Biomedical Engineering Reference
In-Depth Information
A summary of these noise sources is presented in the following table for MOS
CCDs and photodiode arrays. The choice between selecting a photodiode
CCD
Photodiode
Intrinsic
Bulk traps
Leakage
Photon shot
Leakage
Photon shot
Circuit
Transfer inefficiency
(kT/C)
1/2
Johnson-Nyquist
in output reset operation
(kT/C)
1/2
Johnson-Nyquist
in diode reset operation
MOS electrometer
Signal proc. Amp
Fixed pattern
A/D quantizing noise
and CCD for a given AO system application is dependent principally upon
trade-offs between speed of operation, sensitivity, and dynamic range. In
general, for low temporal noise performance (
Q
T
(NES)/
q
< 200 e
−
) and mod-
erate dynamic range (10
2
), CCDs are superior to photodiode arrays princi-
pally because of the low output sensing capacitance at the electrometer as
opposed to the photodiode capacitance. On the other hand, for high clock
rates (≥5 MHz), sensitivity is principally limited to amplifier noise com-
mon to both CCDs and photodiode arrays to about 500-1000 e
−
, making the
higher dynamic range (10
5
) obtainable with photodiodes attractive.” The
PD-capacitor combination responds to optical energy, so the PD response to
short duration pulses, especially those that are shorter than its integration
time, is a function of the pulse width. It τ is the RF ultrasonic propagation
time across the Bragg cell,
T
s
is the integration time, and PW is the input sig-
nal pulse width, then there are three signal conditions [23]:
(3.11)
PW
>
T
,
τ
<
PW
<
T
,
PW
<
T
s
s
s
A graph of PW versus dynamic range is shown in Figure 3.12.
3.5 DynamicRangeEnhancement
Several optoelectronic techniques exist to extend the dynamic range of
the PD. One technique utilizes two linear PD arrays and a beam splitter.
Light intensity is divided unequally between the two detectors. After the
photodetector which receives the majority of the light intensity saturates,
the second detector array output is utilized. In this way the dynamic
range of the two detector arrays can be combined to yield a total detec-
tion range which is the sum of the dynamic range of the PD arrays [24].
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