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
n
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
n
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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4
5
5
5
5
7
8
10
11
CONTENTS
1
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.
1
2
3
2
3
2
3
4
4
2
3
4
2
3
4
7
8
7
9
10
9
11
12
13
14
15
5
6
11
1312
14
15
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
760 to 1100
320 to 1100
760 to 1100
440 to 1100
400 to 1100
760 to 1100
320 to 1100
760 to 1100
760 to 1100
320 to 1100
760 to 1100
760 to 1100
320 to 1100
760 to 1100
320 to 1100
320 to 1100
320 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
6
Interelectrode
resistance
Vb=0.1 V
Type No. Active area Resistance length Package
Spectral response
range
2
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
7
7
Tetra-lateral type
Tetra-lateral type
S1300 Ceramic
13 × 13 Duo-lateral type
S2044 Metal4.7 × 4.7
S1880 12 × 12 10
10
10
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)
1
2
3
5
6
7
8
9
4
1
2
35
67
89
4
128-element PSD array ···························································
Two-dimensional PSD ······························
n
Examples of position detectability (Ta=25 ˚C, λ=890 nm, spot light size: φ200 µm)
l
S1200
KPSDC0017EA
l
S1300
KPSDC0015EA
l
S1880
KPSDC0020EA
l
S5991-01
KPSDC0065EA
l
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 1060
320 to 1100
760 to 1100
320 to 1100
320 to 1100
3
Selection guide
PSD signal processing circuit
l
No complicated adjustments required
Position measurements can be made by just connecting to a PSD and power supply (±15 V).
l
The position (mm) of a spot light from the PSD center is obtained as an output voltage (V).
(except C3683-01)
l
Stable position detection
Accurate position data can be detected independent of incident light intensity.
l
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
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4
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
λ
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
D
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
R
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
5
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
1
to Ix
2
, 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
1
and X
2
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
1
, X
2
is given by
the following formulas.
l
When the center point of PSD is set at the origin:
l
When the end of PSD is set at the origin:
Io : Total photocurrent (I
X1
+ I
X2
)
I
X1
: Output current from electrode X
1
I
X2
: Output current from electrode X
2
L
X
: Resistance length (length of the active area)
X
A
:
Distance from the electrical center of PSD to the light input position
X
B
:
Distance from the electrode X
1
to the light input position
In the above formula, I
X1
and I
X2
are the output currents
obtained from the electrodes shown in Figure 2-2.
l
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
1
, X
2
and Y
1
, Y
2
.
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
I
X2
=
= ............ (1-3)
I
X2
- I
X1
I
X1
+ I
X2
2X
A
L
X
= .............. (1-4)
I
X1
I
X2
L
X
- 2X
A
L
X
+ 2X
A
= ...... (1-7)
I
X2
- I
X1
I
X1
+ I
X2
2X
B
- L
X
L
X
= ........ (2-1)
I
X2
- I
X1
I
X1
+ I
X2
2x
L
X
= ................ (1-8)
I
X1
I
X2
L
X
- X
B
X
B
I
X1
= . Io ............. (1-5)
L
X
- X
B
L
X
I
X2
= . Io ................. (1-6)
X
B
L
X
Characteristic and use
1. Basic principle
2. One-dimensional PSD
3. Two-dimensional PSD
I
X1
=
L
X
2L
X
......... (1-1)
× Io
- X
A
L
X
2L
X
...... (1-2)
× Io
+ X
A
OUTPUT I
X1
PHOTOCURRENT
X
B
INCIDENT
LIGHT
P LAYER
RESISTANCE LENGTH L
X
COMMON
ELECTRODE
X
A
OUTPUT I
X2
ELECTRODE X1ELECTRODE X2
I LAYER
N LAYER
L
X
X
1
X
2
ACTIVE AREA
x
P
D
Cj
Rsh
Rp
: CURRENT GENERATOR
: IDEAL DIODE
: JUNCTION CAPACITANCE
: SHUNT RESISTANCE
: POSITIONING RESISTANCE
PDC
jRsh
Rp
CATHODE
(COMMON)
ANODE (X
2
)
ANODE (X
1
)
6
l
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.
l
Position conversion formula (See Figure 3-4.)
l
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
2x
LX
= ........ (3-2)
IY2 - IY1
IY1 + IY2
2y
LY
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
X2
- I
X1
I
X1
+ I
X2
2x
L
X
= ........ (3-4)
I
Y2
- I
Y1
I
Y1
+ I
Y2
2y
L
Y
= ........ (3-5)
(I
X2
+ I
Y1
) - (I
X1
+ I
Y2
)
I
X1
+ I
X2
+ I
Y1
+ I
Y2
2x
L
X
= ........ (3-6)
(I
X2
+ I
Y2
) - (I
X1
+ I
Y1
)
I
X1
+ I
X2
+ I
Y1
+ I
Y2
2y
L
Y
Characteristic and use
L
X
Y
2
X
2
X
1
L
Y
Y
1
x
y
ACTIVE AREA
L
X
Y
2
X
2
X
1
L
Y
Y
1
x
y
ACTIVE AREA
L
X
Y
2
X
2
X
1
L
Y
Y
1
x
y
ACTIVE AREA *
* Active area is specified at the inscribed square.
