ETC S7105-06

SOLID
STATE D I V I S I O N
PSD
(POSITION SENSITIVE DETECTOR)
What is PSD?
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.
n Features of 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 Applications 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
CONTENTS
Selection guide ················································································································· 1
Description of terms ········································································································· 4
Characteristic and use ······································································································ 5
1. Basic Principle ············································································································ 5
2. One-dimensional PSD ································································································· 5
3. Two-dimensional PSD ································································································· 5
4. Position detection error ······························································································· 7
5. Position resolution ······································································································· 8
6. Response speed ········································································································· 10
7. Saturation photocurrent ································································································ 11
Selection guide
PSD (Position Sensitive Detector) is an optoelectronic position sensor utilizing 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.
One-dimensional PSD
······························
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.
Type No.
S6407
S6515
S4580-04
S4580-06
S4581-04
S4581-06
S3271-05
S4582-04
S4582-06
S3272-05
S4583-04
S4583-06
S3273-05
S7879
S8361 *
S4584-04
S4584-06
S3274-05
S7105-04
S7105-06
S7105-05
S5629
S5629-01
S5629-02
S3979
S3931
S3932
S1352
S3270
2
3
6
7
8
Resistance length
(mm)
1×1
1 × 1.2
(mm)
1
1.2
Interelectrode
resistance
Vb=0.1 V
(kΩ)
200
140
0.8 × 1.5
1.5
140
1×2
2
140
400
1 × 2.5
2.5
140
400
140
1×3
3
400
110
1 × 3.5
3.5
140
400
1 × 4.2
4.2
140
400
1×6
6
1×3
1×6
1 × 12
2.5 × 34
1 × 37
3
6
12
34
37
Works with microscopic spot light detection.
50
300
140
50
50
20
15
4
5
11
14
15
Active area
* High sensitivity in the red region type
1
1
9
12
10
Spectral response
range
(nm)
760 to 1100
760 to 1100
760 to 1100
320 to 1100
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
13
Package
1
2
3
2
3
4
2
3
4
2
3
4
Plastic
5
6
2
3
4
7
8
7
9
10
9
11
TO-5
12
13
14
15
Ceramic
Selection guide
Two-dimensional PSD
······························
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.
4
7
6
1
2
3
8
5
9
(Typ.)
Type No.
Active area
Resistance
length
(mm)
(mm)
Interelectrode
resistance
Vb=0.1 V
(kΩ)
13 × 13
13 × 13
10
12 × 12
22 × 22
14 × 14
26 × 26
10
10
4.7 × 4.7
5.7 × 5.7
S1200
S1300
S1880
S1881
S2044
S5990-01
4×4
4.5 × 4.5
S5991-01
9×9
10 × 10
S7848
2×2
2×2
S7848-01
Works with microscopic spot light detection
Spectral response
range
Structure
(nm)
320 to 1060
320 to 1100
Tetra-lateral type
Duo-lateral type
1
320 to 1060
Pin-cushion type
(improved tetra-lateral type)
3
10
320 to 1060
Pin-cushion type
(improved tetra-lateral type)
7
7
320 to 1100
Pin-cushion type
(improved tetra-lateral type)
100
760 to 1100
320 to 1100
Tetra-lateral type
Package
2
4
Ceramic
Ceramic
5
Metal
6
Ceramic
chip carrier
7
8
9
Plastic
n Examples of position detectability (Ta=25 ˚C, λ=890 nm, spot light size: φ200 µm)
lS1200
lS1300
LINE INTERVAL: 1 mm
lS5991-01
lS1880
LINE INTERVAL: 1 mm
LINE INTERVAL: 1 mm
KPSDC0017EA
LINE INTERVAL: 1 mm
lS7848
KPSDC0015EA
KPSDC0020EA
LINE INTERVAL: 0.2 mm
KPSDC0065EA
KPSDC0084EA
128-element PSD array ···························································
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.
(Typ.)
Type No.
S5681
Active area
(mm)
0.025 × 6.375/
128 elements
Resistance length Interelectrode resistance Spectral response
Vb=0.1 V
range
(nm)
(kΩ)
(mm)
6.375
100
320 to 1100
Package
Ceramic
2
Selection guide
PSD signal processing circuit
Features
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.
DC signal processing circuit
···············································
Designed specifically for DC light detection.
Type No.
