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)

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).

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