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

92 CHAPTER 4
number of pseudo-background-electrons Npseudo to B. (They are not correlated to
the modulation signal and thus contribute to B rather than A.)
L B + Npseudo
∆ L = ⋅
Equation 4.12
8 2 ⋅ A
One obtains this number of pseudo-electrons Npseudo simply by squaring the noiseequivalent
number of noise electrons.
2
2 V
( ) dark noise C
N conv
pseudo # dark noise electrons
q A ⎟
sf
⎟
⎛
⋅ ⎞
= = ⎜
Equation 4.13
⎝ ⋅ ⎠
Example: We assume a measured dark noise of Vdark noise=0.63 mV rms, containing
all noise sources except the photoelectron shot noise, namely the dark current shot
noise and the thermal noise floor. With a typical output amplification of Asf=0.9 and
a conversion capacitance of Cconv=40 fF this corresponds to an equivalent number
of 175 dark noise electrons, leading to 30,000 pseudo-background-electrons.
Vdark noise ≈ 0.63 mV rms
Asf ≈ 0.9
Cconv ≈ 40 fF
V
Q dark noise
equivalent = ⋅ Cconv
≈ 2.8⋅10
Asf
-17 C
Qequivalent
# dark noise electrons = ≈ 175
q
Npseudo=(175) 2 ≈ 30,000.
Demodulation amplitude and effective offset
A closer look at Equation 4.11 and Figure 2.7 shows that the modulated light
source contributes to both offset and demodulation amplitude. Integrated over a
certain integration time Tint, the mean optical power Popt,pixel directly adds a number
of photoelectrons PEopt to the effective offset Beff :
B eff = background + Npseudo
+ PEopt
Equation 4.14
Also the number of demodulation-photoelectrons A can be expressed as a function
of the optical mean power or the total number of photoelectrons per pixel generated
by the modulated light source PEopt. Only the modulation contrast Cmod, a
parameter of the modulated light source, (usually 100%), and the demodulation