PSD type
C3683-01
C4674
C4757
C4758
1-D PSD
Pin-cushion type 2-D PSD
Duo-lateral type 2-D PSD
Tetra-lateral type 2-D PSD
Dimensional outline
(mm)
66 × 56 × 15
90 × 65 × 15
92 × 70 × 15
90 × 65 × 15
AC signal processing circuit ···············································
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
C5923
1-D PSD
C7563
Pin-cushion type 2-D PSD
* Can not be modulated.
3
LED repetition frequency *
(kHz)
3.3
0.33
Dimensional outline
(mm)
110 × 75 × 15
110 × 75 × 15
Description of terms
1. Spectral response
7. Interelectrode resistance: Rie
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.
This is the resistance between opposing electrodes in a
dark state. The interelectrode resistance is an important
factor that determines the response speed, position resolution and saturation photocurrent.
The interelectrode resistance is measured with 0.1 V applied across the opposing electrodes and the common
electrode left open. When measuring the interelectrode resistance of two-dimensional PSDs, the output electrodes
other than the opposing electrodes under measurement
are left open.
2. Photo sensitivity: S
This measure of sensitivity is the ratio of radiant energy
expressed in watts (W) incident on the device, to the resulting 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 spectral response range is defined as the region in which the
relative sensitivity is higher than 5 % of the peak value.
3. Quantum efficiency: QE
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):
QE = S × 1240 × 100 [%]
λ
λ: Wavelength (nm)
S: Photo sensitivity at wavelength λ (A/W)
4. Resistance length: L
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.
8. Dark current: ID
When a reverse voltage is applied to a PSD, a slight current 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.
9. Terminal capacitance: Ct
A capacitor is formed at the PN junction of a PSD and its
capacitance is called the junction capacitance. The terminal capacitance is the sum of the junction capacitance
plus the package stray capacitance, and is a factor in
determining the response speed. The terminal capacitance listed in our PSD data sheets are the total capacitance values measured from all output electrodes.
10. Rise time: tr
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)
5. Position detection error
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 between the
position at which the light is actually incident on the PSD
and the position calculated from the PSD outputs. Measurement conditions for position detection error are as follows:
Light source
: λ=890 nm
Incident spot light: φ200 µm
Photocurrent
: 10 µA
6. Position resolution: ∆R
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
11. Saturation photocurrent: Ist
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 interelectrode resistance, and is defined as the total photocurrent
when the entire active area is illuminated.
12. Maximum reverse voltage: VR Max.
Increasing the reverse voltage applied to a PSD can
cause it to breakdown at a certain level and result in severe deterioration of PSD performance. To avoid this,
the maximum reverse voltage is specified as the absolute maximum rating (this value must not be exceeded
even momentarily) at a reverse voltage somewhat lower
than the breakdown voltage.
4
Characteristic and use
1. Basic principle
A PSD basically consists of a uniform resistive layer
formed on one or both surfaces of a high-resistivity semiconductor 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.
By finding the difference or ratio of Ix1 to Ix2, 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 corresponds to the center-of-gravity of the light beam.
2. One-dimensional PSD
Figure 2-1 Structure chart, equivalent circuit (one-dimensional PSD)
Figure 1-1 PSD sectional view
XA
OUTPUT IX1
OUTPUT IX2
P
INCIDENT
LIGHT
ELECTRODE X1
Rp
ANODE (X1)
XB
Cj Rsh
CATHODE
(COMMON)
ELECTRODE X2
P LAYER
PHOTOCURRENT
D
ANODE (X2)
I LAYER
P : CURRENT GENERATOR
D : IDEAL DIODE
Cj : JUNCTION CAPACITANCE
Rsh: SHUNT RESISTANCE
Rp : POSITIONING RESISTANCE
N LAYER
COMMON
ELECTRODE
KPSDC0006EA
RESISTANCE LENGTH LX
KPSDC0005EA
Figure 2-2 Active area chart (one-dimensional PSD)
LX
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 X1
and X2 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 X1, X2 is given by
the following formulas.
l When the center point of PSD is set at the origin:
LX
LX
- XA
+ XA
2
2
.........
(1-1)
IX2 =
IX1 =
× Io
× Io ...... (1-2)
LX
LX
IX2 - IX1
2XA
IX1 LX - 2XA
=
............ (1-3)
=
.............. (1-4)
IX1 + IX2
LX
IX2 LX + 2XA
l When the end of PSD is set at the origin:
IX1 =
LX - XB .
XB .
