CR-200 Gaussian shaping amplifier:

application guide

Description Pole/Zero Correction

The CR-200 is a single channel shaping amplifier, intended The long decay time of the input pulse creates a small
to be used to read out the signals from charge sensitive overshoot in the shape of the ≠output pulse unless a pole/
preamplifiers. Gaussian shaping amplifiers (also known as zero correction is utilized. This can be done by connecting
pulse amplifiers, linear amplifiers, or spectroscopy amplifiers) a resistor (RP/Z) between pin 1 (input) and pin 2 (P/Z). This
accept a step-like input pulse and produce an output pulse resistor is in parallel with the input capacitor (internal to the
shaped like a Gaussian function. The purposes of this are to CR-200 circuit) and creates a ’zero’ in the amplifier’s transfer
filter much of the noise from the signal of interest and to pro- function which cancels the ’pole’ created by the charge sen-
vide a quickly restored baseline to allow for higher counting sitive preamplifier’s feedback resistor. To achieve proper pole/
rates. The CR-200 is available in 7 different shaping times: zero cancellation, RP/Z should be selected to be equal to
100 ns, 250 ns, 500 ns, 1 μs, 2 μs, 4 μs, and 8 μs. Each has a Rf*Cf/Cin where Rf and Cf are the feedback resistor and feed-
fixed gain of 10. If additional gain is desired, it is recommen- back capacitor of the charge sensitive preamplifier and Cin
ded that this be done with the application of an additional is the value of the input capacitor in the CR-200. The value of
amplifier between the preamplifier and the CR-200 shaping Cin for the CR-200 circuit can be found in the provided table.
amplifier. Cremat offers an evaluation board (CR-160) which Keep in mind that adding RP/Z will likely affect the DC offset
includes a multi-stage variable-gain amplifier, as well as all of the shaping amplifier output. This is because RP/Z directly
necessary connectors. More information on the CR-160 eva- couples the DC offset from the charge sensitive preamplifier
luation board can be found at http://www.fastcomtec.com output into the shaping amplifier input. Some fraction of
this DC offset is amplified along with the pulse. It is recom-
100 mV mended that instrumentation which includes the CR-200
input pulse shap e include a DC offset adjustment to be used to correct for this.
An example of this can be found in the design of the CR-160
output pulse shap e
1.0 V
evaluation board found here: http://www.cremat.com/CR-
160schematic.pdf . You may wish to realize RP/Z as a potenti-
Figure 1. Comparison of sample input and output pulse shapes
ometer so to adjust the value precisely. The effect of RP/Z on
the pulse shape can be seen in the pulse waveforms shown
in Figure 3.
Definition of „Shaping Time“
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 2.4. For example, R P/Z resistance too hig h R P/Z resistance proper R P/Z resistance too lo w
a Gaussian shaping amplifier with a shaping time of 1.0 μs Figure 3.
would have a FWHM of 2.4 μs.

Equivalent circuit diagram Baseline Restoration (BLR)

Figure 2 shows an equivalent circuit. Pin numbers corres- The CR-200 does not contain active baseline restoration
ponding with the CR-200 shaping amplifier are shown. Input circuitry. For this reason there will be a negative ’baseline
components Cin and Rin form a differentiating circuit. The shift’ (change in the output DC offset) at high counting rates.
following circuitry consists of two Sallen and Key filters, pro- In order to determine whether this will be a problem for your
viding 4 poles of integration and signal gain. The numerous application, use the equation (valid for small baseline shifts):
integration stages produce an output pulse that approxima- S/H = R * τ * 2.5 x10-6
tes a Gaussian function.
where S is the negative baseline shift, H is the pulse height,
Cin R is the count rate (counts/sec), and τ is the shaping time
1 8
of the shaping amplifier (in μs). For example, using a 1 μs
2 shaping amplifier we would predict a 0.025 (2.5%) shift in
Rin the baseline at a count rate of 10,000 counts per second. To
3, 6, 7 address this potential problem, Cremat offers the CR-210
baseline restorer. More information on this circuit can be
Figure 2. found at the fastcomtec.com website.

CR-200_20140228 Page 1 FAST ComTec GmbH, Grünwalder Weg 28a, 82041 Oberhaching, phone: 49-(0)89 665180 -0, fax: 49-(0)89 665180 40, http://www.fastcomtec.com
 CR-200 Gaussian shaping amplifier: application guide

Package Specifications Typical Application

The CR-200 circuit is contacted via an 8-pin SIP connection Figure 6 shows the CR-200 in a typical application, coupled
(0.100“ spacing). Pin 1 is marked with a white dot for identi- to a detector via a CR- 110 charge sensitive preamplifier.
fication. 0.88" 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.
0.85"
baseline restore
charge preamplifier shaping amplifier (optional)

