Si photodiodes
CHAPTER 02
1 Si photodiodes
1-1 Operating principle
1-2 Equivalent circuit
1-3 Current vs. voltage characteristics
1-4 Linearity
1-5 Spectral response
1-6 Noise characteristics
1-7 Sensitivity uniformity
1-8 Response speed
1-9 Connection to an op amp
1-10 Application circuit examples
2 PSD (position sensitive detectors)
2-1
2-2
2-3
2-4
2-5
2-6
2-7
Features
Structure and operating principle
Position detection error
Position resolution
Response speed
Saturation photocurrent
How to use
3 Applications
3-1 Particle size analyzers (laser diffraction and
scattering method)
3-2 Barcode readers
3-3 UV sensors
3-4 Rotary encoders
3-5 Color sensors
3-6 VICS (Vehicle Information and Communication
System)
3-7 Triangulation distance measurement
3-8 Direct position detection
1
Si photodiodes
Photodiodes are photosensors that generate a current or voltage when the PN junction in the semiconductor is irradiated
by light. The term photodiode can be broadly defined to include even solar batteries, but it usually means sensors that
accurately detect changes in light level. Hamamatsu Si (silicon) photodiodes can be classified by function and construction
into Si photodiode (PN type), Si PIN photodiode, Si APD (avalanche photodiode), MPPC (multi-pixel photon counter), and
PSD (position sensitive detector). Si photodiodes provide the following features and are widely used to detect the presence
or absence, intensity, and color of light, etc.
Excellent linearity with respect to incident light
Mechanically rugged
Low noise
Compact and lightweight
Wide spectral response range
Long life
The lineup of Si photodiodes we manufacture utilizing our own advanced semiconductor process technologies covers
a broad spectral range from the near infrared to ultraviolet and even to high-energy regions, and features high-speed
response, high sensitivity, and low noise. Hamamatsu Si photodiodes are used in a wide range of applications including
medical and analytical fields, scientific measurements, optical communications, and general electronic products. These
photodiodes are available in various packages such as metal, ceramic, and plastic packages, as well as in surface mount
types. Hamamatsu also offers custom-designed devices to meet special needs.
Hamamatsu Si photodiodes
Type
Si photodiode
Si PIN photodiode
IR-enhanced
Si PIN photodiode
Multi-element Si photodiode
Si photodiode with preamp,
thermoelectrically cooled Si
photodiode
Si photodiode for radiation
detection
PSD
Features
These photodiodes feature high sensitivity and low noise, and
they are specifically designed for precision photometry and
general photometry in the visible range.
Si PIN photodiodes deliver high-speed response when operated
with a reverse voltage applied and are suitable for use in optical
fiber communications, optical disk pickups, etc.
These photodiodes have fine structures fabricated on the back side of
the photosensitive area and feature improved sensitivity in the near
infrared region above 900 nm. Compared to our previous products,
these photodiodes have approximately three times the sensitivity for
YAG laser light (1.06 µm).
Si photodiode arrays consist of multiple elements formed in a linear or
two-dimensional arrangement in a single package. These photodiode
arrays are used in a wide range of applications such as light position
detection and spectrophotometry.
Si photodiodes with preamp incorporate a photodiode and a preamplifier
into the same package, so they are highly immune to external noise
and allow compact circuit design. Thermoelectrically cooled types offer
drastically improved S/N.
These detectors are composed of a Si photodiode coupled to a
scintillator. They are suited for X-ray baggage inspection and nondestructive inspection systems.
These position sensors detect a light spot on the photosensitive
area by using surface resistance.
Because it is not segmented, a PSD provides continuous electrical
signal with high resolution and fast response.
Note: For details on Si APD and MPPC, see chapter 3, “Si APD, MPPC.”
2
Product examples
UV to near infrared range
•For
visible to near infrared range
•For
visible range
•For
color sensor
•RGB
vacuum ultraviolet (VUV) detection
•For
monochromatic light detection
•For
•For electron beam detection
•Cutoff frequency: 10 MHz or more
•For YAG laser monitoring
photodiode
•Segmented
•One-dimensional photodiode array
•For analysis and measurement
with scintillator
•Type
•Large area type
PSD
•One-dimensional
•Two-dimensional PSD
[Figure 1-2] Si photodiode PN junction state
Si photodiodes
Depletion layer
P-layer
Operating principle
Conduction band
[Figure 1-1] Schematic of Si photodiode cross section
Insulation layer
Depletion layer
Incident light
Positive electrode
(anode)
Negative electrode
(cathode)
Short
wavelength
Valence band
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Equivalent circuit
1-2
An equivalent circuit of a Si photodiode is shown in Figure
1-3.
[Figure 1-3] Si photodiode equivalent circuit
,
I
IL :
VD :
ID :
Cj :
Rsh :
I’ :
Rs :
Vo :
Io :
Cj
Rs
Io
Rsh
ID
Load
IL
RL
current generated by incident light (proportional to light level)
voltage across diode
diode current
junction capacitance
shunt resistance
shunt resistance current
series resistance
output voltage
output current
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Using the above equivalent circuit, the output current
(Io) is given by equation (1).
,
,
q VD
Io = IL - ID - I = IL - IS (exp
- 1) - I ............ (1)
kT
Is :
q:
k :
T:
photodiode reverse saturation current
electron charge
Boltzmann’s constant
absolute temperature of photodiode
The open circuit voltage (Voc) is the output voltage when
Io=0, and is expressed by equation (2).
,
Voc = k T ln IL - I + 1 ............ (2)
q
Is
(
)
If I’ is negligible, since Is increases exponentially with
respect to ambient temperature, Voc is inversely proportional
to the ambient temperature and proportional to the
log of IL. However, this relationship does not hold when
detecting low-level light.
The short circuit current (Isc) is the output current when
load resistance (R L)=0 and Vo=0, and is expressed by
equation (3).
Long
wavelength
P-layer
Band gap energy
VD
Figure 1-1 shows a cross section example of a Si photodiode.
The P-type region (P-layer) at the photosensitive surface
and the N-type region (N-layer) at the substrate form a
PN junction which operates as a photoelectric converter.
The usual P-layer for a Si photodiode is formed by selective
diffusion of boron to a thickness of approx. 1 µm or less,
and the intrinsic region at the junction between the
P-layer and N-layer is known as the depletion layer. By
controlling the thickness of the outer P-layer, N-layer,
and bottom N+-layer as well as the dopant concentration,
the spectral response and frequency response described
later can be controlled.
When a Si photodiode is illuminated by light and if the
light energy is greater than the band gap energy, the
valence band electrons are excited to the conduction
band, leaving holes in their place in the valence band
[Figure 1-2]. These electron-hole pairs occur throughout
the P-layer, depletion layer and N-layer materials. In
the depletion layer the electric field accelerates these
electrons toward the N-layer and the holes toward
the P-layer. Of the electron-hole pairs generated in
the N-layer, the electrons, along with electrons that
have arrived from the P-layer, are left in the N-layer
conduction band. The holes are diffused through the
N-layer up to the depletion layer, accelerated, and
collected in the P-layer valence band. In this manner,
electron-hole pairs which are generated in proportion to
the amount of incident light are collected in the N-layer
and P-layer. This results in a positive charge in the P-layer
and a negative charge in the N-layer. When an electrode
is formed from each of the P-layer and N-layer and is
connected to an external circuit, electrons will flow
away from the N-layer, and holes will flow away from
the P-layer toward the opposite respective electrodes,
generating a current. These electrons and holes
generating a current flow in a semiconductor are called
the carriers.
Incident light
1-1
N-layer
Vo
1.
N N+
N-layer
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(
Isc = IL - Is exp
q × Isc × Rs
- 1 - Isc × Rs ...... (3)
Rsh
kT
)
3
Current vs. voltage characteristics
1-3
When a voltage is applied to a Si photodiode in a dark
state, the current versus voltage characteristics observed
are similar to the curve of a rectifier diode as shown by
in Figure 1-4. However, when light strikes the photodiode,
the curve at shifts to and increasing the incident light
level shifts this characteristic curve still further to position
in parallel. As for the characteristics of and , if the Si
photodiode terminals are shorted, a short circuit current Isc
or Isc’ proportional to the light level will flow from the anode
to the cathode. If the circuit is open, an open circuit voltage
Voc or Voc’ will be generated with the positive polarity at the
anode.
Voc changes logarithmically with changes in the light
level but greatly varies with temperature, making it
unsuitable for measurement of light level. Figure 1-5
shows a typical relation between Isc and incident light
level and also between Voc and incident light level.
Current
[Figure 1-4] Current vs. voltage characteristics
Light
(b) Open circuit voltage
Open circuit voltage (mV)
In equation (3), the 2nd and 3rd terms become the cause
that determines the linearity limit of the short circuit
current. However, since Rs is several ohms and Rsh is 107
to 1011 ohms, these terms become negligible over quite a
wide range.
Illuminance (lx)
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Figure 1-6 shows the basic circuit for measuring a
photocurrent. In the circuit shown at (a), the voltage (Io
× RL) is amplified by an amplifier with gain G. A higher
linearity is maintained by applying a reverse voltage
to the photodiode [Figure 1-9 (a), Figure 1-10]. The
circuit shown at (b) uses an op amp to connect to the
photodiode. If we let the open-loop gain of the op amp
be A, the negative feedback circuit allows the equivalent
input resistance (equivalent to load resistance R L) to
be Rf/A which is several orders of magnitude smaller
than RL. Thus this circuit enables ideal measurements
of short circuit current. When necessary to measure the
photocurrent over a wide range, the proper values of RL
and Rf must be selected to prevent output saturation
even when the incident light level is high.