Io ............. (1-5) IX2 =
Io ................. (1-6)
LX
LX
2XB - LX
IX2 - IX1
=
...... (1-7)
IX1 + IX2
LX
IX1 LX - XB
=
................ (1-8)
IX2
XB
Io : Total photocurrent (IX1 + IX2)
IX1: Output current from electrode X1
IX2: Output current from electrode X2
LX : Resistance length (length of the active area)
XA: Distance from the electrical center of PSD to the light input position
XB: Distance from the electrode X1 to the light input position
5
X1
X2
x
ACTIVE AREA
KPSDC0010EA
l Position conversion formula (See Figure 2-2.)
2x
IX2 - IX1
=
........ (2-1)
LX
IX1 + IX2
In the above formula, IX1 and IX2 are the output currents
obtained from the electrodes shown in Figure 2-2.
3. Two-dimensional PSD
Two-dimensional PSDs are grouped by structure into duolateral 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, X1, X2 and Y1, Y2.
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 electrodes 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 opposite that of the other surface is on each surface, to produce 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 offers 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).
Characteristic and use
Figure 3-1 Structure chart, equivalent circuit (duo-lateral type PSD)
Figure 3-4 Active area chart (tetra-lateral type PSD)
LX
CATHODE (Y2)
Rp
ANODE (X1)
Y2
P
D Cj Rsh
ANODE (X2)
Rp
y
X1
X2
LY
CATHODE (Y1)
x
P : CURRENT GENERATOR
D : IDEAL DIODE
Cj : JUNCTION CAPACITANCE
Rsh: SHUNT RESISTANCE
Rp : POSITIONING RESISTANCE
ACTIVE AREA
Y1
KPSDC0007EA
Figure 3-2 Active area chart (duo-lateral type PSD)
LX
KPSDC0011EA
l Position conversion formula (See Figure 3-4.)
2x
IX2 - IX1
=
........ (3-3)
LX
IX1 + IX2
Y2
2y
IY2 - IY1
=
........ (3-4)
LY
IY1 + IY2
y
LY
X1
X2
3-3 Pin-cushion type (improved tetra-lateral type) PSD
x
ACTIVE AREA
Y1
KPSDC0011EA
l Position conversion formula (See Figure 3-2.)
IX2 - IX1
2x
=
........ (3-1)
LX
IX1 + IX2
This is a variant of the tetra-lateral type PSD with an improved active area and reduced interaction between electrodes. 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 distortion 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.
Figure 3-5 Structure chart, equivalent circuit (pin-cushion type PSD)
2y
IY2 - IY1
=
........ (3-2)
LY
IY1 + IY2
ANODE (Y2)
ANODE (X1)
Rp
ANODE (X2)
3-2 Tetra-lateral type PSD
ANODE (Y1)
The tetra-lateral type has four electrodes on the upper
surface, formed along each of the four edges. Photocurrent 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 response. The light input position for the tetra-lateral type
shown in Figure 3-4 is given by conversion formulas (33) and (3-4), which are the same as for the duo-lateral
type.
P
CATHODE
P :
D :
Cj :
Rsh:
Rp :
D
Cj
Rsh
CURRENT GENERATOR
IDEAL DIODE
JUNCTION CAPACITANCE
SHUNT RESISTANCE
POSITIONING RESISTANCE
KPSDC0009EA
Figure 3-6 Active area chart (pin-cushion type PSD)
LX
Y2
LY
Figure 3-3 Structure chart, equivalent circuit (tetra-lateral type PSD)
ANODE (X1)
X1
X2
y
Rp
x
D
Y1
ANODE (Y2)
ACTIVE AREA *
P
ANODE (Y1)
ANODE (X2)
Cj
Rsh
* Active area is specified at the inscribed square.
CATHODE
KPSDC0012EA
l Position conversion formula (See Figure 3-6.)
P : CURRENT GENERATOR
D : IDEAL DIODE
Cj : JUNCTION CAPACITANCE
Rsh: SHUNT RESISTANCE
Rp : POSITIONING RESISTANCE
KPSDC0008EA
2x
(IX2 + IY1) - (IX1 + IY2)
=
........ (3-5)
IX1 + IX2 + IY1 + IY2
LX
2y
(IX2 + IY2) - (IX1 + IY1)
=
........ (3-6)
IX1 + IX2 + IY1 + IY2
LY
6
Characteristic and use
Position detection capability is the most important characteristic 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-ofgravity 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 between the position at which the light is actually incident on
the PSD and the position calculated from the PSD outputs.
Figure 4-1 Cross section of PSD
SPOT
LIGHT
X1
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 detection error estimated from the obtained data is also
shown in the lower graph.
Figure 4-2 Photocurrent output example of onedimensional PSD (S4583-04, etc.)
