SOLID
STATE D I V I S I O N
TECHNICAL INFORMATION
SD-37
Characteristics and use of
Charge amplifier
Table of contents
1. General description ····································································································· 3
2. Principle of operation ···································································································· 3
3. Gain ······························································································································ 3
3-1 Amplifier ··················································································································· 3
3-2 Amplifier with detector ······························································································· 4
4. Characteristics ·············································································································· 4
4-1 Open-loop gain ········································································································· 4
4-2 Noise ······················································································································· 5
5. Applications ·················································································································· 7
5-1 Gamma ray measurement (Direct detection using a PIN photodiode) ···························· 7
5-2 Power and stability measurements from lasers ···························································· 7
6. Specifications ··············································································································· 8
7. Precautions for handling charge amplifier ···································································· 10
2
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Characteristics and use of Charge amplifier
As a result, the signal charge pulses Qs are all integrated to
the feedback capacitance Cf and then output as voltage
If a detector provides a constant charge generation over a time
interval t=0 to to, the output signal charge Qs is given by the
following equation using the Laplace transform.
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capacitance Cf, the output becomes voltage pulses that slowly
discharge with the time constant determined by τ=Cf · Rf.
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-Sto
Qs (S) = Qs
( S1 - Se )
. . . . . (2-1)
Similarly, the transmission coefficient T(S) is given by
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fication then provide a low-impedance output.
1
1
·
Cf S +
1
τ
Thus the output voltage V(S) is expressed using Eqs (2-1) and
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-Sto
( S1 - Se ) · Cf1 · S 1+
e
1
1
Qs 1
=·
·
S
S+ )
Cf ( S S +
V (S) = Qs (S) · T (S) = Qs
1
τ
-Sto
1
τ
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1
τ
. . . . . (2-3)
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connected using a long cable.
. . . . . (2-2)
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the detector capacitance. The output stage is a lowimpedance buffer so as to drive an external stage which is
(τ = Cf · Rf)
(2-2) as follows:
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usually a low-noise FET and its open-loop gain is set
sufficiently high so that the amplification is not influenced by
T (S) = -
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Because of this operation, this type of amplifier is called a
“charge amplifier”. The first stage of a charge amplifier is
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amplifiers have high input impedance, they integrate weak
charge pulses and convert them into voltage pulses for ampli-
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In such applications, operational amplifier mode integrators
using feedback capacitance are commonly used. As these
pulses eout(t). At this point, since the feedback resistance Rf
for direct current is connected in parallel to the feedback
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must be taken into consideration when amplifying this output
signal.
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itself is a capacitive device, its impedance is very high. Therefore, the performance of the preamplifier to be connected,
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rays, the output signal is a weak charge pulse having a pulse
width of several tens of nanoseconds. As the detector element
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When a semiconductor detector such as Si is used for the
measurement of soft X-rays and low to high-energy gamma
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1. General description
-t/τ
eout (t) = - Qs · 1- e
Cf to/τ
0 ≤ t < to
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2. Principle of operation
As a result, the output voltage pulse eout(t) is given by
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Figure 2-1 Principle of operation
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Rf
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Cf
-
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+
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Qs
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to
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eout (t)
AOL
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+
=-
to ≤ t
. . . . . (2-4)
Because generally to << τ, Eq (2-4) can be simplified as follows:
eout (t) = - Q e -t/τ
Cf
. . . . . (2-5)
As can be seen from Eq (2-5), the signal charge pulses Qs are
Qs ,
converted into voltage pulses with amplitude Vout = Cf
which is damped with time constant τ=Cf · Rf.
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3. Gain
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When soft X-rays or gamma rays strike for example a Si
combination.
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3-1 Amplifier
The gain of amplifier Gc, referred to also as “charge gain”, is
given by the following equation:
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( = Cf1 )
[V/coulomb] or [V/pico coulomb]
. . . . . (3-1)
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Gc = Vout
Qs
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potential works through the feedback loop so as to make the
input-end potential zero instantaneously.