Saturation current
[Figure 1-6] Connection examples
Voltage
(a) When load resistor is connected
Light
Light
Increasing
light level
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[Figure 1-5] Output signal vs. incident light level (S2386-5K)
(b) When op amp is connected
(a) Short circuit current
(Typ. Ta=25 ˚C)
103
Short circuit current (μA)
Light
102
101
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100
10-1
10-2
10-1
100
101
102
103
104
Illuminance (lx)
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4
Figure 1-7 is a magnified view of the zero region of curve
shown in Figure 1-4. This proves that the change in
dark current (I D) is approximately linear in a voltage
range of about ±10 mV. The slope in this straight line
indicates the shunt resistance (Rsh), and this resistance
is the cause of thermal noise current described later. For
Hamamatsu Si photodiodes, the shunt resistance values
are obtained using a dark current measured with 10 mV
applied to the cathode.
Dark current
[Figure 1-7] Dark current vs. voltage (enlarged view of
zero region of curve in Figure 1-4)
Voltage (mV)
Rsh =
10 [mV]
[Ω]
ID
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1-4
Linearity
The photocurrent of the Si photodiode is extremely
linear with respect to the incident light level. When the
incident light is within the range of 10-12 to 10-2 W, the
achievable range of linearity is higher than nine orders of
magnitude (depending on the type of photodiode and its
operating circuit, etc.). The lower limit of this linearity is
determined by the noise equivalent power (NEP), while
the upper limit depends on the load resistance, reverse
voltage, etc., and is given by equation (4). As the series
resistance component increases, the linearity degrades.
Psat =
Psat :
VBi :
VR :
RS :
RL :
Sλ :
prevents the connection of large load resistance, and is not
suitable for low-light-level detection. (b) is an example
in which a photodiode is connected directly to the op
amp input terminal and current-to-voltage conversion
is performed using feedback resistance (Rf ). In this
case, the load resistance for the photodiode is the input
resistance to the op amp and is a constant value. Since
the input resistance of the op amp is low (several ohms),
as long as the op amp output does not saturate, the
photocurrent also does not saturate regardless of how
large the feedback resistance is set to. Therefore, (b) is
suitable for low-light-level detection. Figure 1-10 shows
how the upper limit of linearity changes with a reverse
voltage (V R). While application of a reverse voltage to
a photodiode is useful in improving the linearity, it
also increases dark current and noise levels. Since an
excessive reverse voltage may damage the photodiode,
use a reverse voltage that will not exceed the absolute
maximum rating, and make sure that the cathode is
maintained at a positive potential with respect to the
anode.
When laser light is condensed on a small spot, caution
is required because the amount of light per unit area
increases, and linearity deteriorates.
[Figure 1-9] Connection examples
(with reverse voltage applied)
(a)
VBi + VR
............ (4)
(RS + RL) × Sλ
Reverse
voltage
input energy [W] at upper limit of linearity (Psat ≤ 10 mW)
contact voltage [V] (approx. 0.2 to 0.3 V)
reverse voltage [V]
photodiode series resistance (several ohms)
load resistance [Ω]
photosensitivity [A/W] at wavelength λ
RL: load resistance
(b)
Load line with reverse
voltage applied
Current
[Figure 1-8] Current vs. voltage characteristics and load lines
Reverse
voltage
Voc
VR
Voc’ Voltage
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High load line
Isc
Isc’
Increasing
light level
Low load line
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In some cases, applying a reverse voltage is effective in
enhancing the upper limit of linearity. Figure 1-9 shows
connection examples for applying a reverse voltage. (a)
is an example in which the photocurrent is converted
into voltage with load resistance and amplified with
an amplifier. When the load resistance is large, the
upper limit of linearity is limited [Equation (4)]. This
5
[Figure 1-10] Photocurrent vs. illuminance (S1223)
[Figure 1-11] Spectral response (Si photodiodes)
(Typ. Ta=25 ˚C, RL=100 kΩ)
10-3
(Typ. Ta=25 ˚C)
0.8
QE=100%
0.7
Photosensitivity (A/W)
Photocurrent [A]
10-4
VR=5 V
VR=1 V
10-5
VR=0 V
10-6
S3759 (for YAG laser)
S1336-8BQ
0.6
S1337-1010BR
S3590-19
0.5 (high violet sensitivity)
0.4
0.3
S1226-8BQ
(IR sensitivity suppressed)
S1227-1010BR
(IR sensitivity suppressed)
S9219
(visual-sensitivity compensated)
0.2
0.1
10-7
101
102
103
104
0
200
105
400
Illuminance (lx)
600
800
Spectral response
As explained in section 1-1, “Principle of operation,”
when the energy of absorbed light is lower than the band
gap energy of Si photodiodes, the photovoltaic effect
does not occur.
The cutoff wavelength (λc) can be expressed by equation (5).
λc = 1240 [nm] ............ (5)
Eg
Eg: band gap energy [eV]
In the case of Si at room temperature, the band gap
energy is 1.12 eV, so the cutoff wavelength is 1100 nm.
For short wavelengths, however, the degree of light
absorption within the surface diffusion layer becomes
very large [Figure 1-1]. Therefore, the thinner the
diffusion layer is and the closer the PN junction is to the
surface, the higher the sensitivity will be. For normal
Si photodiodes, the cutoff wavelength on the short
wavelength side is 320 nm, whereas it is 190 nm for
UV-enhanced Si photodiodes (S1226/S1336 series, etc.).
The cutoff wavelength is determined by the intrinsic
material properties of the Si photodiode and the spectral
transmittance of the light input window material. For
borosilicate glass and plastic resin coating, wavelengths
below approx. 300 nm are absorbed. If these materials
are used as the window, the short-wavelength sensitivity
will be lost.
When detecting wavelengths shorter than 300 nm, Si
photodiodes with quartz windows are used. Measurements
limited to the visible light region use a visual-sensitive
compensation filter that allows only visible light to pass
through it.
Figure 1-11 shows spectral responses for various types
of Si photodiodes. The BQ type uses a quartz window
and the BR type a resin-coated window. The S9219 is a Si
photodiode with a visual-sensitive compensation filter.
6
1200
Wavelength (nm)
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1-5
1000
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At a given wavelength, the number of electrons or holes that
can be extracted as a photocurrent divided by the number
of incident photons is called the quantum efficiency (QE).
The quantum efficiency is given by equation (6).
QE = S × 1240 × 100 [%] ............ (6)
λ
S: photosensitivity [A/W]
λ: wavelength (nm)
The IR-enhanced Si PIN photodiode features drastically
improved sensitivity in the near infrared region for
wavelengths from 900 nm to 1100 nm.
Since silicon has a large light absorption coefficient in
the visible and ultraviolet regions, even a photodiode
from a thin wafer can sufficiently detect light in these
regions. However, in the near infrared region, the light
absorption coefficient becomes extremely low (allowing
more light to pass through), which lowers the sensitivity.
To achieve high sensitivity with silicon in the near
infrared region, the light absorption layer could be made
thicker by using a thicker silicon wafer, but this causes
shortcomings such as the need for high supply voltage,
increased dark current, and decreased response speed.
With the IR-enhanced Si PIN photodiode, special
micromachining is applied to the backside to achieve
high sensitivity in the near infrared region. For example,
if this technology is applied to a Si photodiode whose
quantum efficiency is 25% at a wavelength of 1.06 µm,
a quantum efficiency of 72% (about three times higher)
can be achieved. This technology allows photodiodes
with high-speed and high sensitivity in the near infrared
region to be produced, which was difficult in the past. The
IR-enhanced Si PIN photodiode is used for monitoring the
YAG laser (1.06 µm).
The lower limit of light detection for Si photodiodes is
usually expressed as the incident light level required to
generate a current equal to the noise current as expressed
in equation (8) or (9), which is termed the noise equivalent
power (NEP).
[Figure 1-12] Spectral response
(IR-enhanced Si PIN photodiode)
(Typ. Ta=25 ˚C)
0.8
0.7
Photosensitivity (A/W)
QE=100%
0.6
NEP = in [W/Hz1/2] ............ (11)
S
0.5
in: noise current [A/Hz1/2]
S : photosensitivity [A/W]
0.4
0.3
0.2
IR-enhanced Si PIN
photodiode
0.1
0
200
In cases where ij is predominant, the relation between
NEP and shunt resistance is plotted as shown in Figure
1-13. This relation agrees with the theoretical data.
Previous product
400
600
800
1000
1200
[Figure 1-13] NEP vs. shunt resistance (S1226-5BK)
Wavelength (nm)
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Noise characteristics
Like other types of photosensors, the lower limits of light
detection for Si photodiodes are determined by their
noise characteristics. The Si photodiode noise current (in)
is the sum of the thermal noise current or Johnson noise
current (ij) of a resistor which approximates the shunt
resistance (Rsh) and the shot noise currents (iSD and iSL)
resulting from the dark current and the photocurrent.
Theoretical line
NEP (W/Hz1/2)
1-6
Shunt resistance (Ω)
in =
ij2 + iSD2 + iSL2 [A] ............ (7)
ij is viewed as the thermal noise of Rsh and is given by
equation (8).
ij =
4k T B [A] ............ (8)
Rsh
k: Boltzmann’s constant
T: absolute temperature of photodiode
B: noise bandwidth
When a reverse voltage is applied as in Figure 1-9, there
is always a dark current. The shot noise iSD of the dark
current is given by equation (9).
isD =
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1-7
This is a measure of the sensitivity uniformity in the
photosensitive area. Si photodiodes offer excellent sensitivity
uniformity; their nonuniformity in 80% of the effective
photosensitive area in the visible to near infrared region is
less than 2%. This is measured with a light beam (e.g., from a
laser diode) condensed to a small spot from a few microns to
dozens of microns in diameter.
[Figure 1-14] Sensitivity uniformity (S1227-1010BQ)
2q ID B [A] ............ (9)
(Typ. Ta=25 ˚C, VR=0 V)
100
The shot noise iSL generated by photocurrent (IL) due to
the incident light is expressed by equation (10).
2q IL B [A] ............ (10)
If IL >> 0.026/Rsh or IL >> ID, the shot noise current iSL of
equation (10) becomes predominant instead of the noise
factor of equation (8) or (9).
The amplitudes of these noise sources are each proportional
to the square root of the noise bandwidth (B) so that they
are expressed in units of A/Hz1/2 normalized by B.
