PSD6407 datasheet(13/16 Pages) ETC

13 / 16 page

Figure 6-1 Response wavelength example of PSD

KPSDC0078EB

2) Diffusion time t2 of carriers generated outside the deple-

tion layer

Carriers are also generated outside the depletion layer

when light is absorbed in the PSD chip surrounding areas

outside the active area or at locations deeper than the de-

pletion layer in the substrate. These carriers diffuse through

the substrate and are extracted as an output. The time t2

required for these carriers to diffuse may be more than sev-

eral microseconds.

The equation below gives the approximate rise time tr of a

PSD. Figure 6-1 shows typical output waveforms in re-

sponse to stepped light input.

Figure 6-2 shows the relation between the rise time and re-

verse voltage measured at different wavelengths. The rise

time can be reduced by increasing the reverse voltage and

using a light beam of shorter wavelengths. Selecting a PSD

with a small Rie is also effective in improving the rise time.

A method for integrating position signals can be used when

detecting pulsed light having a pulse width shorter than the

PSD rise time.

Characteristic and use

6. Response speed

As with photodiodes, the response speed of PSD is the

time required for the generated carriers to be extracted as

current by an external circuit. This is generally expressed

as the rise time tr and is an important parameter when de-

tecting a spot light traveling over the active surface at high

speeds or using pulse-modulated light for subtracting the

background light. The rise time is defined as the time need-

ed for the output signal to rise from 10 to 90 % of its peak

value and is chiefly determined by the following two factors.

1) Time constant t1 determined by the interelectrode resist-

ance, load resistance and terminal capacitance

The interelectrode resistance Rie of PSD basically acts as

load resistance RL, so the time constant t1 is given by the

interelectrode resistance Rie and terminal capacitance Ct,

as follows:

The rise time listed in our PSD datasheets is measured

with a spot light striking the center of the active area with

the interelectrode resistance Rie distributed between the

electrodes. So the time constant t1 is as follows:

t1 = 2.2 . Ct . (Rie + RL) ......... (6-1)

t1 = 0.5 . Ct . (Rie + RL) ......... (6-2)

Figure 6-2 Rise time vs. reverse voltage (S4583-06)

KPSDB0110EA

REVERSE VOLTAGE (V)

(Typ. Ta=25 ˚C)

0.1

100

l=890 nm

l=650 nm

t12 + t22 .................... (6-3)

LIGHT INPUT

OUTPUT WAVEFORM

(t1>>t2)

OUTPUT WAVEFORM

(t2>>t1)

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