S3931 - N/A - Product.Network

PSD

(POSITION SENSITIVE DETECTOR)

SOLID

STATE

DIVISION

Various methods are available for detecting the position of incident light. These

include methods using small discrete detector arrays or multi-element sensors such

as CCD sensors. In contrast to these sensors, PSDs (Position Sensitive Detectors)

are comprised of a monolithic detector with no discrete elements and provide

continuous position data by making use of the surface resistance of the photodiode.

PSDs offer advantages such as high position resolution, high-speed response and

reliability.

What is PSD?

· Excellent position resolution

· Wide spectral response range

· High-speed response

· Detects center-of-gravity position of spot light

· Simultaneously detects light intensity and center-of-gravity position of spot light

· High reliability

Features of PSD

· Position and angle sensing

· Distortion and vibration measurements

· Lens reflection and refraction measurements

· Laser displacement sensing

· Optical remote control

· Optical range finders

· Optical switches

· Camera auto focusing

Applications of PSD

Selection guide

Description of terms

Characteristic and use

1. Basic Principle

2. One-dimensional PSD

3. Two-dimensional PSD

4. Position detection error

5. Position resolution

6. Response speed

7. Saturation photocurrent

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CONTENTS

Hamamatsu provides various types of one-dimensional PSDs designed

for high-precision distance measurement such as displacement meters,

camera auto focusing and optical switches. Our product line includes a

visible-cut type for near infrared detection, a red sensitivity enhanced type

for red light detection, a microscopic spot light (LD beam, etc.) detection

type, and a long, narrow type with an active area exceeding 30 mm.

Selection guide

PSD (Position Sensitive Detector) is an optoelectronic position sensor utiliz-

ing photodiode surface resistance. Unlike discrete element detectors such as

CCD, PSD provides continuous position data (X or Y coordinate data) and

features high position resolution and high-speed response.

1312

One-dimensional PSD ······························

(mm) (mm) (kΩ)(nm)

S6407 1 × 1 1 200 760 to 1100

S6515 1 × 1.2 1.2 140 760 to 1100

S4580-04 760 to 1100

S4580-06 0.8 × 1.5 1.5 140 320 to 1100

S4581-04 760 to 1060

320 to 1060

760 to 1060

760 to 1100

320 to 1100

760 to 1100

320 to 1100

760 to 1100

440 to 1100

400 to 1100

760 to 1100

320 to 1100

760 to 1100

320 to 1100

760 to 1100

320 to 1100

760 to 1100

320 to 1100

700 to 1100

S4581-06 140

S3271-05 1 × 22 400

S4582-04

S4582-06 140

S3272-05 1 × 2.5 2.5 400

S4583-04

S4583-06 140

S3273-05 400

S7879

S8361 *

1 × 33

110

S4584-04

S4584-06 140

S3274-05 1 × 3.5 3.5 400

S7105-04

S7105-06 140

S7105-05 1 × 4.2 4.2 400

S5629

S5629-01 50

S5629-02 1 × 66 300

Plastic

S3979 1 × 3 3 140 TO-5

S3931 1 × 6 6 50

S3932 1 × 12 12 50

S1352 2.5 × 34 34 20

S3270 1 × 37 37 15

Ceramic

* High sensitivity in the red region type

Works with microscopic spot light detection.

12 3 4 5

78910

Interelectrode

resistance

Vb=0.1 V

Type No. Active area Resistance length Package

Spectral response

range

S5681 is a 128-element PSD linear array. By scanning a slit-form light beam right and left based

on the slit light projection method, S5681 allows measuring a 3-D shape of the object.

Two-dimensional PSDs are classified by structure into a tetra-lateral type and a

duo-lateral type. The tetra-lateral type features high-speed response and low

dark current. The duo-lateral type offers small position detection error and high

position resolution. A pin-cushion type, which is a tetra-lateral type with

improved active area and electrodes, has a position detection error as small as

the duo-lateral type while still having the advantages of the tetra-lateral type.

Selection guide

(Typ.)

