation 3.15