Application Note
AFBR-S4E001
Evaluation Kit for the AFBR-S4NxxC01y SiPM
Family
Overview
This application note introduces the evaluation kit AFBR-
S4E001, which has been designed to be used as an
evaluation platform for the AFBR-S4NxxC01y SiPM family.
The document describes in detail the board included in the
evaluation kit and the test setup used for the evaluation of
the optical and electrical characteristics of the mentioned
SiPM family.
Figure 1
shows the front and back sides of the evaluation
board for the AFBR-S4E001.
Evaluation Kit
The evaluation kit includes:
1x Evaluation board
1x AFBR-S4N44C013 SiPM (mounted on a test PCB)
SiPM High Voltage source (up to ~40V)
Dual power supply (±2.5V to ±8.5V)
SMA cables for readout
The evaluation kit does not include:
Applications
Features
Two 50Ω outputs
Designed for applications involving many photons,
typically produced by a scintillator
Operating Temperature range from –20°C to +50°C
RoHS and REACH compliant
Prototyping
Device characterization
X and γ ray spectroscopy
PET
Figure 1: Evaluation Board from Front (Left) and Back (Right)
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AFBR-S4E001
Document_Type
Evaluation Kit for the AFBR-S4NxxC01y SiPM Family
Description
The block diagram is displayed in
Figure 2:
Figure 2: Block Diagram
Figure 3: Evaluation Board Frontside
1
st
stage
HV
Filter
V
+
V
+
50
V
–
Out1
50
SiPM
V
–
V
+
V
–
Out2
+G -
p+n
n+p
GHV
2
nd
stage
The high voltage is filtered before being applied to the SiPM
to minimize the fluctuations in the voltage source. The SiPM
output signal goes into a first stage of amplification based on
an operational amplifier in trans-impedance configuration.
Its output is then split into two lines: the first goes through a
buffer with unity gain and is connected to Out1; the second
passes through a second stage of amplification including a
Pole-Zero (PZ) filtering network and is connected to Out2.
Given a flash of light, Out1 is used to estimate the number
of detected photons while Out2 gives a precise
determination of the timing. Both outputs should be read
through a 50Ω termination resistor.
Figure 4: Evaluation Board Backside
Inputs and Outputs
The board has three inputs (SiPM signal, SiPM high voltage,
and power supply) and two outputs (Out1 and Out2). Two
additional receptacles are included for mechanical stability
only.
Figure 3
and
Figure 4
show a sketch of the board front
and back view, respectively.
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BROADCOM
Out 2
+ HV
Buffer
Out 1
AFBR-S4E001
Document_Type
Evaluation Kit for the AFBR-S4NxxC01y SiPM Family
Detailed Pin Descriptions
SiPM Input
The detector must be connected as shown in
Figure 5.
Figure 5: Connection with SiPM PCB
Board Power Supply
Power for the board is provided by the 3-pole connector
displayed in
Figure 7.
Figure 7: Board Power Supply Connector Detail
+ G −
BROADCOM
p+n
+G -
The anode and cathode of the detector are connected to the
amplifier’s input while the other receptacle has no electrical
connection and is only used for mechanical stability.
SiPM High Voltage
SiPM high voltage is provided with the 2-pole connector
displayed in
Figure 6.
The supply voltage must be positive.
Figure 6: SiPM High Voltage Connector Detail
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Out 2
Broadcom
Out 1
G HV
Suggested starting values:
V
+
: +5V
V
–
: –5V
n+p
With the suggested values, the absorbed current in each
line is approximately 40 mA. The output signals can range
from –3.8V to + 3.8V (±1.2V is used by the op-amp and is
unavailable). Since Out1 is negative and Out2 is positive,
the user might want to shift the output dynamic to avoid
saturation in one of the outputs. Typically, it is crucial to
avoid saturation on the Energy channel rather than the
Timing channel. The reason for this is that the Timing signal
carries the most useful information in the very beginning of
its rising edge, where saturation is not an issue.
To shift the output range, adjust the bias voltages and keep
the overall difference within 11V max. (10V suggested, for
safety). Additionally, each voltage cannot be lower than
±2.5V or higher than ±8.5V.