Interelectrode resistance

Vb=0.1 V

(kΩ)

Resistance length

(mm)

Type No. Package

Spectral response

range

(nm)

S5681 Ceramic

Active area

(mm)

0.025 × 6.375/

128 elements 1006.375

(Typ.)

Interelectrode

resistance

Vb=0.1 V

(kΩ)

S7848

S7848-01 Plastic

2 × 2 2 × 2

10 × 10

4.5 × 4.5

5.7 × 5.7

26 × 26

14 × 14

13 × 13

S1200

100

Tetra-lateral type

S1300 Ceramic

13 × 13 Duo-lateral type

S2044 Metal4.7 × 4.7

S1880 12 × 12 10

S1881 Ceramic

22 × 22 Pin-cushion type

(improved tetra-lateral type)

Pin-cushion type

(improved tetra-lateral type)

Pin-cushion type

(improved tetra-lateral type)

S5990-01 4 × 4

S5991-01 Ceramic

chip carrier

9 × 9

Type No. PackageStructure

Active area

(mm)

Resistance

length

(mm)

Spectral response

range

(nm)

128-element PSD array ···························································

Two-dimensional PSD ······························

Examples of position detectability (Ta=25 ˚C, λ=890 nm, spot light size: φ200 µm)

S1200

KPSDC0017EA

S1300

KPSDC0015EA

S1880

KPSDC0020EA

S5991-01

KPSDC0065EA

S7848

KPSDC0084EA

LINE INTERVAL: 1 mm LINE INTERVAL: 1 mm LINE INTERVAL: 1 mm

LINE INTERVAL: 1 mm LINE INTERVAL: 0.2 mm

Works with microscopic spot light detection

320 to 1060

320 to 1100

320 to 1060

320 to 1100

760 to 1100

320 to 1100

Selection guide

PSD signal processing circuit

No complicated adjustments required

Position measurements can be made by just connecting to a PSD and power supply (±15 V).

The position (mm) of a spot light from the PSD center is obtained as an output voltage (V).

(except C3683-01)

Stable position detection

Accurate position data can be detected independent of incident light intensity.

Compact size

Head amplifiers, signal addition/subtraction circuits, and analog divider are mounted on a

compact PC board.

Designed specifically for DC light detection.

Designed specifically for pulse (AC) signal detection.

Has a synchronous circuit, S/H (sample & hold) circuit and LED driver circuit.

Use of a pulse-driven LED ensures reliable operation even under background light.

Type No. PSD type Dimensional outline

(mm)

C3683-01 1-D PSD 66 × 56 × 15

C4674 Pin-cushion type 2-D PSD 90 × 65 × 15

C4757 Duo-lateral type 2-D PSD 92 × 70 × 15

C4758 Tetra-lateral type 2-D PSD 90 × 65 × 15

Type No.

* Can not be modulated.

PSD type LED repetition frequency *

(kHz) Dimensional outline

(mm)

C5923 1-D PSD 3.3 110 × 75 × 15

C7563 Pin-cushion type 2-D PSD 0.33 110 × 75 × 15

Features

DC signal processing circuit

AC signal processing circuit

···············································

The photocurrent produced by a given level of incident

light varies with the wavelength. This relation between

the photoelectric sensitivity and wavelength is referred to

as the spectral response characteristic and is expressed

in terms of photo sensitivity, quantum efficiency, etc.

The quantum efficiency is the number of electrons or

holes that can be detected as a photocurrent divided by

the number of the incident photons. This is commonly

expressed in percent (%). The quantum efficiency and

photo sensitivity S have the following relationship at a

given wavelength (nm):

λ: Wavelength (nm)

S: Photo sensitivity at wavelength λ (A/W)

1. Spectral response

This measure of sensitivity is the ratio of radiant energy

expressed in watts (W) incident on the device, to the re-

sulting photocurrent expressed in amperes (A). It may be

represented as either an absolute sensitivity (A/W) or as

a relative sensitivity normalized for the sensitivity at the

peak wavelength, usually expressed in percent (%) with

respect to the peak value. For the purpose of our PSD

data sheets (separately available), the photo sensitivity

is represented as the absolute sensitivity, and the spec-

tral response range is defined as the region in which the

relative sensitivity is higher than 5 % of the peak value.