CR-110 CR-200 CR-210

12345678 broadband amplifier 12345678 12345678

(optional)
1 2 3 4 56 7 8 input output
GND
GND
GND
Vs
+Vs

output
P/Z
input

+Vpower +Vpower +Vpower

RP/Z
detector
Figure 4 thickness: 0.14" -Vpower -Vpower -Vpower

Figure 5
Det bias

Choosing the Optimal Shaping Time for your Application

There are a number of considerations in the choice of the Output Characteristics
optimal shaping time for your application. Consider: The CR-200 shaping amplifiers have low output impedance
1. The shaping time must be long enough to collect the (<5Ω) and can source/sink 10 mA of output current. This may
charge from the detector. This may be a limiting factor in slow not be sufficient to drive a terminated cable in your application,
detectors such as gas-based drift chambers or when collecting depending on the size of the signal. For this reason it is best to
the light from slow-decay scintillators. use a cable driver circuit at the CR-200 output to make maxi-
2. The shaping time must be short enough to achieve the high mum use of the CR-200 output voltage swing capability. The
counting rates you require. Assuming randomly spaced pulses, unloaded output voltage swing comes to within 0.5 volt of the
long-shaped pulses have a higher probability of ‚piling up‘ power supply rails. 8 s
250ns
4 s 2 s 1 s 500ns 10 100ns
50ns
than short pulses. Note that ‚pile-up‘ will only be a problem at
25ns
very high count rates; ‚Baseline shift‘ will start to be a prob-
lem at somewhat lower count rates. See the previous section
gain

regarding ‚Baseline Restoration‘.

0.001 0.01 0.1 10
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. Many of these ‚noise components‘ have different 0.1
freq. (MHz)
power spectra, allowing us to use the filtering capability of the
CR-200 Bandpass filtering properties
shaping amplifier to choose a shaping time that minimizes the
noise for the particular detection system under design. Keep
in mind it may be difficult to precisely predict the shaping
time at which the noise will be minimum. The surest method
may be to determine this noise minimum experimentally by
measuring the noise using a variety of shaping times.
Example: Shaping Amplifier used in Nuclear Pulse
Detection
Shown below are two oscilloscope traces: the input (blue) and
output (yellow) of a Gaussian shaping amplifier (1μs shaping
time) reading pulses from a charge sensitive preamplifier in
the presence of noise. Not only does the shaping amplifier
amplify the pulses, but the fall time is quickened, greatly re-
ducing the problem of pulses ‘piling up’ on the tails of prece-
ding pulses. Also, the filtering effects of the shaping amplifier
significantly filters the noise. This allows for pulses to be clearly
detected that would be otherwise buried within the noise.

CR-200_20140228 Page 2 FAST ComTec GmbH, Grünwalder Weg 28a, 82041 Oberhaching, phone: 49-(0)89 665180 -0, fax: 49-(0)89 665180 40, http://www.fastcomtec.com
 CR-200 Gaussian shaping amplifier: application guide

For optional bipolar

shaping (see section on
„Baseline Restoration“
and Figure 4)
part # shaping time output pulse Rin Cin Rout Cout gain
width (FWHM)
CR-200-25ns 25 ns 59 ns 82 Ω 220 pF 50 Ω x pF 4
CR-200-50 ns 50ns 120 ns 220 Ω 220 pF 50 Ω x pF 8
CR-200-100ns 100 ns 240 ns 220 Ω 470 pF 50 Ω 2200 pF 10
CR-200-250 ns 250ns 590 ns 240 Ω 1000 pF 50 Ω 4700 pF 10
CR-200-500ns 500 ns 1.2 μs 510 Ω 1000 pF 50 Ω 0.01 μF 10
CR-200-1μs 1 μs 2.4 μs 1.0 kΩ 1000 pF 50 Ω 0.022 μF 10
CR-200-2μs 2 μs 4.7 μs 2.0 kΩ 1000 pF 50 Ω 0.039 μF 10
CR-200-4μs 4 μs 9.4 μs 1.2 kΩ 3300 pF 50 Ω 0.082 μF 10
CR-200-8μs 8 μs 19 μs 2.4 kΩ 3300 pF 50 Ω 0.15 μF 10
see ‚equivalent circuit diagram‘
on previous page.

Specifications Assume temp =20°C, Vs = 9V, unloaded output power supply voltage (Vs)
- CR-200 units maximum Vs = + - 12 volts
amplification channels 1 - minimum Vs = + - 6 volts
gain 10 -
quiescent power supply 7 mA
polarity non-inverting -
current
operating temperature -40 C to 85 C -
maximum output 10 mA
range
current
input noise voltage maximum output + - 8.5 volts
swing
CR-200-25ns 160 μV RMS
operating temperature - 40 C to 85 C -
CR-200-50ns 115 μV RMS
range
CR-200-100ns 90 μV RMS
CR-200-250ns 60 μV RMS
CR-200-500ns 47 μV RMS
CR-200-1μs 36 μV RMS
CR-200-2μs 30 μV RMS
CR-200-4μs 24 μV RMS
CR-200-8μs 22 μV RMS

output impedance <5 Ω

output offset -40 to +40 mV
output temperature -60 to +60 μV / C
coefficient

CR-200_20140228 Page 3 FAST ComTec GmbH, Grünwalder Weg 28a, 82041 Oberhaching, phone: 49-(0)89 665180 -0, fax: 49-(0)89 665180 40, http://www.fastcomtec.com