CR-200 Gaussian shaping amplifier
(Rev. 2)
:
General Description
The CR-200 is a single channel shaping amplifier, intended to be used
to read out the signals from charge sensitive preamplifiers (e.g. Cremat
CR-110 or equivalent). Gaussian shaping amplifiers (also known as
pulse amplifiers, linear amplifiers, or spectroscopy amplifiers) accept a
step-like input pulse and produce an output pulse shaped like a
Gaussian function. The purposes of this are to filter much of the noise
from the signal of interest and to provide a quickly restored baseline to
allow high counting rates. The CR-200 is available in 4 different
shaping times: 100 ns, 250 ns, 1 s, and 4 s. Each has a fixed gain of
10. If additional gain is desired, it is recommended that this be done
with the application of an additional amplifier between the preamplifier
and the CR-200 shaping amplifier. Cremat offers an evaluation board
(CR-160) which includes a variable gain amplifier of appropriate
bandwidth, as well as all necessary connectors.
Equivalent circuit diagram
application guide
Figure 3 shows an equivalent circuit. Pin numbers corresponding with the CR-200
preamplifier are shown. Input components Cin and Rin form a differentiating circuit. The
following circuitry consists of two Sallen and Key filters, providing 4 poles of integration
and signal gain. The numerous integration stages produce an output pulse that
approximates a Gaussian function.
Cin
1
8
2
Rin
3, 6, 7
Figure 3.
part number
Cin
470 pF
1000 pF
1000 pF
3300 pF
Rin
220
240
1.0 k
1.2 k
shaping time
100 ns
250 ns
1.0 s
4.0 s
100 mV
CR-200-100ns
CR-200-250ns
CR-200-1 s
CR-200-4 s
1.0 V
input pulse shape
output pulse shape
Typical Application
Figure 4 shows the CR-200 in a typical application, coupled to a detector via a CR-110
charge sensitive preamplifier. Depending on the requirements of your application, an AC-
coupled amplifier may be added between the preamplifier and shaping amplifier to further
increase the signal size.
Det bias
Figure 1.
Comparison of sample input and output pulse shapes
Improvements in Revision 2
The new CR-200 'Revision 2' has been improved with a significant
reduction in power consumption, reduced output offset, and an
improved thermal coefficient of output offset. Revision 2 is pin-for-pin
compatible with the previous revision of CR-200, however it should be
noted that the labeling has been changed so that pin 1 appears on the
left side of the label.
Definition of "Shaping Time"
charge preamplifier
filtered bias
shaping amplifier
CR-110
detector
12345678
CR-200
12345678
broadband amplifier
input
output
+Vpower
-Vpower
+Vpower
-Vpower
The shaping time is defined as the time-equivalent of the "standard
deviation" of the Gaussian output pulse. A simpler measurement to
make in the laboratory is the full width of the pulse at half of it's
maximum value (FWHM). This value is greater than the shaping time
by a factor of 2.35. For example, a Gaussian shaping amplifier with a
shaping time of 1.0 s would have a FWHM of 2.35 s.
Pole/Zero Correction
The decay time of the input pulse creates an offset in the baseline of
the output pulse unless a pole/zero correction is utilized. This can be
done by connecting a resistor between pin 1 (input) and pin 2 (P/Z). If
the CR-200 is used to read out the Cremat CR-110 preamplifier, then
this resistor value should be 300 k (CR-200-100ns), 130 k (CR-200-
250ns and CR-200-1 s), and 43 k (CR-200-4 s). If a different
preamplifier is used, then the resistor value should be chosen so that it
is equal to the decay constant (RC) of the preamplifier pulse divided by
the CR-200 shaping amplifier input capacitor Cin (see table).
Package Specifications
The CR-200 circuit is contacted via an 8-pin SIP connection (0.100"
spacing). Pin 1 is marked with a white dot for identification.
P/Z
Figure 4.
Assume temp =20˚C, V
s
=
9V, unloaded output
CR-200
amplification channels
gain
polarity
operating temperature range
input noise voltage
CR-200-100ns
CR-200-250ns
CR-200-1 s
CR-200-4 s
output impedance
output offset
output temperature coefficient
power supply voltage (V
s
)
maximum
minimum
power supply current
maximum output current
(with loaded output)
maximum output swing
units
1
10
non-inverting
-40 C to 85 C
90
90
45
30
<5
-20 to +20
-60 to +60
V
s
= 12
V
s
= 6
7
10
8.5
mV
V/ C
0.88"
Cremat, Inc.
CR-200
Rev.2
1 2 3 4 5 6 7 8
0.14"
V RMS
V RMS
V RMS
V RMS
0.85"
Figure 2.
input
P/Z
GND
Vs
+Vs
GND
GND
output
volts
volts
mA
mA
volts
CR-200 Gaussian Shaping Amplifier User Guide (continued)
Electronic Noise (continued)
The following equation can be used to estimate the noise level in a detection system
based on the CR-110 charge sensitive preamplifier. Estimates have been made for
factors (d) and (e) mentioned previously, assuming short traces on an FR-4 circuit
board (such as those found on Cremat's CR-150-AC-C evaluation board). This
equation may be useful in allowing the user to calculate the optimal shaping time ( in
s) minimizing the electronic noise (ENC in electrons rms) for a given detector
capacitance (C
in
in pF) and detector leakage current (I
d
in pA).