Relative sensitivity (%)
q : electron charge
ID: dark current
isL =
Sensitivity uniformity
Incident light: ϕ7 μm
λ=680 nm
50
Photosensitive
area
(10 × 10 mm)
Nonuniformity is 2% or less within
80% of effective photosensitive area.
0
Position on photosensitive area (1 mm/div.)
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7
1-8
Response speed
The response speed of a photodiode is a measure of how
fast the generated carriers are extracted to an external
circuit as output current, and it is generally expressed
as the rise time or cutoff frequency. The rise time is the
time required for the output signal to change from 10%
to 90% of the peak output value and is determined by the
following factors.
(1) Time constant t 1 of terminal capacitance Ct and
load resistance RL
Ct is the sum of the package capacitance and the photodiode
junction capacitance (Cj). t1 is then given by equation (12).
t1 = 2.2 × Ct × RL .......... (12)
To shorten t1, the design must be such that Ct or RL is made
smaller. Cj is nearly proportional to the photosensitive
area (A) and inversely proportional to the depletion layer
width (d). Since the depletion layer width is proportional
to the second to third root of the product of the reverse
voltage (VR) and the electrical resistivity (ρ) of the substrate
material, this is expressed by equation (13).
Cj ∝ A {(VR + 0.5) × ρ}
-1/2 to -1/3
............ (13)
Accordingly, to shorten t1, a photodiode with a small A and
large ρ should be used with a reverse voltage applied. However,
this is advisable in cases where t1 is a predominant factor
affecting the response speed, so it should be noted that carrier
transit time (t3) in the depletion layer becomes slow as ρ is
made large. Furthermore, applying a reverse voltage also
increases dark current, so caution is necessary for use in lowlight-level detection.
(2) Diffusion time t2 of carriers generated outside the
depletion layer
Carriers may be generated outside the depletion layer
when incident light is absorbed by the area surrounding
the photodiode photosensitive area and by the substrate
section which is below the depletion layer. The time (t2)
required for these carriers to diffuse may sometimes be
greater than several microseconds.
(3) Carrier transit time t3 in the depletion layer
The transit speed (vd) at which the carriers travel in the
depletion layer is expressed using the carrier traveling
rate (µ) and the electric field (E) in the depletion layer, as
in vd = µ E. The average electric field is expressed using
the reverse voltage (VR) and depletion layer width (d), as
in E = VR/d, and thus t3 can be approximated by equation
(14).
t3 =
8
d2 ............
d
=
(14)
VR
vd
To shorten t3, the distance traveled by carriers should be
short or the reverse voltage higher. t3 becomes slower as
the resistivity is increased.
The above three factors determine the rise time of a photodiode.
The rise time (tr) is approximated by equation (15).
tr =
t12 + t22 + t32 ............. (15)
As can be seen from equation (15), the factor that is slowest
among the three factors becomes predominant. As stated
above, t1 and t3 contain the factors that contradict each other.
Making one faster inevitably makes the other slower, so it is
essential to create a well-balanced design that matches the
application.
When a photodiode receives sine wave-modulated light
emitted from a laser diode and the like, the cutoff frequency
(fc) is defined as the frequency at which the photodiode
output drops by 3 dB relative to the 100% output level which
is maintained while the sine wave frequency is increased.
This is roughly approximated from the rise time (tr) as in
equation (16).
fc = 0.35 ............ (16)
tr
Figure 1-15 shows examples of the response waveforms
and frequency characteristics for Si photodiodes.
[Figure 1-15] Examples of response waveforms and
frequency characteristics
(a) Response waveforms
Light input
Output waveform
(t1 and t3 are dominant)
Output waveform
(t2 is dominant)
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(b) Response waveform (S2386-18K)
[Figure 1-16] Cutoff frequency vs. reverse voltage
(S5973, S9055)
Cutoff frequency
Output (5 mV/div.)
(Typ. Ta=25 °C, λ=655 nm, VR=0 V, RL=1 kΩ)
Time (500 ns/div.)
Reverse voltage (V)
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(c) Frequency characteristics (S5973)
Figure 1-17 shows an example of a simple connection with
50 Ω load resistance (measurement device input impedance).
The ceramic capacitor C is used to suppress ripples or noise
which may occur from the reverse voltage power supply,
while the resistor R is used to protect the Si photodiode. The
resistor value is selected such that the extent of the voltage
drop caused by the maximum photocurrent will be sufficiently
smaller than the reverse voltage. The Si photodiode leads,
capacitor leads, and coaxial cable wires carrying high-speed
pulses should be kept as short as possible.
Relative output (dB)
(Typ. Ta=25 °C, λ=830 nm, RL=50 Ω, VR=3.3 V)
[Figure 1-17] Connection example of coaxial cable
Light
Measuring device
50 Ω coaxial cable
Frequency
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PIN photodiodes are designed such that fewer carriers are
generated outside the depletion layer, the terminal capacitance
is small, and the carrier transit time in the depletion layer is
short. They are suited for optical communications and other
applications requiring high-speed response. Hamamatsu PIN
photodiodes exhibit relatively low dark current when reverse
voltage is applied and have excellent voltage resistance.
Figure 1-16 shows changes in the cutoff frequency with
increasing reverse voltage.
Reverse
voltage
Measuring device
input impedance
(should be terminated
with 50 Ω)
PD: high-speed Si PIN photodiode (S5972, S5973, S9055, S9055-01, etc.)
R : 10 kΩ; Voltage drop by photocurrent should be sufficiently lower than reverse voltage.
C : 0.1 μF ceramic capacitor
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1-9
Connection to an op amp
Feedback circuit
Figure 1-18 shows basic connection examples of a Si photodiode
and op amp. When connected with this polarity, in the DC to
low-frequency region, the output voltage Vout is 180 degrees
out of phase with the input current (photodiode short circuit
current Isc) and is given by: Vout = -Isc × Rf. The feedback
resistance Rf is determined by how much the input current
needs to be multiplied. If, however, the feedback resistance is
made greater than the photodiode shunt resistance Rsh, the
op amp equivalent input voltage noise (en) and input offset
voltage will be multiplied by ( 1 + Rf ) and then superimposed
Rsh
on the output voltage Vout. Moreover, the op amp’s bias current
error (described later) will also increase, thus making it not
practical to use an infinitely large feedback resistance. If there
is an input capacitance Ct, the feedback capacitance Cf
9
prevents unstable operation of the circuit in high-frequency
regions. The feedback capacitance and feedback resistance
also form a lowpass filter with a time constant of Cf × Rf, so
their values should be chosen according to the application.
When it is desired to integrate the amount of incident light
in applications such as radiation detection, Rf should be
removed so that the op amp and Cf act as an integrating
circuit. However, a switch is required to discharge Cf in
order to detect continuous signals.
[Figure 1-18] Connection examples of Si photodiode
and op amp
(a)
Rf=10 MΩ
Cf=10 pF
Ct=
100 pF
Rsh=
100 MΩ
+ en IC
Vout
Gain peaking
The high-frequency response characteristics of a Si photodiode
and op amp circuit are determined by the time constant Rf ×
Cf. However, if the terminal capacitance or input capacitance is
large, a phenomenon known as “gain peaking” will sometimes
occur. Figure 1-19 contains examples of frequency response
characteristics showing gain peaking. The output voltage
increases abnormally in the high-frequency region [see the
upper trace in Figure 1-19 (a)], causing significant ringing in the
output voltage waveform in response to the pulsed light input
[Figure 1-19 (b)]. This gain operates in the same manner with
respect to op amp input noise and may result in abnormally
high noise levels [see the upper trace in Figure 1-19 (c)]. This
occurs at the high-frequency region when each reactance
of the input capacitance and the feedback capacitance of
the op amp jointly form an unstable amplifier with respect
to noise. In such a case, adverse effects on light detection
accuracy may result.
KPDC0011EA
[Figure 1-19] Gain peaking
(b)
(a) Frequency characteristics
FET
Reset
Cf=10 pF
Rsh=
100 MΩ
+ en IC
+10
Vout
KPDC0035EA
Relative output (dB)
Ct=
100 pF
(Typ.)
+20
0
-10
-20
-30
IC : op amp
en: equivalent input voltage noise of op amp
-40
Bias current
103
104
105
Frequency (Hz)
Circuit
: Figure 1-18 (a) Upper trace: Cf=0 pF
Op amp
: AD549
Lower trace: Cf=10 pF
Light source: 780 nm
KPDB0019EA
(b) Light pulse response (typical example)
+100
+50
Output voltage (mV)
Since the actual input impedance of an op amp is not
infinite, some bias current will flow into or out of the input
terminals. This may result in error, depending on the
magnitude of the detected current. The bias current which
flows in an FET-input op amp is sometimes lower than 0.1
pA. Bipolar op amps, however, have bias currents ranging
from several hundred picoamperes to several hundred
nanoamperes. In general, the bias current of FET-input op
amps doubles for every 10 °C increase in temperature, while
the bias current of bipolar op amps decreases. In some cases,
the use of a bipolar op amp should be considered when
designing circuits for high-temperature operation. As is the
case with offset voltage, the error voltage attributable to the
bias current can be adjusted by means of a variable resistor
connected to the offset adjustment terminals of the op amp.
Leakage currents on the printed circuit board used to
configure the circuit may be greater than the op amp’s bias
current. Besides selecting the optimal op amp, consideration
must be given to the circuit pattern design and parts layout,
as well as the use of guard rings and Teflon terminals.