2. Photo sensitivity: S

QE = × 100 [%]

S × 1240

This is the resistance between opposing electrodes in a

dark state. The interelectrode resistance is an important

factor that determines the response speed, position res-

olution and saturation photocurrent.

The interelectrode resistance is measured with 0.1 V ap-

plied across the opposing electrodes and the common

electrode left open. When measuring the interelectrode re-

sistance of two-dimensional PSDs, the output electrodes

other than the opposing electrodes under measurement

are left open.

7. Interelectrode resistance: Rie

When a reverse voltage is applied to a PSD, a slight cur-

rent flows even in a dark state. This is termed the dark

current and is a source of noise. The dark current listed in

our PSD data sheets (separately available) are the total

dark current values measured from all output electrodes.

8. Dark current: I

A capacitor is formed at the PN junction of a PSD and its

capacitance is called the junction capacitance. The ter-

minal capacitance is the sum of the junction capacitance

plus the package stray capacitance, and is a factor in

determining the response speed. The terminal capaci-

tance listed in our PSD data sheets are the total capaci-

tance values measured from all output electrodes.

9. Terminal capacitance: Ct

The rise time is defined as the time required for the PSD

output to rise from 10 to 90 % of the steady output level,

when a step function light is input to the PSD. The rise

time depends on the incident light wavelength, load

resistance, light incident position and reverse voltage,

and is measured under the following conditions.

· Light source : λ=890 nm

· Incident spot light : φ1 mm

· Incident light position: Center point of PSD

· Load resistance : 1 kΩ

(connected to all output electrodes)

10. Rise time: tr

This is the maximum photocurrent value obtained from a

PSD as long as it still functions as a position sensor.

This value depends on the reverse voltage and interelec-

trode resistance, and is defined as the total photocurrent

when the entire active area is illuminated.

11. Saturation photocurrent: Ist

Increasing the reverse voltage applied to a PSD can

cause it to breakdown at a certain level and result in se-

vere deterioration of PSD performance. To avoid this,

the maximum reverse voltage is specified as the abso-

lute maximum rating (this value must not be exceeded

even momentarily) at a reverse voltage somewhat lower

than the breakdown voltage.

12. Maximum reverse voltage: V

Max.

3. Quantum efficiency: QE

If a light beam strikes the electrical center of a PSD, the sig-

nal currents extracted from the output electrodes are equal.

When this electrical center is viewed as the origin, the posi-

tion detection error is defined as the difference between the

position at which the light is actually incident on the PSD

and the position calculated from the PSD outputs. Measure-

ment conditions for position detection error are as follows:

Light source : λ=890 nm

Incident spot light: φ200 µm

Photocurrent : 10 µA

5. Position detection error

This is the minimum detectable displacement of a spot light

incident on a PSD, and is expressed as a distance on the

PSD surface. Resolution is mainly determined by the S/N

and given by “resistance length × noise / signal”. The

resolution values listed in our PSD data sheets (separately

available) are calculated based on the RMS values for

noise measured under the following conditions.

· Interelectrode resistance: Typical value

(listed in the data sheets)

· Photocurrent : 1 µA

· Frequency bandwidth : 1 kHz

· Equivalent noise input voltage to circuit: 1 µV

6. Position resolution: ∆R

This is the distance between electrodes on a PSD and is

used to calculate the position from the PSD outputs. The

resistance length is equivalent to the active area size,

except for the pin-cushion type (improved tetra-lateral

type) whose resistance length is expressed by the

distance actually used to calculate the position.

4. Resistance length: L

Description of terms

A PSD basically consists of a uniform resistive layer

formed on one or both surfaces of a high-resistivity semi-

conductor substrate, and a pair of electrodes formed on

both ends of the resistive layer for extracting position

signals. The active area, which is also a resistive layer,

has a PN junction that generates photocurrent by means

of the photovoltaic effect.