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polarity appears at the output end. However because the
amplifier's open-loop gain is sufficiently large, the output-end
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to this charge generation, the input-end potential of the charge
amplifier rises and at the same time, a potential with reverse
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semiconductor detector, signal charge pulses Qs are generated, with an amplitude according to the particle energy. Due
The gain of charge amplifier is given in one of two ways: the
gain for amplifier or the gain for a detector/amplifier
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KACCC0015EA
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Cj : Semiconductor detector capacitance
Cf : Feedback capacitance
Rf : Feedback resistance
AOL : Open-loop gain of amplifier
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Cj
Qs (e to/τ -1) -t/τ
·
e
to/τ
Cf
3
Characteristics and use of Charge amplifier
4-1 Open-loop gain
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There are various semiconductor detectors used for the detection of soft X-rays and gamma rays. Even among Si detectors
for example, a variety of types are used to match the applica-
capacitance. However, when the same Si detector is used
for the detection of soft X-rays and gamma rays, the amount of
generated charge must be the same if the particle energy of
the soft X-rays and gamma rays is equivalent. Therefore,
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tion, which have different active areas and depletion layer
thicknesses. Furthermore, detectors also differ in regards to
charge amplifier must provide a constant gain regardless of
the capacitance value. In fact as shown in Eq (2-5) in
“Principle of operation”, the output from the detector is
independent of the junction capacitance Cj. This is because
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ε : Energy required to create one electron/hole pair.
For example with silicon, Qs ranges from 3.62 eV
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E : Particle energy (MeV)
e-: Elementary charge 1.6 × 10-19 (coulomb)
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as soft X-rays and gamma rays and also by the material of the
semiconductor.
Qs = E · e (coulomb) or (pico coulomb)
. . . . . (3-2)
ε
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The amplitude of the signal charge obtained with a semiconductor detector is determined by the input particle energy such
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than “gain”. Sensitivity is expressed in the output voltage (mV)
per one MeV of particle energy irradiated onto a detector.
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In this case we usually use the term called “sensitivity” rather
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3-2 Amplifier with detector
the open-loop gain of the charge amplifier is very high.
Figure 4-1 Equivalent circuit
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Thus, from Eqs (3-1) and (3-2), sensitivity is given by
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(at 300 K) to 3.71 eV (at 77 K).
Cf
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Qs
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. . . . . (3-3)
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= e · 1 (mV/MeV)
Cf ε
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ε
e-
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Cf
Rs = Vout =
Qs ·
E
eout
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AOL
Cj
(Qs)
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Zin
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For example when using a Si detector and an H4083 charge
amplifier (Cf=2 pF), the sensitivity at room temperatures Rs
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KACCC0016EA
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-19
= 1.6 × 10-12 · 1
2 × 10
3.62
In this equivalent circuit, when seen from the amplifier's input,
the input impedance Zin is given by
Zin =
. . . . . (4-1)
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If signal charge Qs is generated in the Si detector, the voltage
ein at the amplifier's input becomes
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ein =
Qs
jω Cj + {1 + AOL (jω)} · jω Cf
. . . . . (4-2)
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● Low noise
Thus the output voltage eout is expressed using Eq (4-2), as
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follows:
eout = AOL (jω) · ein
= AOL (jω) ·
Qs
jωCj + {1 + AOL (jω)} jωCf
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The following sections discuss major characteristics of charge
amplifier.
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● High temperature stability, etc.
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● High-speed rise time
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● Excellent integration linearity
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● High gain
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amplifier used for the detection of soft X-rays and low to highenergy gamma rays.
1 + AOL (jω)
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In general, the following characteristics are required of charge
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4. Characteristics
1
jωCf
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. . . . . (3-4)
When a charge amplifier is connected with a Si detector, its
equivalent circuit is like that shown in Figure 4-1.
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= 2.2 × 10 -8 (V/eV)
= 22 (mV/MeV)
Cj : Semiconductor detector capacitance
Cf : Feedback capacitance
AOL : Open-loop gain of amplifier
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becomes
Rs = e · 1
Cf ε
4
=
Qs
jωCf + AOL jω
(Cf + Cj)
(jω)
. . . . . (4-3)
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Thermal noise of the first-stage FET, en1, is given by
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en1 =
( V/
Hz
)
. . . . . (4-5)
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. . . . . (4-4)
8 KT
3 gm
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Qs
eout =
jω Cf
◆Thermal
Thermal noise of first-stage FET
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Here, assuming that AOL >> 0, in other words, the open-loop
gain of the amplifier is very large, then eout can be simplified
as follows:
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Characteristics and use of Charge amplifier
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K : Boltzmann constant
T : Absolute temperature
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gm : Mutual conductance of first-stage FET
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As discussed above, the output voltage eout of charge
amplifier is not dependent on the capacitance of Si detectors.