-50
102
0
-50
-100
-150
-200
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Time (ms)
Circuit : Figure 1-18 (a) Light source: 780 nm
Op amp: AD549
Cf=0 pF
10
KPDB0020EA
(c) Frequency characteristics of noise output
(typical example)
[Figure 1-20] Graphical representation of gain peaking
106
107
108
102
103
104
Gain
peaking
Ci
rc
ui
tg
ai
n
Circuit gain, open-loop gain
105
e
pl
m
p xa
)
am e
Hz
n
i
p
M
O ga
1
=
op
th
-lo
id
w
en
nd
op
ba
n
ai
(g
Output noise voltage (V/Hz1/2)
104
105
Frequency (Hz)
Frequency (Hz)
KPDB0016EA
To eliminate gain peaking, take the following measures:
Circuit : Figure 1-18 (a) Upper trace: Cf=0 pF
Op amp: AD549
Lower trace: Cf=10 pF
KPDB0021EA
Elimination of gain peaking
To achieve a wide frequency characteristic without gain
peaking and ringing phenomena, it is necessary to select
the optimal relationship between the photodiode, op amp,
feedback resistance, and feedback capacitance. It will prove
effective in this case to reduce the terminal capacitance (Ct),
as was previously explained in section 1-8, “Response speed.”
In the op amp, the higher the speed and the wider the
bandwidth, the less the gain peaking that occurs. However,
if adequate internal phase compensation is not provided,
oscillation may be generated as a result. Connect the
feedback elements in parallel, not only the resistance
but also the feedback capacitance, in order to avoid gain
peaking. The above measures can be explained as follows,
using the circuit shown in Figure 1-18 (a).
As shown in Figure 1-20, the circuit gain of the op amp is
determined for the low-frequency region simply by the
resistance ratio of Rsh to Rf.
Rsh + Rf
From the frequency f1 =
, gain begins to
2π Rsh Rf (Cf + Ct)
increase with frequency as shown in region . Next, at
1
the frequency f2 =
, and above, the circuit gain of
2π Cf Rf
the op amp enters a flat region which is determined by
the ratio of Ct and Cf. At the point of frequency f3 where
circuit gain contacts the open-loop gain line (normally,
rolloff is 6 dB/octave) of the op amp, region is entered. In
this example, f1 and f2 correspond to 160 Hz and 1.6 kHz,
respectively, under the circuit conditions of Figure 1-18
(a). If Cf is made 1 pF, f2 shifts to f2’ and the circuit gain
increases further. What should be noted here is that, since
the setting of increasing circuit gain in region exceeds the
open-loop gain line of the op amp, region actually does
not exist. As a result, gain peaking occurs in the frequency
characteristics of the op amp circuit, and ringing occurs in
the pulsed light response characteristics, then instability
results [Figure 1-19].
(1) Determine Rf and Cf so that the flat region in Figure
1-20 exists.
(2) When f2 is positioned to the right of the open-loop
gain line of the op amp, use the op amp having a high
frequency at which the gain becomes 1 (unity gain
bandwidth), and set region .
(3) Replace a photodiode with a low Ct value. In the example
Ct
shown in Figure 1-20, ( 1 + Cf ) should be close to 1.
The above measures (1) and (2) should reduce or prevent
gain peaking and ringing. However, in the high-frequency
region , circuit gain is present, and the input noise of the
op amp and feedback resistance noise are not reduced,
but rather, depending on the circumstances, may even be
amplified and appear in the output. Measure (3) can be
used to prevent this situation.
Using the above procedures, the S/N deterioration caused
by gain peaking and ringing can usually be solved. However,
regardless of the above measures, if load capacitance from
several hundred picofarads to several nanofarads or more
(for example, a coaxial cable of several meters or more and a
capacitor) is connected to the op amp output, oscillation may
occur in some types of op amps. Thus the load capacitance
must be set as small as possible.
1 - 10
Application circuit examples
Ultra-low-light detection circuit
Ultra-low-light detection circuits require measures for
reducing electromagnetic noise in the surrounding area,
AC noise from the power supply, and internal op amp
noise, etc.
Figure 1-21 shows some measures for reducing electromagnetic
noise in the surrounding area.
11
[Figure 1-21] Ultra-low-light sensor head
(a) Using shielded cable to connect to photodiode
Metal package
PD
Shielded
cable
10-turn
potentiometer
Metal shielded box
BNC
coaxial
cable and
the like
leakage from the surface of the circuit board, try using a
guard pattern or aerial wiring with teflon terminals for the
wiring from the photodiode to op amp input terminals and
also for the feedback resistor (Rf) and feedback capacitor
(Cf) in the input wiring.
Hamamatsu offers the C6386-01, C9051, and C9329
photosensor amplifiers optimized for use with photodiodes
for ultra-low-light detection.
[Figure 1-22] Photosensor amplifiers
KSPDC0051EC
(a) C6386-01
(b) C9051
(b) Using metal shielded box that contains entire circuit
10-turn
potentiometer
Metal shielded box
KSPDC0052EB
(c) C9329
(c) Using optical fiber
Rf1
Rf2
PD
Optical fiber ISC
Cf
-
IC1
+
SW1
SW2
+
10
+
10
-
IC2
+
+5 V
0
-5 V
Photodiodes and coaxial cables
with BNC-to-BNC plugs are sold
separately.
Vo
10-turn
potentiometer
Metal shielded box
KSPDC0053EB
Bold
IC1 :
IC2 :
Cf :
Rf :
SW:
PD :
lines should be within guarded layout or on Teflon terminals.
FET-input op amp and the like
OP07 and the like
10 pF to 100 pF polystyrene capacitor
10 GΩ max.
reed relay or switch with low leakage current
S1226/S1336/S2386 series, S2281, and the like
Vo = Isc × Rf [V]
Terminating the photosensitive area of the photodiode
to the ground to use it as a shield layer and extracting the
photodiode signal from the cathode terminal is another
effective means. An effective countermeasure against AC
noise from the power supply is inserting an RC filter or an
LC filter in the power supply line. Using a dry cell battery
for the power supply also proves effective against power
supply noise. Op amp noise can be reduced by selecting
an op amp having a low 1/f noise and low equivalent
input noise current. Moreover, high-frequency noise can
be reduced by using a feedback capacitor (Cf ) to limit the
frequency bandwidth of the circuit to match the signal
frequency bandwidth.
Output errors (due to the op amp input bias current and
input offset voltage, routing of the circuit wiring, circuit
board surface leakage current, etc.) must next be reduced.
Select an FET-input op amp or a CMOS input op amp with
low 1/f noise, both of which allow input bias currents below
a few hundred femtoamperes. In addition, it will be effective
to use an op amp that provides input offset voltages below
several millivolts and has an offset adjustment terminal.
Also use a circuit board made from materials having high
insulation resistance. As countermeasures against current
12
Light-to-logarithmic voltage conversion circuit
The voltage output from a light-to-logarithmic voltage
conversion circuit [Figure 1-23] is proportional to the
logarithmic change in the detected light level. The log
diode D for logarithmic conversion should have low dark
current and low series resistance. The base-emitter (B-E)
junction of a small signal transistor or the gate-source
(G-S) junction of a junction FET can also be used as
the log diode. IB is the current source that supplies bias
current to the log diode D and sets the circuit operating
point. Unless this IB current is supplied, the circuit will
latch up when the photodiode short circuit current Isc
becomes zero.
[Figure 1-23] Light-to-logarithmic voltage conversion circuit
D:
IB :
R:
Io :
IC:
diode of low dark current and low series resistance
current source for setting circuit operating point, IB << Isc
1 GΩ to 10 GΩ
saturation current of D, 10-15 to 10-12 A
FET-input op amp and the like
Vo ≈ -0.06 log (
Isc + IB
+ 1) [V]
Io
KPDC0021EA
After calibration, the output should be 1 mV/lx in the L range,
and 100 mV/lx in the M range on the C9329.
Light level integration circuit
This light level integration circuit uses an integration circuit
made up of a photodiode and an op amp. This is used to
measure the amount of integrated light or average amount
of a light pulse train with irregular pulse heights, cycles, and
widths.
The IC and C in Figure 1-24 make up the integrator that
accumulates short circuit current Isc generated by each light
pulse in the integration capacitor C. By measuring the output
voltage Vo immediately before reset, the average short circuit
current can be obtained from the integration time (to) and
the capacitance C. A low dielectric absorption type capacitor
should be used as the capacitance C to eliminate reset errors.
The switch SW is a CMOS analog switch.
[Figure 1-25] Simple illuminometer (1)
Photosensor
amplifier
PD
ISC
1k
C9329
Coaxial cable
E2573
VR
1k
CW
V
500
Externally connected
voltage divider circuit
PD: S9219 (4.5 μA/100 lx)
KSPDC0054EB
Simple illuminometer (2)
[Figure 1-24] Light level integration circuit
+15 V
10 k
C
13
1
1k
2 SW
14
Isc
2
3
Reset input
Isc
+15 V
7
IC
+
4
-
PD
1k
7
6
VO
t
VO
-15 V
t
Reset input
to
t
Reset input: Use TTL "low" level to reset.
IC
: LF356 and the like
SW
: CMOS 4066
PD
: S1226/S1336/S2386 series and the like
C
: polycarbonate capacitor and the like
VO = Isc × tO ×
1
[V]
C
KPDC0027EB
Simple illuminometer (1)
This is a simple illuminometer circuit using an op amp
current-voltage conversion circuit and the S7686 Si
photodiode with sensitivity corrected to match human
eye sensitivity. This circuit can measure illuminance up
to a maximum of 10000 lx by connecting to a voltmeter in
the 1 V range.
Use a low current consumption type op amp that operates
from a single power supply and allows low input bias currents.
A simple calibration can be performed using a 100 W white
light source.
To calibrate this circuit, first select the 10 mV/lx range and
short the op amp output terminal to the sliding terminal
of the variable resistor for meter calibration. Next turn on
the white light source, and adjust the distance between the
white light source and the S7686 so that the voltmeter reads
0.45 V. (The illuminance on the S7686 surface at this time is
approx. 100 lx.) Then adjust the variable resistor for meter
calibration until the voltmeter reads 1 V. The calibration is
now complete.
[Figure 1-26] Simple illuminometer (2)
A simple illuminometer circuit can be configured by using
the Hamamatsu C9329 photosensor amplifier and the S9219
Si photodiode with sensitivity corrected to match human eye
sensitivity. As shown in Figure 1-25, this circuit can measure
illuminance up to a maximum of 1000 lx by connecting the
output of the C9329 to a voltmeter in the 1 V range via an
external resistive voltage divider.