KPSDC0010EA

KPSDC0006EA

By finding the difference or ratio of Ix

to Ix

, the light input

position can be obtained by the formulas (1-3), (1-4), (1-7)

and (1-8) irrespective of the incident light intensity level

and its changes. The light input position obtained here cor-

responds to the center-of-gravity of the light beam.

Figure 1-1 shows a sectional view of a PSD using a simple

illustration to explain the operating principle. The PSD has

a P-type resistive layer formed on an N-type high-resistive

silicon substrate. This P-layer serves as an active area for

photoelectric conversion and a pair of output electrodes

are formed on the both ends of the P-layer. On the

backside of the silicon substrate is an N-layer to which a

common electrode is connected. Basically, this is the

same structure as that of PIN photodiodes except for the

P-type resistive layer on the surface.

When a spot light strikes the PSD, an electric charge

proportional to the light intensity is generated at the

incident position. This electric charge is driven through the

resistive layer and collected by the output electrodes X

and X

as photocurrents, while being divided in inverse

proportion to the distance between the incident position

and each electrode.

The relation between the incident light position and the

photocurrents from the output electrodes X

, X

is given by

the following formulas.

When the center point of PSD is set at the origin:

When the end of PSD is set at the origin:

Io : Total photocurrent (I

+ I

)

: Output current from electrode X

: Resistance length (length of the active area)

Distance from the electrical center of PSD to the light input position

Distance from the electrode X

to the light input position

In the above formula, I

and I

are the output currents

obtained from the electrodes shown in Figure 2-2.

Position conversion formula (See Figure 2-2.)

Two-dimensional PSDs are grouped by structure into duo-

lateral and tetra-lateral types. Among the tetra-lateral type

PSDs, a pin-cushion type with an improved active area

and electrodes is also provided. (See “3-3”.) The position

conversion formulas slightly differ according to the PSD

structure. Two-dimensional PSDs have two pairs of output

electrodes, X

, X

and Y

, Y

3-1 Duo-lateral type PSD

On the duo-lateral type, the N-layer shown in the sectional

view of Figure 1-1 is processed to form a resistive layer,

and two pair of electrodes are formed on both surfaces as

X and Y electrodes arranged at right angles. (See Figure

3-1.) The X position signals are extracted from the X elec-

trodes on the upper surface, while the Y position signals

are extracted from the Y electrodes on the bottom surface.

As shown in Figure 3-1, a photocurrent with a polarity op-

posite that of the other surface is on each surface, to pro-

duce signal currents twice as large as the tetra-lateral type

and achieve a higher position resolution. In addition, when

compared to the tetra-lateral type, the duo-lateral type of-

fers excellent position detection characteristics because

the electrodes are not in close proximity. The light input

position can be calculated from conversion formulas (3-1)

and (3-2).

Figure 2-1 Structure chart, equivalent circuit (one-dimensional PSD)

Figure 2-2

Active area chart (one-dimensional PSD)

KPSDC0005EA

Figure 1-1 PSD sectional view

= ............ (1-3)

- I

+ I

= .............. (1-4)

- 2X

+ 2X

= ...... (1-7)

- I

+ I

- L

= ........ (2-1)

- I

+ I

= ................ (1-8)

- X

= . Io ............. (1-5)

- X

= . Io ................. (1-6)

Characteristic and use

1. Basic principle

2. One-dimensional PSD

3. Two-dimensional PSD

......... (1-1)

× Io

- X

...... (1-2)

× Io

+ X

OUTPUT I

PHOTOCURRENT

INCIDENT

LIGHT

P LAYER

RESISTANCE LENGTH L

COMMON

ELECTRODE

OUTPUT I

ELECTRODE X1ELECTRODE X2

I LAYER

N LAYER

ACTIVE AREA

Rsh

: CURRENT GENERATOR

: IDEAL DIODE

: JUNCTION CAPACITANCE

: SHUNT RESISTANCE

: POSITIONING RESISTANCE

PDC

jRsh

CATHODE

(COMMON)

ANODE (X

)

ANODE (X

)

Position conversion formula (See Figure 3-2.)