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Figure 4-2 Open-loop gain (H4083)
◆Shot
Shot noise caused by gate current of first-stage
FET and dark current of detector
The shot noise in is given by
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OPEN-LOOP VOLTAGE GAIN (dB)
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100
in =
2q (IG + ID) ( A/ Hz )
. . . . . (4-6)
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80
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q : Elementary charge
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60
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IG : Gate leakage current of first-stage FET
ID : Dark current of detector
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40
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10 k
100 k
1M
10 M
The thermal noise en2 caused by the feedback resistance Rf
is given by
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1k
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0
100
◆Thermal
Thermal noise caused by feedback resistance
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20
en2 =
4KTRf
( V/
Hz
)
. . . . . (4-7)
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FREQUENCY (Hz)
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KACCB0022EA
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Figure 4-3 shows the noise equivalent circuit of a charge
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4-2 Noise
From Eqs (4-5), (4-6) and (4-7), the total noise ent (jω) becomes as follows:
Cin 2
en2 2
1
2
ent2 (jω) = en 1 · 1 +
+ in2 +
. . . . . (4-8)
(jω Cf)2
Rf
Cf
(
) { ( )}
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en2
In Eq (4-8) above, the first term component is constant over
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Figure 43 Noise equivalent circuit of the charge amplifier
4-3
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amplifier.
where Rf is the feedback resistance.
second term component is constant regardless of the input
capacitance Cin, but decreases with increasing frequency.
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Rf
the entire frequency range and amplified by the noise gain
(1+Cin/Cf) determined by the input capacitance Cf . The
Figure 4-4 and 4-5 show noise characteristics of the H4083
charge amplifier.
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Cf
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en1
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ent
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-
Cin=Cj // Cs
+
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in
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Cj : Capacitance of semiconductor detector
Cs: Input capacitance of charge-sensitive amplifier
Cf : Feedback capacitance
Rf : Feedback resistance
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sources:
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Noise in charge amplifier comes from the following three major
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KACCC0017EA
5
Characteristics and use of Charge amplifier
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Figure 4-4 Noise spectrum
amplifier are carried out using a measurement system like that
shown in Figure 4-6.
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Figure 4-6 Noise measurement system
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1000
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100
PULSE HEIGHT
ANALYZER
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PULSE-SHAPING
AMPLIFIER
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10
Several µs
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About 50 µs
1k
10 k
100 k
1M
10 M
PULSE GENERATOR
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1
100
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NOISE VOLTAGE (nV/Hz1/2)
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10000
Generally, the noise measurement and evaluation for charge
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FREQUENCY (Hz)
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KACCC0018EA
In this system, a charge Q is supplied from the pulse generator
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Figure 45 Input capacitance vs. number of noise electrons
4-5
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KACCB0024EA
The output from the charge amplifier is amplified once with the
pulse-shaping amplifier and input to the pulse height analyzer.
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Then the pulse height distribution is measured to obtain the
noise based on the half width.
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1000
The pulse-shaping amplifier improves the S/N of the charge
amplifier and also serves as a filter. As various types of pulseshaping amplifiers are available depending on the circuit
configurations, choose the best amplifier for your application.
100
10
100
1000
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1
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RMS NOISE ELECTRONS
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10000
via the capacitance connected to the input end of a charge
amplifier.
Figure 4-7 Frequency characteristic
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KACCB0023EA
shaping circuit. Its typical frequency characteristic and output
waveform are shown in Figure 4-7 and 4-8, respectively.
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INPUT CAPACITANCE (pF)
One of the more popular pulse-shaping amplifiers is a
Gaussian
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80
GAIN (dB)
40
20
0
1k
10 k
100 k
1M
10 M
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60
6
FREQUENCY (Hz)
KACCB0025EA
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5-2 Power and stability measurements
from lasers
Figure 5-2 shows a typical pulse height distribution when the
H4083 charge amplifier combined with S3590-01 Si PIN
photodiode is irradiated from a laser having a pulse width of
about 100 ns and a wavelength at 830 nm. The peak channel
of the pulse height distribution indicates the average laser
power and the half width represents the fluctuation in the
power level.