A standard light source is normally used to calibrate this
circuit, but if not available, then a simple calibration can
be performed with a 100 W white light source.
To calibrate this circuit, first select the L range on the C9329
and then turn the variable resistor VR clockwise until it stops.
Block the light to the S9219 while in this state, and rotate
the zero adjustment knob on the C9329 so that the voltmeter
reads 0 V. Next turn on the white light source, and adjust the
distance between the white light source and the S9219 so
that the voltmeter display shows 0.225 V. (The illuminance
on the S9219 surface at this time is approx. 100 lx.) Then
turn the VR counterclockwise until the voltmeter display
shows 0.1 V. The calibration is now complete.
1M
10 mV/lx
100 k
1 mV/lx
10 k
0.1 mV/lx
100 p
VR
2
3
PD
Isc
-
IC
+
4
500
1k
7
6
8
1k
006 p
(9 V)
V Voltmeter
VR: variable resistor for meter calibration
IC : TLC271 and the like
PD: S7686 (0.45 A/100 lx)
KPDC0018EE
13
[Figure 1-28] Absorptiometer
Light balance detection circuit
+15 V
(Sample)
Figure 1-27 shows a light balance detector circuit utilizing
two Si photodiodes, PD1 and PD2, connected in reverseparallel and an op amp current-voltage converter circuit.
The photosensitivity is determined by the value of the
feedback resistance Rf. The output voltage Vo becomes
zero when the light levels incident on PD1 and PD2 are
equal. Since two diodes D are connected in reverse in
parallel, Vo will be limited to about ±0.5 V when the light
levels on PD1 and PD2 are in an unbalanced state, so that
only the light level near a balanced state can be detected
with high sensitivity. If a filter is used, this circuit can
also be utilized to detect a light level balance in specific
wavelength regions.
Rf
D
D
ISC1
PD2
2
-
IC
PD1
3
+
+15 V
7
6
4
PD
Vo
A
+
Filter
Isc2
100 p
-15 V
A : log amp
PD: S5870 and the like
Vo = log (ISC1/ISC2) [V]
KPDC0025EC
Total emission measurement of LED
Since the emitting spectral width of LED is usually as
narrow as dozens of nanometers, the amount of the
LED emission can be calculated from the Si photodiode
photosensitivity at a peak emission wavelength of the
LED. In Figure 1-29, the inner surface of the reflector
block B is mirror-processed and reflects the light emitted
from the side of the LED toward the Si photodiode,
so that the total amount of the LED emission can be
detected by the Si photodiode.
[Figure 1-27] Light balance detection circuit
ISC2
Isc1
Vo
-15 V
PD : S1226/S1336/S2386 series and the like
IC : LF356 and the like
D : ISS226 and the like
[Figure 1-29] Total emission measurement of LED
Vo = Rf × (Isc 2 - Isc1) [V]
(Note that Vo is within ±0.5 V.)
Isc
IF
KPDC0017EB
Po
LED
PD
A
Absorptiometer
This is a light absorption meter that obtains a logarithmic
ratio of two current inputs using a dedicated IC and two
Si photodiodes [Figure 1-28]. By measuring the light level
of the light source and the light level transmitting through
a sample using two Si photodiodes and then comparing
them, light absorbance by the sample can be measured.
To make measurements, the optical system such as an
aperture diaphragm should first be adjusted so that the
short circuit currents of the two Si photodiodes are equal
and the output voltage Vo is set to 0 V. Next, the sample is
placed on the light path of one photodiode. The output
voltage at this point indicates the absorbance of the
sample. The relation between the absorbance A and the
output voltage Vo is expressed by A=-Vo [V].
If necessary, a filter is placed in front of the light source
as shown in Figure 1-28 in order to measure the spectral
absorbance of a specific wavelength region or monochromatic
light.
14
B
A :
PD :
B :
S :
ammeter, 1 mA to 10 mA
S2387-1010R
aluminum block with inner surface gold-plated
Si photodiode photosensitivity
See characteristics table in our datasheet.
S2387-1010R: S ≈ 0.58 A/W at 930 nm
Po : total amount of emission
Po ≈
Isc
[W]
S
KPDC0026EA
High-speed light detection circuit (1)
This is a high-speed light detection circuit using a lowcapacitance Si PIN photodiode with a reverse voltage
applied and a high-speed op amp current-voltage converter
circuit [Figure 1-30]. The frequency band of this circuit is
limited by the op amp device characteristics to less than
about 100 MHz.
When the frequency band exceeds 1 MHz in this circuit, the
lead inductance of each component and stray capacitance
from feedback resistance Rf exert drastic effects on device
response speed. That effect can be suppressed by using chip
components to reduce the component lead inductance,
and connecting multiple resistors in series to reduce stray
capacitance.
The photodiode leads should be kept as short as possible,
and the pattern wiring to the op amp should be made
as short and thick as possible. This will lower the effects
from the stray capacitance and inductance occurring on
the circuit board pattern of the op amp inputs and also
alleviate effects from photodiode lead inductance. To
enhance device performance, a ground plane structure
using the entire surface of the board copper plating as the
ground potential will be effective.
A ceramic capacitor should be used for the 0.1 µF capacitor
connected to the op amp power line, and it should be
connected to the nearest ground point in the shortest
distance.
Hamamatsu provides the C8366 photosensor amplifier
for PIN photodiodes with a frequency bandwidth up to
100 MHz.
[Figure 1-32] High-speed light detection circuit (2)
[Figure 1-30] High-speed light detection circuit (1)
This is an AC light detection circuit [Figure 1-33] that
uses load resistance RL to convert the photocurrent from
a low-capacitance Si PIN photodiode (with a reverse
voltage applied) to a voltage, and amplifies the voltage
with a high-speed op amp. In this circuit, there is no
problem with gain peaking due to phase shifts in the op
amp. A circuit with a frequency bandwidth higher than
100 MHz can be fabricated by selecting the correct op
amp.
Points for caution in the components, pattern, and
structure are the same as those listed for the “High-speed
light detection circuit (1).”
51 Ω
PD: high-speed PIN photodiode (S5971, S5972, S5973, etc.)
Rf : Two or more resistors are connected in series to eliminate parallel capacitance.
IC : AD745, LT1360, HA2525, etc.
Vo = -Isc × Rf [V]
KPDC0020ED
10 k
PD
0.1
+ 10
+5 V
0.1
3 +7 6
A
IC
2 - 4 0.1
Isc
RL
R
51 Ω
Vo
Rf
-5 V
PD
: high-speed PIN photodiode
(S5971, S5972, S5973, S9055, S9055-01, etc.)
R L, R, Rf: adjusted to meet the recommended conditions of op amp
IC
: AD8001 and the like
Vo = Isc × R L × (1 +
Rf
) [V]
R
KPDC0015EE
AC light detection circuit (1)
[Figure 1-31] Photosensor amplifier C8366
[Figure 1-33] AC light detection circuit (1)
10 k
PD
0.1
Isc
RL
+ 10
+5 V
0.1
3 +7 6
IC
A
2 - 4 0.1
C
r
R
51 Ω
Vo
Rf
-5 V
PD
: high-speed PIN photodiode
(S5971, S5972, S5973, S9055, S9055-01, etc.)
R L, R, Rf, r: adjusted to meet the recommended conditions of op amp
IC
: AD8001 and the like
Vo = Isc × R L × (1 + Rf ) [V]
R
KPDC0034EA
High-speed light detection circuit (2)
This high-speed light detection circuit [Figure 1-32] uses
load resistance RL to convert the short circuit current from
a low-capacitance Si PIN photodiode (with a reverse voltage
applied) to a voltage, and amplifies the voltage with a highspeed op amp. In this circuit, there is no problem with gain
peaking due to phase shifts in the op amp. A circuit with a
frequency bandwidth higher than 100 MHz can be fabricated
by selecting the correct op amp. Points for caution in the
components, pattern, and structure are the same as those
listed for the “High-speed light detection circuit (1).”
AC light detection circuit (2)
This AC light detection circuit utilizes a low-capacitance
PIN photodiode with a reverse voltage applied and an
FET serving as a voltage amplifier [Figure 1-34]. Using a
low-noise FET allows producing a small and inexpensive
low-noise circuit, which can be used in light sensors
for FSP (free space optics), optical remote control, etc.
In Figure 1-34, the signal output is taken from the FET
drain. However, to interface to a next-stage circuit having
low input resistance, the signal output should be taken
from the source or a voltage-follower should be added.
15
[Figure 1-34] AC light detection circuit (2)
2.
+15 V
10 k
+
+ 10
1k
PD
0.1
0.1
1000 p
Vo
FET
ISC
RL
PD :
RL :
RS :
FET:
1M
PSD (position sensitive detectors)
10
RS
0.1
high-speed PIN photodiode (S2506-02, S5971, S5972, S5973, etc.)
determined by photodiode sensitivity and terminal capacitance
determined by FET operating point
2SK362 and the like
KPDC0014EE
Various methods are available for detecting the position of
incident light, including methods using an array of many
small detectors and a multi-element detector (e.g., image
sensor). In contrast to these, the PSD is a monolithic device
designed to detect the position of incident light.
Since the PSD is a non-segmented photosensor that makes
use of the surface resistance of the photodiode, it provides
continuous electrical signals and offers excellent position
resolution, fast response, and high reliability.
The PSD is used in a wide range of fields such as measurements
of position, angles, distortion, vibration, and lens reflection/
refraction. Applications also include precision measurement
such as laser displacement meters, as well as optical remote
control devices, distance sensors, and optical switches.
2-1
Features
•
Excellent position resolution
•
Wide spectral response range
•
High-speed response
•
Simultaneous detection of light level and center-ofgravity position of light spot
•
High reliability
2-2
16
Structure and operating 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
photosensitive area, which is also a resistive layer, has a
PN junction that generates photocurrent by means of the
photovoltaic effect.
