PI0256HSN, PI0512HSN, PI1024HSN

Engineering Data Sheet

Page 3 of 8

March 13, 2002

0.0E+00

5.0E-05

1.0E-04

1.5E-04

2.0E-04

2.5E-04

100

300

500

700

900

Wavelength (nm)

QE=20%

QE=40%

QE=60%

QE=80%

Figure 4. Predicted spectral response.

The 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°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 sig-

nal and the photo-generated signal combined result

in the total signal charge.

Selectable Charge Capacity

The HSN devices have the unique feature of having a

selectable charge capacity.

There is a bank of ca-

pacitors with one capacitor for each photodiode pixel.

When the capacitors are connected to the photodi-

odes, they give the photodiodes a charge capacity of

typically 65 pC. This large charge capacity is useful in

applications that demand high dynamic range and high

signal-to-noise ratios.

With the capacitors discon-

nected, the photodiodes have an intrinsic charge

capacity of typically 25 pC.

With a reduced capaci-

tance, the photodiode array can operate with a lower

reset (kTC) noise.

The ADDCAP pin is provided to control the connection

of the capacitors.

When ADDCAP is high, all the

capacitors are connected. When ADDCAP is low, all

the capacitors are disconnected. It is advised that all

the photodiodes are reset after each toggle of

ADDCAP. This is simply done by clocking one lines-

can of the photodiode array.

Anti-Blooming Circuit

Each photodiode pixel has a built-in anti-blooming

circuit structure. Without an anti-blooming circuit, it is

possible that a fraction of the excess charge from one

pixel will flow into neighboring pixels.

The anti-

blooming circuit prevents this by redirecting the excess

current into the anti-blooming drain before the photodi-

ode is too full. A self-biased anti-blooming gate sets

the level at which the charge begins to flow into the

drain. Think of it this way. If the photodiode were your

bathroom sink, then the anti-blooming circuit would be

your sink’s overflow drain.

The anti-blooming circuit may be disabled by ground-

ing the anti-blooming gate. This would in effect raise

the drain level.

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.