Figure 5-2
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Figure 5-1
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tion source.
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Figure 5-1 shows a typical pulse height distribution when the
H4083 charge amplifier combined with S3590-05 Si PIN
photodiode is irradiated with gamma rays from an 241Am radia-
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(Direct detection using Si PIN photodiode)
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5-1 Gamma ray measurement
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5. Applications
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level that matches the input range (0 to 10 V) of the pulse
height analyzer.
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Since the output signal from a charge amplifier is as small as
several tens of millivolts even after being integrated. The
pulse-shaping amplifier is also used to amplify this signal to a
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Figure 4-8 Output waveform
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Characteristics and use of Charge amplifier
7
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Characteristics and use of Charge amplifier
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increased compactness and light weight offered by this configuration make it easy to incorporate in with other test
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from increased capacitance because it solders directly onto
the rear side of H4083 thus reducing wire length. The
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For optimum operation it is recommended that H4083 be used
with Hamamatsu Si PIN photodiodes (S3590/S3204 series,
etc.). In particular, S3590 series eliminates the risk of trouble
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measurements with good energy resolution.
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detectors. Even when combined with a detector having a
relatively large terminal capacitance, H4083 can make
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and low to high energy gamma-ray spectrometers. H4083 is
used in conjunction with semiconductor detectors such as Si
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Charge amplifier H4083 is a hybrid low-noise amplifier that
can be used in a wide range of application, including soft X-ray
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6. Specifications (H4083)
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equipment, etc.
■ Specifications
Parameter
Amplification method
Input/output polarity
Charge gain
Noise characteristic
Negative feedback constant
Power supply
Power consumption
Configuration
Dimensional outline
Charge-sensitive type
Inverted
0.5 V/pC
22 mV/MeV (Si)
550 electrons/FWHM
50 M9//2 pF
±12 V
150 mW
9-pin, single line type
24 (W) × 19 (H) × 4 (T) mm
■ Pin connections
8
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Content
parallel connections, directly replace the resistor (Rf) and
capacitor (Cf) shown at the right, which serve as feedback
resistance and capacitance.
Replacement components shoud be 2012 size.
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● Changing the feedback resistance and capacitance
If you want to change the feedback constant without making
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● Using pin No. 2
Use pin No. 2 when changing the feedback constant which
is typically 50 MΩ/2 pF. Connect feedback resistance and
capacitance between pin No. 2 and No. 9. Note that this
connection is made parallel to the internal feedback resistance and capacitance.
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The left end pin is designated No. 1 when viewed from the
component side with the pins facing downwards.
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Input terminal
Feedback constant adjustment terminal
Input ground terminal
Last pulse input terminal
Power and output ground terminal
Power terminal
Power terminal
Output ground terminal
Output terminal
Anode connection terminal
Cathode connection terminal
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Pin No.
Symbol
➀
IN
➁
GAIN
➂
GND *
➃
CAL
➄
GND *
➅
-12 V
➆
+12 V
➇
GND *
➈
OUT
Through-hole A
P
Through-hole B
N
* GND is internally connected.
Specification
● Using through-holes A and B
When H4083 is not covered with protective coating, a photodiode (such as S3590 series) can be directly soldered on
the board. In this case, insert the photodiode leads into the
through-holes from the solder side.
Characteristics and use of Charge amplifier
■ Component side diagram
■ Equivalent circuit
GAIN
2 pF
Cf
Rf
THROUGH-HOLE B
50 MΩ
THROUGH-HOLE A
+12 V
0.01 µF
IN
1
2
3
4
5
6
7
8
OUT
9
-12 V
KACCC0136EA
1.5 pF
,
CAL
,
GND
KACCC0019EA
■ Circuit example 1 (external connection to photodiode)
■ Circuit example 2 (direct connection to photodiode)
POWER
SUPPLY
POWER
SUPPLY
-HV
-HV
7
6
7
5
6
5
RL
RL
1
9
TH. P
8
9
1
MULTICHANNEL
ANALYZER
8
TH P
MULTICHANNEL
ANALYZER
TH N
TH. N
3
3
4
4
50Ω
50Ω
PULSE
GENERATOR
PULSE
GENERATOR
KACCC0020EA
KACCC0021EA
■ Dimensional outline (unit: mm)
COMPONENT SIDE
SOLDER SIDE
5
THROUGH-HOLE
FOR PHOTODIODE
5 MIN.
(7)
19 MAX.
(7)
(8 ×) 2.54
0.5
0.25
4 MAX.
KACCA0017EB
9
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● When configuring a measurement system using Si PIN
photodiode and a charge amplifier, the following points
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other components from being damaged by excessive heat.