Figure 2-1 is a schematic view of a PSD cross section showing
the operating principle. On an N-type high-resistivity silicon
substrate, a P-type resistive layer is formed that serves as
a photosensitive area for photoelectric conversion and
a resistive layer. A pair of output electrodes is formed on
both ends of the P-type resistive layer. 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 light spot strikes the PSD, an electric charge
proportional to the light level is generated at the light incident
position. This electric charge flows as photocurrents through
the resistive layer and is extracted from the output electrodes
X1 and X2, while being divided in inverse proportion to
the distance between the light incident position and each
electrode.
[Figure 2-1] Schematic of PSD cross section
[Figure 2-3] Photosensitive area (one-dimensional PSD)
XB
XA
Output current IX1
Output current I X2
Incident light
Output
electrode X1
Output electrode X2
P-type resistive layer
photocurrent
Photosensitive
area
I-layer
KPSDC0010EB
N-layer
Incident position conversion formula (See also Figure 2-3.)
Common electrode
IX2 - IX1 2XA ........
=
(9)
IX1 + IX2
LX
Resistance length L X
KPSDC0005EB
In Figure 2-1, the relation between the incident light
spot position and the output currents from the output
electrodes X1 and X2 is as follows:
When the center point of the PSD is set as the origin:
IX1
LX
- XA
2
=
× Io .... (1)
LX
IX2
2XA ........
IX2 - IX1
=
(3)
IX1 + IX2
LX
LX
+ XA
2
=
× Io ... (2)
LX
LX - 2XA .......
IX1
=
(4)
IX2
LX + 2XA
LX - XB
× Io ...... (5)
LX
IX2 =
IX2 - IX1 2XB - LX ...
=
(7)
IX1 + IX2
LX
IX1:
IX2:
Io :
LX :
XA :
XB :
The shapes of the photosensitive area and electrodes of
two-dimensional PSDs have been improved to suppress
interactions between the electrodes. Besides the advantages
of small dark current, high-speed response, and easy
application of reverse voltage, the peripheral distortion
has been greatly suppressed. Incident position conversion
formulas are shown in equations (10) and (11).
[Figure 2-4] Structure and equivalent circuit
(two-dimensional PSD)
When the end of the PSD is set as the origin:
IX1 =
Two-dimensional PSD
XB
× Io ........... (6)
LX
LX - XB ...........
IX1
=
(8)
IX2
XB
output current from electrode X1
output current from electrode X2
total photocurrent (IX1 + IX2)
resistance length (length of photosensitive area)
distance from electrical center position of PSD to light incident position
distance from electrode X1 to light incident position
Rp
Anode (X2)
Anode (Y1)
Cathode (common)
P
D Cj Rsh
P : current source
D : ideal diode
Cj : junction capacitance
Rsh: shunt resistance
Rp : positioning resistance
KPSDC0009EC
[Figure 2-5] Photosensitive area (two-dimensional PSD)
LX
Y2
X1
LY
By finding the values of IX1 and IX2 from equations (1),
(2) , (5), and (6) and substituting them into equations
(3), (4), (7), and (8), the light incident position can be
obtained irrespective of the incident light level and
its changes. The light incident position obtained here
corresponds to the center-of-gravity of the light spot.
Anode (Y2)
Anode (X1)
YA
X2
XA
One-dimensional PSD
Photosensitive
area*
[Figure 2-2] Structure and equivalent circuit
(one-dimensional PSD)
Y1
* Photosensitive area is specified as the inscribed square.
KPSDC0012EC
Anode (X1)
Incident position conversion formulas (See also Figure 2-5.)
Anode (X2)
(IX2 + IY1) - (IX1 + IY2) 2XA ........
=
(10)
IX1 + IX2 + IY1 + IY2
LX
Cathode (common)
P : current source
D : ideal diode
Cj : junction capacitance
Rsh: shunt resistance
Rp : positioning resistance
(IX2 + IY2) - (IX1 + IY1) 2YA ........
=
(11)
IX1 + IX2 + IY1 + IY2
LY
KPSDC0006EA
17
Position detection error
The position of a light spot incident on the PSD surface
can be measured by making calculations based on the
photocurrent extracted from each output electrode. The
position obtained with the PSD is the center-of-gravity of
the light spot, and it is independent of the light spot 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 one of the most important characteristics of
a PSD.
If a light spot strikes the PSD surface and the photocurrents
extracted from the output electrodes are equal, the
position of the incident light spot on the PSD is viewed
as the electrical center position. Using this electrical
center position as the origin point, 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 photocurrents.
Figures 2-7 shows the photocurrent measurement example
using a one-dimensional PSD with a resistance length of 3
mm (e.g., S4583-04). The position detection error determined
from the data is also shown in Figure 2-8.
[Figure 2-7] Photocurrent measurement example of
one-dimensional PSD (e.g., S4583-04)
(Ta=25 ˚C)
1.0
Photocurrent (relative value)
2-3
IX2
IX1
0.5
0
-1.5
0
+1.5
Position on PSD (mm)
KPSDB0114EA
[Figure 2-8] Position detection error example of onedimensional PSD (e.g., S4583-04)
[Figure 2-6] Schematic of PSD cross section
Output
electrode X1
Electrical center
position
Output electrode X2
P-type resistive layer
I-layer
N-layer
Light incident position Xi
Calculated position Xm
Common electrode
KPSDC0071EB
A position detection error is calculated as described below.
In Figure 2-6, which shows the electrical center position as
the reference position (origin point), if the actual position of
incident light spot is Xi, the photocurrents obtained at the
output electrodes are IX1 and IX2, and the position calculated
from the photocurrents is Xm, then the difference in distance
between Xi and Xm is defined as the position detection error (E).
E = Xi - Xm [μm] ............. (12)
Xi : actual position of incident light [μm]
Xm: calculated position [μm]
Xm =
IX2 - IX1 LX ........
×
(13)
IX1 + IX2
2
The position detection error is measured under the following
conditions.
· Light source: λ=830 nm
· Light spot size: ϕ200 µm
· Total photocurrent: 10 µA
· Reverse voltage: specified value listed in our datasheets
18
Position detection error (μm)
Light spot
Resistance length LX
Position on PSD (mm)
KPSDB0005EA
Specified area for position detection error
The light spot position can be detected over the entire
photosensitive area of a PSD. However, if part of the
light spot strikes outside the PSD photosensitive area
as shown in Figure 2-9, a positional shift in the centerof-gravity occurs between the entire light spot and the
light spot falling within the photosensitive area, making
the position measurement unreliable. It is therefore
necessary to select a PSD whose photosensitive area
matches the incident light spot.
[Figure 2-9] Center-of-gravity of incident light spot
IX2 + ΔI =
Light spot
XB + Δx
× Io ......... (14)
LX
Δ I: change in output current
Δ x: small displacement of light spot
Output
electrode X1
Photosensitive area
Output
electrode X2
Center-of-gravity of
light spot within
photosensitive area
Then, Δx can be expressed by equation (15).
Δx = LX ×
Center-of-gravity of
entire light spot
ΔI .........................
(15)
Io
KPSDC0073EA
The areas used to measure position detection errors are
specified as shown in Figure 2-10.
[Figure 2-10] Specified area for position detection error
(a) One-dimensional PSD (resistance length ≤ 12 mm)
Output
electrode X1
Photosensitive area
Output
electrode X2
When the positional change is infinitely small, the noise
component contained in the output current IX2 determines
the position resolution. If the PSD noise current is In, then the
position resolution (ΔR) is generally expressed by equation
(16).
ΔR = LX ×
In ........................
(16)
Io
Figure 2-11 shows the connection example when using a
one-dimensional PSD with current-to-voltage conversion
op amps. The noise model for this circuit is shown in Figure
2-12.
Specified area
Lx × 0.75
Resistance length LX
KPSDC0074EA
[Figure 2-11] Connection example of one-dimensional PSD
and current-to-voltage conversion op amps
(b) One-dimensional PSD (resistance length > 12 mm)
Output
electrode X1
Photosensitive
area
Output
electrode X2
Specified area
Lx × 0.90
Resistance length Lx
KPSDC0075EA
KPSDC0076EA
(c) Two-dimensional PSD
[Figure 2-12] Noise model
Zone A
Zone B
Photosensitive area
Zone A: within a circle with a diameter equal to 40% of one side length of the photosensitive area
Zone B: within a circle with a diameter equal to 80% of one side length of the photosensitive area
KPSDC0063EA
On two-dimensional PSDs, the position detection error
along the circumference is larger than that in the center of
the photosensitive area, so the error is specified separately
in Zone A and Zone B.
2-4
Position resolution
Io :
ID :
Rie:
Cj :
Rf :
Cf :
in :
en :
Vo :
photocurrent
dark current
interelectrode resistance
junction capacitance
feedback resistance
feedback capacitance
equivalent input current noise of op amp
equivalent input voltage noise of op amp
output voltage
KPSDC0077EA
Position resolution is defined as the minimum detectable
displacement of a light spot incident on a PSD, and it is
expressed as a distance on the PSD photosensitive area.
Position resolution is determined by the PSD resistance length
and the S/N. Using equation (6) for position calculation as
an example, equation (14) can be established.
19
•
Noise current
Thermal noise voltage VRf generated by feedback resistance
4k T B [V] .......................... (24)
Rf
VRf = Rf ×
Noise currents that determine the position resolution are
described below.
•
Noise voltage Vin due to op amp equivalent input current
(1) When Rf >> Rie
If the feedback resistance (Rf ) of the current-to-voltage
converter circuit is sufficiently greater than the PSD
interelectrode resistance (Rie), the noise current is calculated
using equation (19). In this case, 1/Rf can be ignored since
it is sufficiently smaller than 1/Rie.
•
Shot noise current Is originating from photocurrent
and dark current
2q × (IO + ID) × B [A] ............ (17)
Is =
q :
Io :
ID :
B :
•
electron charge [C]
photocurrent [A]
dark current [A]
bandwidth [Hz]
Thermal noise current (Johnson noise current) Ij
generated from interelectrode resistance
4k T B [A] ............ (18)
Rie
Ij =
k : Boltzmann's constant [J/K]
T : absolute temperature [K]
Rie: interelectrode resistance [Ω]
Vin = Rf × in ×
B [V] ........................... (25)
in: op amp equivalent input current noise [A/Hz1/2]
The op amp output noise voltage (Vn) is then expressed
as an effective value (rms) by equation (26).