The tetra-lateral type has four electrodes on the upper

surface, formed along each of the four edges. Photocur-

rent is divided into 4 parts through the same resistive

layer and extracted as position signals from the four

electrodes. Compared to the duo-lateral type, interaction

between the electrodes tends to occur near the corners

of the active area, making position distortion larger. But

the tetra-lateral type features an easy-to-apply reverse

bias voltage, small dark current and high-speed re-

sponse. The light input position for the tetra-lateral type

shown in Figure 3-4 is given by conversion formulas (3-

3) and (3-4), which are the same as for the duo-lateral

type.

3-2 Tetra-lateral type PSD

Figure 3-1 Structure chart, equivalent circuit (duo-lateral type PSD)

Figure 3-2 Active area chart (duo-lateral type PSD)

KPSDC0007EA

KPSDC0011EA

This is a variant of the tetra-lateral type PSD with an im-

proved active area and reduced interaction between elec-

trodes. In addition to the advantages of small dark current,

high-speed response and easy application of reverse bias

that the tetra-lateral type offers, the circumference distor-

tion has been greatly reduced. The light input position of

the pin-cushion type shown in Figure 3-6 is calculated from

conversion formulas (3-5) and (3-6), which are different

from those for the duo-lateral and tetra-lateral types.

Position conversion formula (See Figure 3-4.)

Position conversion formula (See Figure 3-6.)

3-3 Pin-cushion type (improved tetra-lateral type) PSD

Figure 3-3 Structure chart, equivalent circuit (tetra-lateral type PSD)

KPSDC0008EA

Figure 3-4 Active area chart (tetra-lateral type PSD)

KPSDC0011EA

= ........ (3-1)

IX2 - IX1

IX1 + IX2

= ........ (3-2)

IY2 - IY1

IY1 + IY2

Figure 3-5 Structure chart, equivalent circuit (pin-cushion type PSD)

Figure 3-6 Active area chart (pin-cushion type PSD)

KPSDC0012EA

KPSDC0009EA

= ........ (3-3)

- I

+ I

= ........ (3-4)

- I

+ I

= ........ (3-5)

+ I

) - (I

+ I

)

+ I

= ........ (3-6)

+ I

) - (I

+ I

)

+ I

Characteristic and use

ACTIVE AREA

ACTIVE AREA *

* Active area is specified at the inscribed square.

ANODE (X

)

ANODE (X

)

CATHODE (Y

)

CATHODE (Y

)Rp

PDC

jRsh

Rsh

: CURRENT GENERATOR

: IDEAL DIODE

: JUNCTION CAPACITANCE

: SHUNT RESISTANCE

: POSITIONING RESISTANCE

ANODE (X

)ANODE (Y

)

ANODE (X

)

ANODE (Y

)CATHODE

CjRsh

Rsh

: CURRENT GENERATOR

: IDEAL DIODE

: JUNCTION CAPACITANCE

: SHUNT RESISTANCE

: POSITIONING RESISTANCE

PDCjRsh

Rsh

ANODE (X

)

ANODE (Y

)

ANODE (X

)ANODE (Y

)

CATHODE

: CURRENT GENERATOR

: IDEAL DIODE

: JUNCTION CAPACITANCE

: SHUNT RESISTANCE

: POSITIONING RESISTANCE

Figure 4-2 shows the photocurrent output example from

electrodes of a one-dimensional PSD with a resistance

length of 3 mm (S4583-04, etc.), measured when a light

beam is scanned over the active surface. The position de-

tection error estimated from the obtained data is also

shown in the lower graph.

The light beam position can be detected over the entire ac-

tive area of PSD. However, if part of the light beam strikes

outside the active area, a positional shift in the center-of-

gravity occurs between the entire light beam and the light

spot falling within the active area, making the position

measurement unreliable. It is therefore necessary to select

a PSD whose active area matches the incident spot light.

Figure 4-1 Cross section of PSD

KPSDB0005EA

Figure 4-2 Photocurrent output example of one-

dimensional PSD (S4583-04, etc.)