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amplifier or the amplifier onto a PC board, carry out the
soldering as quickly as possible to prevent the amplifier and
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● When soldering Si PIN photodiode S3590 series onto a charge
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the impedance at the input end is very high.
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into a non-conductive sponge material or wrap it with
aluminum foil to protect it against static electricity because
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● For proper storage of the charge amplifier, insert its leads
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with bare hands when handling H4083.
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photodiode of S3590 series can be directly soldered on it,
no protective coating is provided. Avoid touching any part
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● Because charge amplifier H4083 is designed so that Si PIN
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7. Precautions for handling charge amplifier
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Characteristics and use of Charge amplifier
The output end of the amplifier should be kept away
from the input end as much as possible.
●
In the power supply line connect a ceramic capacitor of
about 0.1 µF between the amplifier and the ground.
●
In the bias line for the photodiode, use a bias resistor
and capacitor which can adequately withstand the bias
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Si PIN photodiodes are capacitive elements with high
impedance. Provide appropriate shielding to prevent
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When mounting Si PIN photodiode on a charge amplifier,
take care to minimize stray cpacitance.
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should be observed.
To increase the bias voltage applied to Si PIN
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against visible light when measuring soft X-rays or
gamma rays.
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Because Si PIN photodiodes also have high sensitivity
in the visible range, provide sufficient light-shielding
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the optimum level.
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photodiode, slowly increase it while monitoring the
output so that the output amplitude does not swing past
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● The following points must also be observed during
operation:
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using a series power supply with minimum noise
characteristics.
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As a power supply for the system, we recommend
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external noise from interfering with the signal.
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voltage applied.
Information furnished by HAMAMATSU is believed to be reliable. However, no responsibility is assumed for possible inaccuracies or omissions.
Specifications are subject to change without notice. No patent rights are granted to any of the circuits described herein. ©2001 Hamamatsu Photonics K.K.
HAMAMATSU PHOTONICS K.K., Solid State Division
1126-1 Ichino-cho, Higashi-ku, Hamamatsu City, 435-8558 Japan, Telephone: (81) 53-434-3311, Fax: (81) 53-434-5184, http://www.hamamatsu.com
U.S.A.: Hamamatsu Corporation: 360 Foothill Road, P.O.Box 6910, Bridgewater, N.J. 08807-0910, U.S.A., Telephone: (1) 908-231-0960, Fax: (1) 908-231-1218
Germany: Hamamatsu Photonics Deutschland GmbH: Arzbergerstr. 10, D-82211 Herrsching am Ammersee, Germany, Telephone: (49) 08152-3750, Fax: (49) 08152-2658
France: Hamamatsu Photonics France S.A.R.L.: 19, Rue du Saule Trapu, Parc du Moulin de Massy, 91882 Massy Cedex, France, Telephone: 33-(1) 69 53 71 00, Fax: 33-(1) 69 53 71 10
United Kingdom: Hamamatsu Photonics UK Limited: 2 Howard Court, 10 Tewin Road, Welwyn Garden City, Hertfordshire AL7 1BW, United Kingdom, Telephone: (44) 1707-294888, Fax: (44) 1707-325777
North Europe: Hamamatsu Photonics Norden AB: Smidesvägen 12, SE-171 41 Solna, Sweden, Telephone: (46) 8-509-031-00, Fax: (46) 8-509-031-01
Cat.
Italy: Hamamatsu Photonics Italia S.R.L.: Strada della Moia, 1/E, 20020 Arese, (Milano), Italy, Telephone: (39) 02-935-81-733, Fax: (39) 02-935-81-741
No. KACC9001E01
Oct. 2001 DN
10