CSPA1X_20210623
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FAST ComTec GmbH, Grünwalder Weg 28a, 82041 Oberhaching, phone: 49-(0)89 665180 -0, fax: 49-(0)89 665180 40,
Description
FAST ComTec’s CSPA10...13 is a single channel charge
sensitive preamplifier module intended for use with various
types of radiation detectors including semiconductor
detectors (e.g. CdTe and CZT), p-i-n photodiodes,
avalanche photodiodes (APDs), and various gas-based
detectors.
The CSPA10 is one of a series of four charge sensitive
preamplifiers offered by FAST ComTec, which differ from
each other most notably by their gain. A guide to selecting
the best charge sensitive preamplifier for your application
can be found at our web site: http://www.fastcomtec.com.
The CSPA1X is a successor model of CSP1X, with easier
classic power supply. Other improvements include
integrated protection circuits and better variability.
Equivalent circuit diagram
Figure 2 shows a simplified equivalent circuit diagram of the
CSPA10, which is a two stage amplifier. The first stage is
high gain, and the second stage is low gain with an
emphasis on supplying sufficient output current to drive a
terminated coaxial cable. R
f
(100 MΩ) and C
f
(1.4 pF) are
the feedback resistor and capacitor respectively (t
decay
=
140μs). The feedback values for the other models are:
R
f
=10 MΩ and C
f
=15 pF, t
decay
= 150μs (CSPA11),
R
f
=680 kΩ and C
f
=75 pF, t
decay
= 50μs (CSPA12),
R
f
=68 kΩ and C
f
=750 pF, t
decay
= 50μs (CSPA13).
Theory of operation
Charge sensitive preamplifiers are used when radiation is
detected as a series of pulses, resulting in brief bursts of
current flowing into or out of the preamplifier input.
Depending on the type of detector, this burst of current may
be very brief (<1 ns) or as long as a few μs. For an
idealized detection current pulse taking the form of a delta
function, the detected charge (time integral of the input
current) will ideally take the form of a step function.
The output waveform of an actual charge sensitive
preamplifier will of course have a non-zero rise time: for the
CSPA10 this figure is approximately 7 ns. Furthermore,
capacitance at the preamplifier input (i.e. detector
capacitance) will further slow the rise time at a rate of
0.4 ns / pF. Keep in mind the output rise time will also be
limited by the speed of the detector. For example, the
detection current pulse from a CsI(Tl)/photodiode
scintillation detector has a duration of approximately a
couple μs, so the expected rise time of the charge sensitive
preamplifier output will be at least that long.
The output waveform of the CSP10 using a capacitively
coupled fast square wave pulser at the input is shown below
to the left. At long time domains, the output decays due to
the discharge of the feedback capacitor through the
feedback resistor, with an RC time constant of 140 μs. This
decay of the output waveform is also shown below, to the
right.
CSPA1X charge sensitive preamplifier