Position detection error example of one-

dimensional PSD (S4583-04, etc)

Specific area for position detection error

KPSDC0071EA

Position detection capability is the most important charac-

teristic of a PSD. The position of a spot light incident on the

PSD surface can be measured by making calculations

based on the photocurrent extracted from each electrode.

The position obtained here with the PSD is the center-of-

gravity of the spot light, and is independent of the spot light

size, shape and intensity.

However, the calculated position usually varies slightly in

each PSD from the actual position of the incident light. This

difference is referred to as the “position detection error” and

is explained below.

If a light beam strikes the electrical center of a PSD, the

signal currents extracted from the output electrodes are

equal. When this electrical center is viewed as the origin,

the position detection error is defined as the difference be-

tween the position at which the light is actually incident on

the PSD and the position calculated from the PSD outputs.

Characteristic and use

4. Position detection error

KPSDC0073EA

Figure 4-3 Center-of-gravity of incident spot light

In Figure 4-1 above, if the actual position of incident light

is Xi and the position calculated by the photocurrents

(IX1 and IX2) from electrodes X1 and X2 is Xm, then the

difference in distance between Xi and Xm is defined as

the position detection error as calculated below.

Position detection error E = Xi - Xm [µm] ........ (4-1)

Xi : Actual position of incident light (µm)

Xm: Calculated position of incident light (µm)

The position detection error is measured under the follow-

ing conditions.

· Light source : λ=890 nm

· Spot light size : φ200 µm

· Total photocurrent: 10 µA

Reverse voltage : Specified value (listed in data sheets)

Xm = .

- I

+ I

2........ (4-2)

POSITION ON PSD (mm)

+1.50

-1.5

0.5

1.0

RELATIVE PHOTOCURRENT OUTPUT

+50

-50 -1.5 -1.0 -0.5 +0.5 +1.00 +1.5

POSITION ON PSD (mm)

POSITION DETECTION ERROR (µm)

OUTPUT

ELECTRODE X

OUTPUT

ELECTRODE X

CENTER-OF-GRAVITY

OF SPOT LIGHT FALLING

WITHIN ACTIVE AREA

CENTER-OF-GRAVITY

OF ENTIRE SPOT LIGHT

SPOT

LIGHT

ACTIVE

AREA

COMMON ELECTRODE



RESISTANCE LENGTH L

SPOT

LIGHT

CALCULATED POSITION Xm

ACTUAL POSITION Xi

ELECTRICAL

CENTER B

N LAYER

I LAYER

P-TYPE

RESISTIVE LAYER

The position detection error is usually measured with a

light beam of φ200 µm, so the specified areas shown in

Figures 4-4 to 4-6 are used for position detection error.

Position detection error for two-dimensional PSDs is sep-

arately measured in two areas: Zone A and Zone B. Two

zones are used because position detection error in the

circumference is larger than that in the center of the ac-

tive area,

· Zone A: Within a circle with a diameter equal to 40 % of

one side length of the active area.

· Zone B: Within a circle with a diameter equal to 80 % of

one side length of the active area.

Figure 4-4

Specific area for one-dimensional PSD position

detection error (resistance length ≤ 12 mm)

Figure 4-5

Specific area for one-dimensional PSD position

detection error (resistance length > 12 mm)

Figure 4-6 Specific area for two-dimensional PSD

position detection error

KPSDC0063EA

KPSDC0074EA

KPSDC0075EA

Figure 5-1 Basic connection example of one-dimensional

PSD and current-to-voltage conversion type

operational amplifier

KPSDC0076EA

Position resolution is the minimum detectable displace-

ment of a spot light incident on PSD, expressed as a dis-

tance on the PSD surface. Resolution is determined by the

PSD resistance length and the S/N. Using formula (1-6) as

an example, the following equation can be established.

In cases where the positional displacement is infinitely

small, the noise component contained in the output current

IX2 clearly determines the position resolution. Generally, if

the PSD noise current is In, then the position resolution ∆R

is given as follows:

Figure 5-1 shows the basic connection example when us-

ing a PSD in conjunction with current-to-voltage amplifiers.