1.0
0.5
X2
0
RESISTANCE LENGTH LX
ELECTRICAL
CENTER B
IX2
IX1
RELATIVE PHOTOCURRENT OUTPUT
4. Position detection error
0
-1.5
+1.5
Xi
POSITION ON PSD (mm)
Xm
P-TYPE
RESISTIVE LAYER
Position detection error example of onedimensional PSD (S4583-04, etc)
+50
N LAYER
ACTUAL POSITION Xi
CALCULATED POSITION Xm
COMMON ELECTRODE
KPSDC0071EA
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)
IX2 - IX1 . LX ........ (4-2)
Xm =
IX1 + IX2 2
The position detection error is measured under the following conditions.
· Light source
: λ=890 nm
· Spot light size
: φ200 µm
· Total photocurrent: 10 µA
· Reverse voltage : Specified value (listed in data sheets)
POSITION DETECTION ERROR (µm)
I LAYER
0
-50
-1.5
-1.0
-0.5
0
+0.5
+1.0
POSITION ON PSD (mm)
+1.5
KPSDB0005EA
Specific area for position detection error
The light beam position can be detected over the entire active area of PSD. However, if part of the light beam strikes
outside the active area, a positional shift in the center-ofgravity 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 4-3 Center-of-gravity of incident spot light
ACTIVE
AREA
SPOT
LIGHT
OUTPUT
ELECTRODE X1
CENTER-OF-GRAVITY
OF SPOT LIGHT FALLING
WITHIN ACTIVE AREA
CENTER-OF-GRAVITY
OF ENTIRE SPOT LIGHT
OUTPUT
ELECTRODE X2
7
KPSDC0073EA
Characteristic and use
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.
Figure 4-4 Specific area for one-dimensional PSD position
detection error (resistance length ≤ 12 mm)
OUTPUT
ELECTRODE X1
ACTIVE AREA
OUTPUT
ELECTRODE X2
5. Position resolution
Position resolution is the minimum detectable displacement of a spot light incident on PSD, expressed as a distance 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.
XB + ∆x .
Io ......... (5-1)
LX
IX2 + ∆I =
∆x: Small displacement
∆I: Change in output current
SPECIFIED RANGE
LX × 0.75
RESISTANCE LENGTH LX
Then, ∆x can be expressed by the following equation.
KPSDC0074EA
Figure 4-5 Specific area for one-dimensional PSD position
detection error (resistance length > 12 mm)
OUTPUT
ELECTRODE X1
ACTIVE AREA
OUTPUT
ELECTRODE X2
∆I
........................... (5-2)
Io
∆x = LX .
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:
In
.......................... (5-3)
Io
∆R = LX .
SPECIFIED RANGE
LX × 0.90
RESISTANCE LENGTH LX
KPSDC0075EA
Figure 4-6 Specific area for two-dimensional PSD
position detection error
Figure 5-1 shows the basic connection example when using a PSD in conjunction with current-to-voltage amplifiers.
The noise model for this circuit is shown in Figure 5-2.
Figure 5-1 Basic connection example of one-dimensional
PSD and current-to-voltage conversion type
operational amplifier
ZONE A
ZONE B
Rf
ACTIVE AREA
Cf
Rf
PSD
Cf
-
-
+
+
A
KPSDC0063EA
Position detection error for two-dimensional PSDs is separately 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 active 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.
A
VR
KPSDC0076EA
Figure 5-2 Noise model
Rf
Cf
PSD
IO
ID
Rie
Cj
in
en
~
A
Vo
+
KPSDC0077EA
Io : Photocurrent
ID : 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
8
Characteristic and use
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
2q . (Io + ID) . B [A] ............ (5-4)
q : Electron charge (1.60 × 10-19 C)
Io: Signal photocurrent (A)
ID: Dark current (A)
B : Bandwidth (Hz)
2) Thermal noise current (Johnson noise current) Ij generated
from interelectrode resistance (This can be ignored as Rsh
>> Rie.)
Ij =
4 kTB [A] ............ (5-5)
Rie
k : Boltzmann constant (1.38 × 10-23 J/K)
T : Absolute temperature (K)
Rie: Interelectrode resistance (W)
3) Noise current Ien by equivalent noise input voltage of
operational amplifier
Ien =
en
Rie
Figure 5-3 Shot noise vs. signal photocurrent
(Typ. Ta=25 ˚C)
B [A] ............ (5-6)
en: Equivalent noise input voltage of operational amplifier
(V/Hz1/2)
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:
In =
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/Hz1/2)
· Frequency bandwidth: 1 kHz
Is2 + Ij2 + Ien2 [A] ............ (5-7)
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:
10
SHOT NOISE CURRENT (A/Hz1/2)
Is =
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 kW, the operational amplifier becomes a crucial
factor in determining the noise current, so a low-noisecurrent operational amplifier must be used. When using a
PSD with an interelectrode resistance exceeding 100 kW,
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.
