S2044 datasheet(14/16 Pages) ETC

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Photocurrent saturation must be taken into account when a

PSD is used outdoors, in locations where the background

light level is high, or the signal light amount is extremely

large. Figure 7-1 shows typical photocurrent output of a

PSD in a non-saturated state. This PSD is operating nor-

mally with good output linearity over the entire active area.

If the background light level is excessively high or the sig-

nal light amount is extremely large, the PSD photocurrent

will saturate. A typical output from a saturated PSD is

shown in Figure 7-2. The output linearity of the PSD is im-

paired so the correct position cannot be detected in this

case.

Photocurrent saturation of a PSD depends on the interelec-

trode resistance and reverse voltage, as shown in Figure 7-

3. The saturated photocurrent is measured as the total

photocurrent of a PSD when the entire active area is illumi-

nated. If a small spot light is focused on the active area, the

photocurrent that is generated is concentrated only on a lo-

calized portion, so saturation occurs at a lower level.

To avoid the saturation effect, use the following methods.

· Reduce the background light level by using an optical filter.

· Use a PSD with a small active area.

· Increase the reverse voltage.

· Decrease the interelectrode resistance.

· Avoid concentrating the light beam on a small area.

Figure 7-3 Saturation photocurrent vs. interelectrode resistance

(entire active area fully illuminated)

KPSDB0085EA

KPSDB0086EA

Figure 7-2 Photocurrent output example of saturated

PSD (S5629)

Characteristic and use

7. Saturation photocurrent

KPSDB0087EA

Figure 7-1 Photocurrent output example of PSD in

normal operation (S5629)

INCIDENT POSITION (mm)

(Ta=25 ˚C)

-2

-4

120

100

-1

-3

IX1

IX2

IX1 + IX2

CENTER OF

ELECTRICITY

INCIDENT POSITION (mm)

(Ta=25 ˚C)

-2

-4

120

100

-1

-3

IX1

IX2

IX1 + IX2

IX2

CENTER OF

ELECTRICITY

INTERELECTRODE RESISTANCE (k

Ω)

(Typ. Ta=25 ˚C)

100

0.01

0.1

1000

VR=5 V

VR=2 V

VR=1 V

VR=0 V

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