ANODE (X
1
)
ANODE (X
2
)
CATHODE (Y
2
)
CATHODE (Y
1
)Rp
PDC
jRsh
P
D
Cj
Rsh
Rp
Rp
: CURRENT GENERATOR
: IDEAL DIODE
: JUNCTION CAPACITANCE
: SHUNT RESISTANCE
: POSITIONING RESISTANCE
ANODE (X
1
)ANODE (Y
2
)
ANODE (X
2
)
ANODE (Y
1
)CATHODE
PD
CjRsh
P
D
Cj
Rsh
Rp
Rp
: CURRENT GENERATOR
: IDEAL DIODE
: JUNCTION CAPACITANCE
: SHUNT RESISTANCE
: POSITIONING RESISTANCE
Rp
PDCjRsh
P
D
Cj
Rsh
Rp
ANODE (X
2
)
ANODE (Y
1
)
ANODE (X
1
)ANODE (Y
2
)
CATHODE
: CURRENT GENERATOR
: IDEAL DIODE
: JUNCTION CAPACITANCE
: SHUNT RESISTANCE
: POSITIONING RESISTANCE
7
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
X2
- I
X1
I
X1
+ I
X2
L
X
2........ (4-2)
POSITION ON PSD (mm)
+1.50
-1.5
0
0.5
1.0
RELATIVE PHOTOCURRENT OUTPUT
I
X2
I
X1
+50
0
-50 -1.5 -1.0 -0.5 +0.5 +1.00 +1.5
POSITION ON PSD (mm)
POSITION DETECTION ERROR (µm)
OUTPUT
ELECTRODE X
1
OUTPUT
ELECTRODE X
2
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
X
SPOT
LIGHT
Xm
Xi
X
1
X
2
CALCULATED POSITION Xm
ACTUAL POSITION Xi
ELECTRICAL
CENTER B
N LAYER
I LAYER
P-TYPE
RESISTIVE LAYER
8
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
LX
∆x = LX . ........................... (5-2)
∆I
Io
∆R = LX . .......................... (5-3)
In
Io
Characteristic and use
5. Position resolution
Io : Photocurrent
I
D
: 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
L
X
× 0.75
RESISTANCE LENGTH L
X
OUTPUT
ELECTRODE X
1
OUTPUT
ELECTRODE X
2
ACTIVE AREA
SPECIFIED RANGE
L
X
× 0.90
RESISTANCE LENGTH L
X
OUTPUT
ELECTRODE X
1
OUTPUT
ELECTRODE X
2
ACTIVE AREA
ACTIVE AREA
ZONE B
ZONE A Rf
Cf PSD
-
+
-
+
Rf
Cf
AA
VR
~
-
+
Rf
Cf
in en AVo
PSD
I
OIDRie Cj
9
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
2)
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
W
, 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
W
,
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
D
) . B[A] ............ (5-4)
q : Electron charge (1.60 × 10
-19
C)
Io: Signal photocurrent (A)
I
D
: Dark current (A)
B : Bandwidth (Hz)
k : Boltzmann constant (1.38 × 10
-23
J/K)
T : Absolute temperature (K)
Rie: Interelectrode resistance (
W
)
en: Equivalent noise input voltage of operational amplifier
(V/Hz
1/2
)
Ij = [A] ............ (5-5)
4 kTB
Rie
Ien = B [A] ............ (5-6)
en
Rie
In = Is
2
+ Ij
2
+ Ien
2
[A] ............ (5-7)
Vs = Rf . 2q . (Io + I
D
) . B[V] ............ (5-8)
Vj = Rf . [V] .............................. (5-9)
4 kTB
Rie
B
Ve
n
= 1 + . en . [V] .............. (5-10)
Rf
Rie
B
V
R
f
= Rf . [V] ............................ (5-11)
4 kTB
Rf
Vi
n
= Rf . in . [V] ............................ (5-12)
Vn = Vs
2
+ Vj
2
+ Ve
n2
+ V
R
f
2
+ Vi
n2
[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
10
1
110
SHOT NOISE CURRENT (A/Hz
1/2
)
INTERELECTRODE RESISTANCE (kΩ)
(Typ. Ta=25 ˚C)
10010
0.01
0.1
10
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
10
Figure 6-1 Response wavelength example of PSD
KPSDC0078EB
2) Diffusion time t
2
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
2
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
1
determined by the interelectrode resist-
ance, load resistance and terminal capacitance
The interelectrode resistance Rie of PSD basically acts as
load resistance R
L
, so the time constant t
1
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
1
is as follows:
t
1
= 2.2 . Ct . (Rie + R
L
) ......... (6-1)
t
1
= 0.5 . Ct . (Rie + R
L
) ......... (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
0
10
8
6
4
2
10 100
l
=890 nm
l
=650 nm
t
12
+ t
22
.................... (6-3)tr
LIGHT INPUT
OUTPUT WAVEFORM
(t
1
>>t
2
)
OUTPUT WAVEFORM
(t
2
>>t
1
)
11
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
0
20
40
120
100
80
60
02-1-3 1 3 4
I
X1
I
X2
I
X1
+ I
X2
CENTER OF
ELECTRICITY
INCIDENT POSITION (mm)
(Ta=25 ˚C)
RELATIVE PHOTOCURRENT (%)
-2-4
0
20
40
120
100
80
60
02-1-3 1 3 4
I
X1
I
X2
I
X1
+ I
X2
I
X2
CENTER OF
ELECTRICITY
INTERELECTRODE RESISTANCE (kΩ)
(Typ. Ta=25 ˚C)
10010
0.01
0.1
10
1
1000
SATURATION PHOTOCURRENT (µA)
V
R
=5 V
V
R
=2 V
V
R
=1 V
V
R
=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.
© 2002 Hamamatsu Photonics K.K.
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