The noise model for this circuit is shown in Figure 5-2.

∆x: Small displacement

∆I: Change in output current

Then, ∆x can be expressed by the following equation.

IX2 + ∆I = . Io ......... (5-1)

XB + ∆x

∆x = LX . ........................... (5-2)

∆I

∆R = LX . .......................... (5-3)

Characteristic and use

5. Position resolution

Io : Photocurrent

: Dark current

Rie: Interelectrode resistance

Cj : Junction capacitance

Rf : Feedback resistance

Cf : Feedback capacitance

en : Equivalent noise input voltage of operational amplifier

in : Equivalent noise input current of operational amplifier

Vo : Output voltage

KPSDC0077EA

Figure 5-2 Noise model

SPECIFIED RANGE

× 0.75

RESISTANCE LENGTH L

OUTPUT

ELECTRODE X

OUTPUT

ELECTRODE X

ACTIVE AREA

SPECIFIED RANGE

× 0.90

RESISTANCE LENGTH L

OUTPUT

ELECTRODE X

OUTPUT

ELECTRODE X

ACTIVE AREA

ZONE B

ZONE A Rf

Cf PSD

in en AVo

PSD

OIDRie Cj

Noise currents are calculated below, assuming that the

feedback resistance Rf of the current-to-voltage conversion

circuit is sufficiently greater than the PSD interelectrode

resistance Rie. In this case, 1/Rf can be ignored since it is

sufficiently small compared to 1/Rie. Position resolution as

listed in our PSD data sheets is calculated by this method.

1) Shot noise current Is originating from photocurrent and

dark current

Thermal noise current (Johnson noise current) Ij generated

from interelectrode resistance (This can be ignored as Rsh

>> Rie.)

3) Noise current Ien by equivalent noise input voltage of

operational amplifier

By taking the sum of equations (5-4), (5-5) and (5-6), the

PSD noise current can be expressed as an RMS value

as follows:

If Rf cannot be ignored versus Rie (as a guide, Rie/Rf >

0.1), then the equivalent noise output voltage must be

taken into account. In this case, equations (5-4), (5-5)

and (5-6) are converted into output voltages as follows:

The thermal noise from the feedback resistance and the

equivalent noise input current of the operational amplifier

are also added as follows:

The equivalent noise input voltage of the operational am-

plifier is then expressed as an RMS value by the following

equation.

Figure 5-3 shows the shot noise current plotted along the

signal photocurrent value when Rf >>Rie. Figure 5-4

shows the thermal noise current and the noise current by

the equivalent noise input voltage of the operational

amplifier, plotted along the interelectrode resistance value.

When using a PSD with an interelectrode resistance of

about 10 k

, the operational amplifier becomes a crucial

factor in determining the noise current, so a low-noise-

current operational amplifier must be used. When using a

PSD with an interelectrode resistance exceeding 100 k

the thermal noise generated from the interelectrode

resistance of the PSD itself will be predominant.

As explained above, PSD position resolution is determined

by interelectrode resistance and light intensity. This is the

point in which the PSD greatly differs from discrete type

position detectors.

The following methods are effective for increasing the PSD

position resolution.

· Increase the signal photocurrent Io.

· Increase the interelectrode resistance Rie.

· Shorten the resistance length L.

· Use a low noise operational amplifier.

The position resolution listed in our PSD data sheets is

measured under the following conditions.

· Photocurrent: 1 µA

· Circuit input noise: 1 µV (31.6 nV/Hz

1/2

)

· Frequency bandwidth: 1 kHz

Is = 2q . (Io + I

) . B[A] ............ (5-4)

q : Electron charge (1.60 × 10

-19

Io: Signal photocurrent (A)

: Dark current (A)

B : Bandwidth (Hz)

k : Boltzmann constant (1.38 × 10

-23

J/K)

T : Absolute temperature (K)

Rie: Interelectrode resistance (

)

en: Equivalent noise input voltage of operational amplifier

(V/Hz

1/2

)

Ij = [A] ............ (5-5)