Vn =
Vs2 + Vj2 + Ven2 + VRf2 + Vin2 [V] ............ (26)
Figure 2-13 shows the shot noise current plotted versus the
photocurrent value when Rf>>Rie. Figure 2-14 shows the
thermal noise and the noise current by the op amp equivalent
input voltage noise plotted versus the interelectrode resistance
value. When using a PSD with an interelectrode resistance
of about 10 kΩ, the op amp characteristics become a crucial
factor in determining the noise current, so a low-noise
current op amp must be used. When using a PSD with an
interelectrode resistance exceeding 100 kΩ, the thermal
noise generated from the interelectrode resistance of the
PSD itself will be predominant.
[Figure 2-13] Shot noise current vs. photocurrent
Note: Rsh can be usually ignored as Rsh >> Rie.
Ien =
en
Rie
B [A] ............ (19)
en: equivalent input voltage noise of op amp [V/Hz1/2]
By taking the sum of equations (17), (18), and (19), the
PSD noise current (In) can be expressed as an effective
value (rms) by equation (20).
In =
Is2 + Ij2 + Ien2 [A] ............ (20)
(2) If Rf cannot be ignored with respect to Rie
Rie
> approx. 0.1)
(when
Rf
The noise current is calculated by converting it to an
output noise voltage. In this case, equations (17), (18),
and (19) are respectively converted into output voltages
as follows:
Vs = Rf ×
2q × (Io + ID) × B [V] ............. (21)
Vj = Rf ×
4k T B [V] ............................... (22)
Rie
Ven = 1 + Rf
Rie
× en ×
B [V] ............. (23)
The thermal noise from the feedback resistance and the
op amp equivalent input current noise are also added as
follows:
20
Shot noise current (pA/Hz1/2)
Noise current Ien by op amp equivalent input voltage noise
Photocurrent (μA)
KPSDB0083EB
[Figure 2-14] Noise current vs. interelectrode resistance
(Typ. Ta=25 ˚C)
10
Noise current (pA/Hz1/2)
•
1
Thermal noise current Ij generated
from interelectrode resistance
Noise current by equivalent input
voltage noise (en=10 nV) of op amp
Noise current by equivalent input
voltage noise (en=30 nV) of op amp
0.1
0.01
10
100
1000
Interelectrode resistance (kΩ)
KPSDB0084EA
As explained, the position resolution of a PSD is determined
by the interelectrode resistance and photocurrent. This is
the point in which the PSD greatly differs from segmented
type position detectors.
The following methods are effective for improving the
PSD position resolution.
[Figure 2-15] Examples of PSD response waveforms
Light input
· Increase the interelectrode resistance (Rie).
· Increase the signal photocurrent (Io).
· Shorten the resistance length (Lx).
· Use an op amp with appropriate noise characteristics.
2-5
Response speed
As with photodiodes, the response speed of a PSD is the
time required for the generated carriers to be extracted as
current to an external circuit. This is generally expressed as
the rise time and is an important parameter when detecting
a light spot moving on the photosensitive area at high speeds
or when using a signal light source driven by pulse for
background light subtraction. The rise time is defined as the
time needed for the output signal to rise from 10% to 90%
of its peak and is mainly determined by the following two
factors:
(1) Time constant t1 determined by the interelectrode
resistance, load resistance, and terminal capacitance
Output waveform
(t2 is dominant)
KPSDC0078EA
Figure 2-16 shows the relation between the rise time and
reverse voltage for incident light at different wavelengths.
As seen from the figure, the rise time can be shortened by
using light of shorter wavelengths and increasing the reverse
voltage. Selecting a PSD with a small interelectrode resistance
is also effective in improving the rise time.
[Figure 2-16] Rise time vs. reverse voltage (typical example)
Rise time (μs)
Hamamatsu measures and calculates the position
resolution under the conditions that the photocurrent is
1 µA, the circuit input noise is 1 µV (31.6 nV/Hz1/2), and
the frequency bandwidth is 1 kHz.
Output waveform
(t1 is dominant)
The interelectrode resistance (Rie) of a PSD basically acts
as load resistance (RL), so the time constant t1 determined
by the interelectrode resistance and terminal capacitance
(Ct) is expressed as in equation (27).
λ=890 nm
λ=650 nm
Reverse voltage (V)
KPSDB0110EB
t1 = 2.2 × Ct × (Rie + RL) ......... (27)
The interelectrode resistance of a PSD is distributed between
the electrodes. Hamamatsu measures the response speed
with a light spot incident on the center of the photosensitive
area, so equation (27) roughly becomes equation (28).
t1 = 0.5 × Ct × (Rie + RL) ......... (28)
(2) Diffusion time t2 of carriers generated outside the
depletion layer
Carriers are also generated outside the depletion layer when
light enters the PSD chip peripheral areas outside the
photosensitive area or when light is absorbed 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.
Equation (29) gives the approximate rise time (tr) of a PSD,
and Figure 2-15 shows output waveform examples.
tr ≈
t12 + t22 .......................... (29)
2-6
Saturation photocurrent
Photocurrent saturation must be taken into account when
a PSD is used in locations such as outdoors where the
background light level is high, or when the signal light level
is extremely large. Figure 2-17 shows an output example of
a non-saturated PSD. This PSD is operating normally with
good output linearity over the entire photosensitive area.
Figure 2-18 shows an output example of a saturated PSD. This
PSD does not function correctly since the output linearity is
lost.
Photocurrent saturation of a PSD depends on the interelectrode
resistance and reverse voltage [Figure 2-19]. The saturation
photocurrent is specified as the total photocurrent measurable
when the entire photosensitive area is illuminated. If a
small light spot is focused on the photosensitive area, the
photocurrent will be concentrated only on a localized portion,
so saturation will occur at a lower level than specified.
The following methods are effective to avoid the saturation
effect.
21
· Reduce the background light level by using an optical filter.
· Use a PSD with a small photosensitive area.
· Increase the reverse voltage.
· Decrease the interelectrode resistance.
· Make the light spot larger.
Relative photocurrent (%)
[Figure 2-17] Photocurrent output example of PSD in
normal operation (S5629)
Electrical
center
Incident light position (mm)
KPSDB0087EA
[Figure 2-18] Photocurrent output example of saturated
PSD (S5629)
2-7
How to use
Recommended circuits
(1) Operating circuit examples
The output of a PSD is current, which is usually converted
to a voltage signal using an op amp and then arithmetically
processed with a dedicated IC. Typical circuits are shown
in Figures 2-20 and 2-21. If a light spot is incident on the
photosensitive area of the PSD, the calculated position
output does not change even if the incident light level
fluctuates due to changes in the distance between the PSD
and the light source or in the light source brightness.
If background light exists, use a pulse-driven light source
to eliminate the photocurrent caused by background light,
and only AC signal components should be extracted by
AC-coupling the PSD to current-to-voltage converters like
the circuit shown in Figure 2-21.
Figure 2-22 shows the block diagram of an operating circuit
with a digital output that allows data transfer to a PC. This
circuit arithmetically processes the PSD output current with
the microcontroller after performing current-to-voltage
conversion and A/D conversion.
[Figure 2-20] DC-operating circuit examples
Relative photocurrent (%)
(a) For one-dimensional PSD
PSD
Cf
R1
R2
- Rf
IC1
+
Cf
- Rf
IC2
+
Electrical
center
4.7
R5
+IC3
R3
R6
R4
IC4
+
R7
IC5
ΔX
X
Position signal
ΔX/ΣI
ΣI
VR
R1-R7 : same resistance value
Rf
: determined by input level
IC1-IC4: low-drift op amp (e.g., TL071)
IC5
: analog divider
KPSDC0085EC
Incident light position (mm)
KPSDB0086EB
(b) For two-dimensional PSD
[Figure 2-19] Saturation photocurrent vs. interelectrode resistance
(entire photosensitive area fully illuminated)
Rf
+
PSD
IC1
Rf
+ IC2
Rf
Saturation photocurrent
VR
+
IC3
Rf
IC4
R9
R6
R7
R13
+ IC5
R10
R14
+
R
IC6 15
R11
R16
+
IC7 R17
R12
R8
+
R2
R3
R4
R5
IC8 R18
R19
+ IC9
R 20
ΔY
ΣI
R 24 - R 25
- R 21
+
+ IC10
IC12
ΔX
R
22
+ IC11
ΔY
ΣI
Y0
IC13
ΔX
ΣI
X0
IC14
R 23
R1-R 25 : same resistance value
Rf
: determined by input level
IC1-IC12 : low-drift op amp (e.g., TL071)
IC13, IC14: analog divider
KPSDC0026EE
Interelectrode resistance (kΩ)
KPSDB0003EA
22
+
R1
Hamamatsu also offers a digital-output signal processing
circuit that uses a microcontroller to perform all position
calculations such as addition/subtraction and division.
Stable position output can be obtained in measurements
where the incident light level is high but brightness changes
are small. This processing circuit is easy to handle as it
operates on an AC power adapter.