Achieving the Desired Gain
The CR-200 series of amplifiers have a fixed gain of 10. It is expected
that many users will want a gain greater than this for their application.
This should be done by adding one or more amplification stages
between the preamplifier and the shaping amplifier. The noise level of
this voltage amplification stage should be kept reasonably low in order
to take full advantage of the low noise of the preceding preamplifier
stage. Assuming the CR-110 charge sensitive preamplifier is used in
the application, a noise voltage of 10 nV / Hz is adequate. Because
there is typically a DC offset at the preamplifier output, this amplifier
should be AC coupled to preamplifier. The bandwidth of these
amplifiers should span ranges at least as wide as that shown in the
following table in order to make optimal use of the signal:
part number
CR-200-100ns
CR-200-250ns
CR-200-1 s
CR-200-4 s
f low
1 kHz
1 kHz
1 kHz
1 kHz
f high
15 MHz
6 MHz
1.5 MHz
400 kHz
Output Pulse Shapes
The following scope traces were recorded using the four different models of the
CR-200. In each case the input pulse was a 100 mV step.
CR-200-100ns:
CR-200-250ns:
The Cremat CR-160 evaluation board accepts the CR-200 shaping
amplifier as a plug-in module and also contains a low noise variable
gain amplifier and the appropriate connectors needed for the user to
easily evaluate the CR-200 shaping amplifier. A schematic diagram of
the amplifier on the CR-160 board is included, and this can serve as a
template for users to design and build their own amplifier for the CR-
200 in their own applications.
Choosing the Optimal Shaping Time for your Application
There are a number of considerations in the choice of the optimal
shaping time for your application. Consider:
1.
The shaping time must be long enough to collect the charge from the
detector. This may be a limiting factor in slow detectors such as gas-
based drift chambers or when collecting the light from slow scintillator
materials.
2.
The shaping time must be short enough to achieve the high counting
rates you require. Assuming randomly spaced pulses, long-shaped
pulses have a higher probability of 'piling up' than short pulses.
3.
Choose a shaping time that filters as much of the electronic noise as
possible. Electronic noise at the preamplifier output is created by a
number of different aspects of the detection system. Some of these
'noise components' have different frequency distributions, allowing us
to use the filtering capability of the shaping amplifier to choose a
shaping time that minimizes the noise for the particular detection
system under design. The principal sources of electronic noise in a
detection system are
a)
the thermal noise of the input JFET in the
preamplifier (which is proportional to the total capacitance to ground at
the input node),
b)
the thermal noise of the feedback resistor and any
'biasing' resistor attached to the detector,
c)
the 'shot noise' of the
detector leakage current,
d)
the electrical contact-related 1/f noise of
the detector and preamplifier input JFET, and
e)
the 'f noise' caused by
the proximity of lossy dielectric material near the preamplifier input
node.
Of the noise components mentioned above, the noise from factor (a)
is more heavily filtered with longer shaping times. More precisely, the
electronic noise due to this factor is inversely proportional to the
shaping time. The electronic noise due to factor (b), on the other hand,
is proportional to the shaping time, as is factor (c). Factors (d) and (e)
are generally difficult to predict, which means it is difficult to predict the
exact noise performance of a detection system. Fortunately, both of
these factors are independent of shaping time, so they have no impact
on the determination of the optimal shaping time. The optimal shaping
time can be predicted by considering only factors (a), (b) and (c). The
subject of noise in detection systems using charge sensitive
preamplifiers is addressed in more detail in these articles:
Bertuccio G; Pullia A; "A Method for the Determination of the Noise
Parameters in Preamplifying Systems for Semiconductor Radiation
Detectors", Rev. Sci. Instrum., 64, p. 3294, (1993).
Radeka V; "Low-Noise Techniques in Detectors", Ann. Rev. Nucl. Part.
Sci., 38, p. 217, (1988).
Goulding
FS;
Landis
DA;
"Signal
Processing
for
Semiconductor Detectors", IEEE Trans. Nuc. Sci., NS-29, p. 1125,
(1982).
200mV / div
200ns / div
200mV / div
400ns / div
CR-200-1 s:
CR-200-4 s:
200mV / div
2 s / div
200mV / div
10 s / div
Output Characteristics
The CR-200 shaping amplifiers have low output impedance (<5 ) and can
source/sink 10 mA of output current. This may not be sufficient to drive a terminated
cable in your application, depending on the size of the signal. For this reason it is best
to use a cable driver circuit at the CR-200 output to make maximum use of the CR-
200 output voltage swing capability. The unloaded output voltage swing comes to
within 0.5 volt of the power supply rails.
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