In summary:
G HV
+ 2.5V ≤ V
+
≤ +8.5V;
–2.5V ≥ V
–
≥ –8.5V;
V
+
– V
–
≤ 11V;
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Out1: Energy
Out1 is the output of the first stage of amplification.
The polarity is negative. The gain of the amplification stage
(also accounting for termination) is G = 83 V/A. An offset of
a few tens of millivolts with respect to ground is common
and is due to the operational amplifier chip. The exact value
changes from sample to sample and can be positive or
negative.
Out1 preserves the signal shape and thus can be integrated
to obtain the delivered charge, a quantity that is proportional
to the number of detected photons. In a typical application
with a scintillator, this quantity is also proportional to the
energy of the detected X or γ ray. Hence, Out1 is referred to
as the Energy signal.
Figure 8: Typical Test Setup Scheme
Radioactive
Source
X–
Rays
Digital
Oscilloscope
Scintillator
Interaction
Point
Visible
Photons
SiPM
Power
supply
SiPM HV
Out1
Evaluation
board
Out2
Out2: Timing
Out2 is the output of the second stage of amplification. The
polarity is positive. The gain of the amplification stage is not
defined because of the presence of the PZ network.
The PZ filter is designed to change the signal shape.
Specifically, it suppresses the recovery tail of the SiPM
signal thus reducing the baseline fluctuations due to dark
counts. As a consequence, the extraction of the timing
information (when using a simple leading edge discriminator
[LED], as an example) is more accurate and allows for
improved timing resolution performance. Hence, Out2 is
referred to as the Timing signal.
The scintillator converts the energy from the X – γ ray into
many visible photons which are then detected by the SiPM.
The interaction between the radiation and the scintillator
can be of two types: photoelectric effect or Compton
scattering. In the first case, the X – γ ray is absorbed and all
the energy is deposited. With the Compton scattering,
however, the X – γ ray is not absorbed and only a fraction of
its energy is released into the scintillator.
Each detection event is randomly a Compton scattering or a
photoelectric effect, according to a distribution that strongly
depends on the atomic number of the scintillator and the
radiation energy. Different materials have different
characteristics in terms of density, number and wavelength
of produced photons, and emission time. The choice of the
material strongly depends on the energy of the radiation that
has to be detected.
The following example signals can be obtained with a typical
test setup. The scintillator is a commercially available LSO
crystal with dimensions of 4 mm x 4 mm x 20 mm. The
employed radioactive source is
22
Na and emits two primary
gamma lines at energies of 511 keV and 1274 keV. The
setup is completed by the digital oscilloscope from LeCroy
(model WavePro 760Zi-A with 6 GHz of bandwidth and a
40 GS/s sampling rate).
A persistence of signals from Out1 is displayed on
Figure 9.
The two most populated groups of events represent the
511 keV and 1274 keV energies. Events between the two
families are referred to as Compton scattering interactions.
Example of Operation and Sample
Signals
The typical test setup is pictured in
Figure 8.
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Figure 9: Example Output Signals from Out1
Absolute Maximum Ratings
Stresses in excess of the absolute maximum ratings can
cause damage to the circuit. Limits apply to each parameter
in isolation. Absolute maximum ratings are those values
beyond which damage to the board may occur if these limits
are exceeded for more than a short period of time.
Table 1: Absolute Maximum Ratings
Parameter
Symbol
TSTG
V±
V
+
– V
–
I±
Min.
–20
–20
±2.5
Max.
+60
+50
±8.5
11
100
Unit
°C
°C
V
V
mA
One example signal from Out2 is displayed in
Figure 10.
It
can be seen that Pole-Zero filtering produces a much
steeper signal optimized for the extraction of the timing
information. The oscillations after the first peak are due to
photons reaching the detector after the first bunch and
oscillations of the op-amp. They do not deteriorate the
performance because the best information about the time of
arrival of the gamma ray is carried by the very first photons
that contribute to the very first part of the first rising edge.
For optimal operation, it is thus suggested to set a low
threshold (as close as possible to the baseline fluctuation
level), and consider only the first threshold crossing.
Figure 10: Example Output Signal from Out2
Storage Temperature
Op-amp Dual Voltage
Op-amp Total Voltage
Difference
Supply Current
Operating Temperature TA
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