[Figure 2-21] AC-operating circuit example
(for two-dimensional PSD)
Rf
PSD
IC1
C2 Rb +
IC2
Rf
C3
VR
Rb +
Rf
C4
Rb +
R9
R1
Rf
R2
R3
R4
R5
IC3
R6
R7
IC4
R8
+
R13
IC5
R10
R14
R19
+
+
IC9
R 20
ΔY
ΣI
IC6 R15 - R 21
R11 R16 +
IC10
R 22 Δ X
R17
+ IC7
R12
+ IC11
R18
+ IC8
R 23
R1-R 24 : same resistance value
Rf
: determined by input level
IC1-IC11 : low-drift op amp (e.g., TL071)
IC12, IC13: analog divider
ΔY
ΣI
IC12
DC restoration circuit
C1
Rb +
Y0
ΔX
ΣI
IC13
X0
P1
P2
Δ X, Δ Y, Σ 1
0V
DC restoration circuit
R1 R 3
S/H A
S/H B
P1
P2
R2
R4
P2 P1 R1=R 2=R 3=R4
KPSDC0029EE
[Figure 2-22] Block diagram of DC-operating circuit
with digital output (C9069)
LED for light level monitor
Lo OK Hi
Digital output
PSD input
X1
X2
Y1
Y2
I/V
converter
Op amp
reference
voltage
VR1 adjustment
circuit
PSD bias
voltage output
VR
GND
A/D
converter
Reset
circuit
Microcontroller
RS-232C
line driver
SD
R
Oscillator
circuit
Amplitude
adjustment
VR2 circuit VREFH
Amplitude
adjustment
VR3 circuit VREFL
Power supply circuit
D/A conversion
output
D/A
converter
X
Y
Σ
Power input
DC+12 V
GND
KACCC0223EA
(2) PSD signal processing circuits
[Figure 2-23] Two-dimensional PSD signal processing
circuit C9069
Hamamatsu provides various types of PSD signal processing
circuits to help users easily evaluate one-dimensional and
two-dimensional PSDs. These include a DC signal processing
circuit assembled on a compact board that contains a
current-to-voltage converter, addition/subtraction circuit,
and analog divider circuit similar to the DC-operating circuit
examples described above. Also available is an AC signal
processing circuit that contains a sync circuit and LED
driver circuit in addition to the AC-operating circuit example
described above, so measurement can be started by simply
connecting to a power supply (±15 V) and an LED.
23
3.
Applications
3-1
Particle size analyzers
(laser diffraction and
scattering method)
speed response and high sensitivity, and it must also be able
to detect the reflected light accurately. Hamamatsu Si PIN
photodiodes meet all these needs, and their photosensitive
area has small variations in sensitivity and so can detect
light with high stability at any position on the photosensitive
area. Hamamatsu also uses advanced technologies for
mounting filters that block extraneous light and mounting
components in a compact manner, which help reduce the
size of barcode readers.
The laser diffraction and scattering method is a particle size
measurement technique offering features such as a short
measurement time, good reproducibility, and measurement of
the flowing particles. Irradiating a laser beam (monochrome
collimated beam) onto the particles for measurement
generates a light level distribution pattern from spatially
diffracted and scattered light. This distribution pattern
changes with the size of the particles. Large area sensors
with high resolution are needed to detect the diffracted
and scattered light.
Hamamatsu multi-element Si photodiodes have superb
sensitivity and small characteristic variations between
elements. These photodiodes are manufactured using our
sophisticated “large chip mounting/processing” technology.
Many of them are used in sensor units (forward diffracted/
scattered light sensors & side and back scattering light
sensors) which are the core of the particle size analyzers.
These photodiodes are also incorporated in particle size
analyzers capable of measuring particles from 10 nm to
300 µm, and so are used for environmental measurements.
[Figure 3-1] Structure of particle size analyzer
[laser diffraction and scattering method]
3-3
UV sensors
Ultraviolet light is high in energy and exhibits sterilizing
effects and photocatalysis. On the other hand, ultraviolet
rays deteriorate the materials that absorb them.
Si photodiodes also have high sensitivity in the ultraviolet
region and so are widely used for detecting ultraviolet light.
For example, a product consisting of Si photodiode and
ultraviolet monochromatic band-pass filters mounted in the
highly reliable package is widely used in devices that detect
organic contamination, which is a kind of water pollution.
Sensitivity may degrade as a result of received ultraviolet
light reacting with the outgas that is emitted from the resin
in the package depending on the operating environment.
Hamamatsu has also developed packaging technology that
does not use resin and Si photodiode chips highly resistant
to ultraviolet light. These are used to produce high-reliability
UV Si photodiodes.
3-4
Rotary encoders
Forward diffraction and
scattering light sensor
Diffracted and
scattered image
Condenser lens
Particle cluster
Semiconductor
laser
Diffracted and
scattered light
Collimator
Side and back scattering
light sensor
KSPDC0056EA
3-2
Barcode readers
In a barcode reader, the light source such as an LED or
laser diode emits light onto the barcode surface, and the
lens focuses the light reflected from that surface, which is
then detected by the photosensor. The detected pattern
is compared with the registered patterns and then
decoded into characters and numbers, etc.
The photosensor in the barcode reader must have high24
Rotary encoders are widely used in FA (factory automation)
and industrial control equipment. Rotary encoders contain
a rotary slit disk and fixed slit plate between a light emitter
and a photosensor (photodiode). The rotation of the rotary
slit disk serves to pass or block light from the light emitter,
and changes in this light are detected by the photosensor as
rotations.
The photosensor must have high-speed response and high
chip position accuracy in order to convert the number of
shaft rotations (analog values) into pulses (digital values).
Multi-element Si PIN photodiodes made by Hamamatsu
are suitable for detecting high-speed changes in the optical
signal. These photosensors deliver stable detection because
there is small variation in sensitivity and response speed
between elements. To ensure low photosensor noise, patterning
technology may be applied to block light to sections other
than the photosensitive areas.
[Figure 3-2] Example of rotary encoder structure
Infrared LED
3-6
Fixed slit plate
Si PIN photodiode
Encoder shaft
Rotary slit disk
KSPDC0062EA
3-5
Color sensors
Separately detecting the three primary colors of light,
which are red (R), green (G), and blue (B) color signals,
not only simplifies color identification but also makes
it possible to authenticate paper money, identify paint
colors, and manage printed matter and textile product
colors, and so on. Si photodiodes have sensitivity over a
wide wavelength range. However, combining them with
filters allows detecting the individual RGB wavelengths.
Hamamatsu Si photodiodes for RGB color sensors are
small since each of the RGB sensors is integrated on the
same chip and allows easy detection of color signals.
Color sensor modules with Hamamatsu Si photodiodes
are used in the detection of RGB colors of LEDs in order to
adjust the effects of color changes caused by the temperature
characteristics or deterioration of the RGB-LEDs of TFT
LCD backlight.
VICS
(Vehicle Information and Communication System)
VICS is a system used in Japan for providing information such
as traffic congestion, road construction, traffic regulations,
and required time, etc. by media such as FM multiplex
broadcasts, radio waves, and light.
Information supplied by light (optical media) makes use of
optical beacons (in-vehicle devices) mounted in the vehicle
and optical beacons (roadside devices) mounted at major
points on the road to carry out two-way communication
by near infrared light. One advantage of this method is that
unlike other communication media, information can be
exchanged in both directions. A disadvantage however is
that only pinpoint information can be provided since the
communication area is limited. The uplink (in-vehicle device
→ roadside device) communication range is different from
the downlink (roadside device → in-vehicle device) range.
[Figure 3-5] Optical beacons used by VICS
Optical beacon (roadside device)
Traffic
control
center
Communication
zone
Uplink
Downlink
Optical beacon
(in-vehicle device)
[Figure 3-3] Color adjustment of TFT-LCD backlight using
RGB-LED (application example of C9303 series)
Color sensor module
C9303 series
KLEDC0029EA
Color controller
Luminance and chromatic
coordinate settings
Red driver
Green driver
RGB-LED
Blue driver
KACCC0212EE
[Figure 3-4] Color sensor module C9303 series
The optical beacon contains an LED and a photodiode. The
in-vehicle device must be compact to avoid installation
space problems and uses a surface mount type photodiode.
The in-vehicle device will have to operate under harsh
environmental conditions, so the design specifications
must allow for a wider operating and storage temperature
range than in ordinary photodiodes.
In early-stage VICS systems, the LED array and the photodiode
were almost always mounted separately. Currently, however,
both are integrated into one compact device [Figure 3-6].
[Figure 3-6] Light emitting/receiving module P8212 for VICS
25
3-7
Triangulation distance measurement
correction optical system.
[Figure 3-8] Example of direct position detection
The principle of triangulation distance measurement is
shown in Figure 3-7. Light emitted from a light source
(LED or LD) is focused by a light projection lens to strike
the target object, and light reflecting from that object is
input via a light receiving lens onto the PSD photosensitive
surface. If we let the distance between the PSD and light
source (baseline length) be B, the lens focal distance be f,
and the amount of movement of the light spot from the
center on the PSD be X, then the distance L to the target
object is expressed as L = (1/X) × f × B. This method offers
a great advantage: the distance can be found regardless of
the reflectance of the target object and variations in the
light source power. This principle is also applied in laser
displacement meters.
I1 output
Uniform emission
pattern LED (DC light)
Movable
slit plate
PSD
I2 output
Paper or other object
KPSDC0080EB
[Figure 3-9] Optical camera-shake correction
(a) State with no camera shake
[Figure 3-7] Principle of triangulation distance measurement
Correction optical
system (using PSD)
Driver circuit
Light source
Subject side
Image sensor
Shake correction controller
KPSDC0087EA
(b) State when camera shake occurred
Correction optical
system (using PSD)
Target object
KPSDC0086EA
Image shift
Subject side
3-8
Figure 3-8 shows the direct position detection principle.
The light source (LED or LD, etc.) emits light which passes
through a slit and irradiates onto the photosensitive area
of the PSD. The position where the light strikes the PSD
surface shifts according to the slit movement. Calculating
that position information allows finding the amount of slit
displacement.
Figure 3-9 shows how this is applied to optical camerashake correction. When a camera lens shake occurs due
to shaky hands, the correction optical system (using a
PSD) causes a horizontal movement in the direction of
the shake so that the center of the image returns to a
position at the center of the image sensor photosensitive
area. The PSD is utilized to detect and control movement
(position information) of the slit which is built into the
26
Image sensor
Direct position detection
Shake correction controller
KPSDC0088EA
(c) State when camera shake was corrected
(by moving the correction optical system)
Correction optical
system (using PSD)
Subject side
Corrected
light beam
Image sensor
Shake correction controller
KPSDC0089EA