1
0.1
0.01
0.01
0.1
1
10
SIGNAL PHOTOCURRENT (µA)
Vs = Rf .
2q . (Io + ID) . B [V] ............ (5-8)
KPSDB0083EA
Figure 5-4 Noise current vs. interelectrode resistance
4 kTB [V] .............................. (5-9)
Rie
Ven = 1 + Rf . en .
Rie
B [V] .............. (5-10)
The thermal noise from the feedback resistance and the
equivalent noise input current of the operational amplifier
are also added as follows:
VRf = Rf .
Vin = Rf . in .
4 kTB [V] ............................ (5-11)
Rf
9
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
1
0.1
B [V] ............................ (5-12)
The equivalent noise input voltage of the operational amplifier is then expressed as an RMS value by the following
equation.
Vn =
(Typ. Ta=25 ˚C)
10
NOISE CURRENT (pA/Hz1/2)
Vj = Rf .
Vs2 + Vj2 + Ven2 + VRf2 + Vin2 [V] ............ (5-13)
0.01
10
100
1000
INTERELECTRODE RESISTANCE (kΩ)
KPSDB0084EA
Characteristic and use
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 detecting 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 needed 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 resistance, 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:
t1 = 2.2 . Ct . (Rie + RL) ......... (6-1)
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 = 0.5 . Ct . (Rie + RL) ......... (6-2)
2) Diffusion time t2 of carriers generated outside the depletion 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 depletion 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 several microseconds.
Figure 6-2 shows the relation between the rise time and reverse 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.
Figure 6-2 Rise time vs. reverse voltage (S4583-06)
(Typ. Ta=25 ˚C)
10
8
RISE TIME (µs)
6. Response speed
6
l=890 nm
4
2
l=650 nm
0
0.1
1
10
100
REVERSE VOLTAGE (V)
KPSDB0110EA
A method for integrating position signals can be used when
detecting pulsed light having a pulse width shorter than the
PSD rise time.
The equation below gives the approximate rise time tr of a
PSD. Figure 6-1 shows typical output waveforms in response to stepped light input.
tr
t12 + t22 .................... (6-3)
Figure 6-1 Response wavelength example of PSD
LIGHT INPUT
OUTPUT WAVEFORM
(t1>>t2)
OUTPUT WAVEFORM
(t2>>t1)
KPSDC0078EB
10
Characteristic and use
Figure 7-3 Saturation photocurrent vs. interelectrode resistance
(entire active area fully illuminated)
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 normally with good output linearity over the entire active area.
If the background light level is excessively high or the signal 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 impaired so the correct position cannot be detected in this
case.
Photocurrent saturation of a PSD depends on the interelectrode resistance and reverse voltage, as shown in Figure 73. The saturated photocurrent is measured as the total
photocurrent of a PSD when the entire active area is illuminated. If a small spot light is focused on the active area, the
photocurrent that is generated is concentrated only on a localized 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 7-1 Photocurrent output example of PSD in
normal operation (S5629)
(Ta=25 ˚C)
120
RELATIVE PHOTOCURRENT (%)
IX1 + IX2
100
IX1
IX2
80
60
CENTER OF
ELECTRICITY
40
20
0
-4
-3
-2
-1
0
1
2
3
4
INCIDENT POSITION (mm)
KPSDB0087EA
Figure 7-2 Photocurrent output example of saturated
PSD (S5629)
(Ta=25 ˚C)
120
RELATIVE PHOTOCURRENT (%)
IX2
100
IX1
80
IX2
IX1 + IX2
60
40
CENTER OF
ELECTRICITY
20
0
-4
-3
-2
-1
0
1
2
3
4
INCIDENT POSITION (mm)
11
KPSDB0086EA
(Typ. Ta=25 ˚C)
10
SATURATION PHOTOCURRENT (µA)
7. Saturation photocurrent
VR=5 V
VR=2 V
1
0.1
VR=1 V
VR=0 V
0.01
10
100
1000
INTERELECTRODE RESISTANCE (kΩ)
KPSDB0085EA
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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
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· 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.
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Cat. No. KPSD0001E01
Jan. 2002 DN
Printed in Japan (7,000)