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PI0128WSN, PI0256WSN, PI0512WSN
Engineering Data Sheet
Page 3 of 8
Revised March 21, 2002
0.0E+00
5.0E-05
1.0E-04
1.5E-04
2.0E-04
2.5E-04
3.0E-04
3.5E-04
4.0E-04
4.5E-04
5.0E-04
100
300
500
700
900
Wavelength (nm)
R
2
)
QE=20%
QE=40%
QE=60%
QE=80%
Figure 4. Predicted spectral response.
Note: Quantum Efficiency (QE) can be calculated by
dividing the responsivity by the area of the sensor's
element and multiplying the resulting ratio by the
energy per photon in electron volts (eV).
The dark current is typically 0.2 pA at 25
o
C and varies
as function of temperature. The dark current will con-
tribute dark-signal charges and these charges will
increase linearly with integration time. The dark signal
and the photogenerated signal combined result in the
total signal charge.
Self-Scanning Circuit
Figure 5 shows a simplified electrically equivalent
circuit diagram of the photodiode array. An MOS read
switch connects every photodiode in the array to a
common output video line. Incident photons generate
electron charge that is collected on each imaging
photodiode while the switch is open. The shift register
is activated by the start pulse. A pulse propagates
through each shift register stage and activates the
MOS read switches sequentially. As the shift register
sequentially closes each read switch, the negative
stored charge, which is proportional in amount to the
light exposure, from the corresponding photodiode is
readout onto the video line, QOUT. Typically, an
external charge-integrating amplifier senses the nega-
tive output charge on the video line from each photodi-
ode pixel. The shift register continues scanning the
photodiodes in sequence, until the last shift register
stage is reach, at which time the fourth and last dummy
pixel is read out and end-of-scan (EOS) output is held
high for one clock cycle. The next start pulse can then
restart the shift register.
I/O Pins
Although the WSN package has 22 pins, as shown in
Figure 1, there are only 6 functionally active I/O pins in
addition to the supply pins, as shown in Figure 5. In
essence, only two clocks, CLK and START, are re-
quired for controlling the timing of the sensor's video
readout. The remaining I/O descriptions are for the
Figure 5. Simplified circuit diagram of a WSN photodiode array.
SHIFT REGISTER