™
Peripheral
Imaging
Corporation
PI0128WSN, PI0256WSN, PI0512WSN
50-m
m-Pitch Wide Aperture Spectroscopic Photodiode Arrays
Engineering Data Sheet
Description
Peripheral Imaging Corporation's WSN series is family
of self-scanning photodiode solid-state linear imaging
arrays.
These photodiode sensors employ PIC’s
proprietary CMOS Image Sensing Technology to
integrate the sensors into a single monolithic chip.
These sensors are optimally designed for applications
in spectroscopy. Accordingly, these sensors contain a
linear array of photodiodes with an optimized geomet-
rical aspect ratio (50-m aperture pitch x 2500-m
m
m
aperture width) for helping to maintain mechanical
stability in spectroscopic instruments and for providing
a large light-capturing ability. The family of sensors
consists of photodiode arrays of various lengths, 128,
256, and 512 pixels.
The WSN photodiode arrays are mounted in 22-pin
ceramic side-brazed dual-in-line packages that fit in
standard DIP sockets. A diagram of its pinout configu-
ration is seen in Figure 1.
Features
·
·
·
·
·
·
·
·
·
·
65-pC saturation capacity for wide dynamic range.
Wide spectral response (180 – 1000 nm) for UV
and IR response.
NP junction photodiodes with superior resistance
to UV damage.
Low dark current.
Integration time up to 9 seconds at room tempera-
ture.
Integration time extended to hours by cooling.
High linearity.
Low power dissipation (less than 1 mW).
Geometrical structure for enhanced stability and
registration.
Standard 22-lead dual-in-line IC package.
Figure 1. Pinout configuration.
Sensor Characteristics
The Peripheral Imaging Corporation's self-scanned
WSN photodiodes are spaced on a 50-m pitch. The
m
line density is 20 diodes/mm and accordingly the
overall die lengths of the different arrays vary with the
number of photodiodes. For example, the 128-pixel
array is 6.4-mm long, the 256-pixel array is 12.8-mm
long, and the 512-pixel array is 25.6-mm long. Each
array has four additional dummy photodiodes. On each
side, there are one dark (non-imaging) dummy photo-
diode and one imaging dummy photodiode. The height
of the sensors is 2500
m
m. The tall, narrow apertures
make these sensors desirable for use in monochro-
mators and spectrographs.
Page 1 of 8
Revised March 21, 2002
PI0128WSN, PI0256WSN, PI0512WSN
Engineering Data Sheet
Figure 2. Geometry and layout of photodiode pixels.
During normal operation, the photons incident in or
near the NP photodiode junction generate free charges
that are collected and stored on the junction's depletion
capacitance. The number of collected charges is
proportional to the light exposure. Figure 3 shows the
stored signal charge as function of light exposure at a
wavelength of 575 nm. The exposure is the product of
the light intensity in nW/cm
2
and integration time in
seconds. The charge accumulates linearly until reach-
ing the saturation charge, and the corresponding
exposure is the saturation exposure.
The responsivity may be calculated as the saturation
charge divided by saturation exposure. The predicted
typical responsivity of a photodiode is 3.5´10
-4
C/J/cm
2
at 575 nm. Figure 4 shows the predicted responsivity
of the photodiodes as a function of wavelength.
80
Output Charge (pC)
70
60
50
40
30
20
10
0
0
50
100
150
200
250
Saturation
Charge
Saturation
Exposure
Exposure (nJ/cm
2
)
Figure 3. Stored signal charge as function of exposure at
a wavelength of 575 nm.
Page 2 of 8
Revised March 21, 2002
PI0128WSN, PI0256WSN, PI0512WSN
Engineering Data Sheet
SHIFT REGISTER
5.0E-04
Responsivity (C/J/cm
2
)
4.5E-04
4.0E-04
3.5E-04
3.0E-04
2.5E-04
2.0E-04
1.5E-04
1.0E-04
5.0E-05
0.0E+00
100
QE=80%
QE=60%
QE=40%
QE=20%
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.
300
500
700
900
Wavelength (nm)
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.
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
I/O Pins
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.
Page 3 of 8
Revised March 21, 2002
PI0128WSN, PI0256WSN, PI0512WSN
Engineering Data Sheet
video signal output, the end-of-scan signal, the two
temperature diodes, and the supply biases. QOUT
must be biased externally to Vbias (see Recom-
mended Operating Conditions section below). Each
temperature diode is operated with a small constant
current that forward-biases its PN junction. By meas-
uring the forward-bias voltage, one can track the silicon
die temperature. The temperature diodes may be
disabled by connecting their anodes to VSS. These
I/Os are listed with their acronym designators and
functional descriptions in the following Table 1.
Table 1. Symbols and functions and I/O pins.
Symbol
VSS
VDD
START
CLK
EOS
QOUT
TD1
TD2
NC
Ground.
+5.0 Volts.
Start Pulse: Input to start the line scan.
Clock Pulse: Input to clock the shift register.
End Of Scan: Output from the shift register to indicate the completion of one
line scan.
Video Charge Output: Output from the photodiodes pixels.
Temperature Diode 1: Anode of temperature diode 1.
Temperature Diode 2: Anode of temperature diode 2.
No Connection
Function and Description
Page 4 of 8
Revised March 21, 2002
PI0128WSN, PI0256WSN, PI0512WSN
Engineering Data Sheet
Clock and Voltage Requirements
The clocking requirements are relatively simple. As it
was indicated in Figure 5 and Table 1, there are only
two input signals that require clocked inputs. They are
CLK, the clock for the shift register, and START, the
shift register start pulse. The timing specifications and
the symbol definition for Figure 6 are listed in Table 2.
The control clock amplitudes for I/Os are compatible
with the 5-Volt CMOS devices.
Figure 6. Timing diagram.
Table 2. Symbol definitions and timing specifications for timing diagram.
Item
Clock cycle time
Clock high pulse width
Clock low pulse width
Clock duty cycle
Data setup time
Data hold time
EOS low-to-high delay
EOS high-to-low delay
Signal delay time
Signal settling time
Signal settle to clock edge
Symbol
to
twh
twl
tds
tdh
telh
tehl
tsd
tsh
tsch
Min
666 (128WSN)
666 (256WSN)
1000 (512WSN)
566 (128WSN)
566 (256WSN)
900 (512WSN)
100
1
100
100
50
Typical
10000
Max
Units
ns
ns
50
99
400
400
566 (128WSN)
566 (256WSN)
900 (512WSN)
ns
%
ns
ns
ns
ns
ns
ns
ns
0
Page 5 of 8
Revised March 21, 2002