4 kTB

Rie

Ien = B [A] ............ (5-6)

Rie

In = Is

+ Ij

+ Ien

[A] ............ (5-7)

Vs = Rf . 2q . (Io + I

) . B[V] ............ (5-8)

Vj = Rf . [V] .............................. (5-9)

4 kTB

Rie

= 1 + . en . [V] .............. (5-10)

Rie

= Rf . [V] ............................ (5-11)

4 kTB

= Rf . in . [V] ............................ (5-12)

Vn = Vs

+ Vj

+ Ve

+ V

+ Vi

[V] ............ (5-13)

Characteristic and use

Figure 5-3 Shot noise vs. signal photocurrent

Figure 5-4 Noise current vs. interelectrode resistance

KPSDB0083EA

KPSDB0084EA

SIGNAL PHOTOCURRENT (µA)

(Typ. Ta=25 ˚C)

0.10.01

0.01

0.1

110

SHOT NOISE CURRENT (A/Hz

1/2

)

INTERELECTRODE RESISTANCE (kΩ)

(Typ. Ta=25 ˚C)

10010

0.01

0.1

1000

NOISE CURRENT (pA/Hz

1/2

)

Thermal noise current Ij generated from

interelectrode resistance

Noise current (en=10 nV) by equivalent

noise input voltage of operational amplifier

Noise current (en=30 nV) by equivalent

noise input voltage of operational amplifier

Figure 6-1 Response wavelength example of PSD

KPSDC0078EB

2) Diffusion time t

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 t

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 t

determined by the interelectrode resist-

ance, load resistance and terminal capacitance

The interelectrode resistance Rie of PSD basically acts as

load resistance R

, so the time constant t

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 t

is as follows:

= 2.2 . Ct . (Rie + R

) ......... (6-1)

= 0.5 . Ct . (Rie + R

) ......... (6-2)

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

KPSDB0110EA

REVERSE VOLTAGE (V)

(Typ. Ta=25 ˚C)

RISE TIME (µs)

10.1

10 100

=890 nm

=650 nm

+ t

.................... (6-3)tr

LIGHT INPUT

OUTPUT WAVEFORM

>>t

)

OUTPUT WAVEFORM

>>t

)

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)

RELATIVE PHOTOCURRENT (%)

-2-4

120

100

02-1-3 1 3 4

+ I

CENTER OF

ELECTRICITY

INCIDENT POSITION (mm)

(Ta=25 ˚C)

RELATIVE PHOTOCURRENT (%)

-2-4

120

100

02-1-3 1 3 4

+ I

CENTER OF

ELECTRICITY

INTERELECTRODE RESISTANCE (kΩ)

(Typ. Ta=25 ˚C)

10010

0.01

0.1

1000

SATURATION PHOTOCURRENT (µA)

=5 V

=2 V

=1 V

=0 V

Notice

· The information contained in this catalog does not represent or create any warranty, express or implied, including

any warranty of merchantability or fitness for any particular purpose.

The terms and conditions of sale contain complete warranty information and is available upon request from your

local HAMAMATSU representative.

· The products described in this catalog should be used by persons who are accustomed to the properties of

photoelectronics devices, and have expertise in handling and operating them.

They should not be used by persons who are not experienced or trained in the necessary precations surrounding their

use.

· The information in this catalog is subject to change without prior notice.

· Information furnished by HAMAMATSU is believed to be reliable. However, no responsibility is assumed for

possible inaccuracies or ommission.

· No patent rights are granted to any of the circuits described herein.

Cat. No. KPSD0001E01

Jan. 2002 DN

Printed in Japan (7,000)

HAMAMATSU PHOTONICS K.K., Solid State Division

1126-1, Ichino-cho, Hamamatsu City, 435-8558, Japan

Telephone: (81)53-434-3311, Fax: (81)53-434-5184

Homepage: http://www.hamamatsu.com

Information in this catalog is

believed to be reliable. However,

no responsibility is assumed for

possible inaccuracies or omission.

Specifications are subject to

change without notice. No patent

rights are granted to any of the

circuits described herein.

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