FAST ComTec's CSPA1X 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...13 is one of a series of four charge sensitive preamplifiers offered by FAST ComTec, which differ from each other most notably by their gain. The CSPA1X is small shielded metal case with a input connector for power supply.
CSPA1x is a successor model of CSP1x, which includes easier supply with classic plug-in power supply.
Other improvements include integrated protection circuit and better variability.
Equivalent circuit diagram
Figure 2 shows a simplified equivalent circuit diagram of the CSPA1X, 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.
Rf (100 MΩ) and Cf (1.4 pF) are the feedback resistor and capacitor respectively (tdecay= 140μs).
The feedback values for the other models are:
Rf =10 MΩ and Cf =15 pF, tdecay= 150μs (CSPA11),
Rf =680 kΩ and Cf =75 pF, tdecay= 50μs (CSPA12),
Rf =68 kΩ and Cf =750 pF, tdecay= 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 CSP10 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 CSPA1X 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.
(@ 20oC, ±12 volts, unloaded output)
Preamplification channels: 1
Equivalent noise charge (ENC)*:
ENC RMS: 200 electrons, 0.03 femtoCoul.
Equivalent noise in silicon: 1.7 keV (FWHM)
Equivalent noise in CdZnTe: 2.4 keV (FWHM)
ENC slope: 4 electrons RMS /pF
Gain: see table 1
Rise time**: 7 ns (see table 1)
Decay time constant: 140 μs (150 μs, 50 μs, 50 μs resp.)
Unsaturated output swing: -3 to +3 volts
Maximum charge detectable per event: (see table 1)
Power supply voltage (Vs): ± 12 volts nominal (± 2 volts)
Power supply current: < 10 mA
Power dissipation: < 240 mW
Operating temperature: -40 to +85 oC
Output offset: +0.2 to -0.2 volts
Output impedance: 50 ohms
Physical: Net weight: 250 gr.
Size without connectors: 126 mm x 80 mm x 30 mm
Size with connectors: 165 mm x 80 mm x 30 mm
* Measured with input unconnected, using Gaussian shaping amplifier with time constant =1 μs. With a detector attached to the input, noise from the detector capacitance, leakage current, and dielectric losses will add to this figure.
** Pulse rise time (defined as the time to attain 90% of maximum value) has a linear relationship with input capacitance. Value cited in the table assumes zero added input capacitance. Tocalculate pulse rise time for practical situations, use the equation: tr =0.4 Cd + 7 ns, where tr isthe pulse rise time in ns, and Cd is the added capacitance (e.g. detector capacitance) in pF. Keep in mind that others factors within the detection system may further limit this value.
|preamp model||gain (mV /picoCoul.)||max. detect. pulse (e¯)||Equiv. noise (ENC) in silcon keV (FWHM)|
|preamp model||noise (ENC) in e¯ RMS*||noise (ENC) slope e¯/pF||rise time (Cd= 0pF)||rise time slope|
|CSPA10||200 e¯||4 e¯/pF||7 ns||0.4ns/pF|
|CSPA11||630 e¯||3.7 e¯/pF||3 ns||0.25ns/pF|
|CSPA12||6,800 e¯||28 e¯/pF||6 ns||0.25ns/pF|
|CSPA13||24,000 e¯||27 e¯/pF||20 ns||0.25ns/pF|
|Model No.||Description||Order No.|
|CSPA10-2S||Charge sensitive preamp, SHV, 2kV/10nF, 1.4V/pC||CSPA102S|
|CSPA11-2S||Charge sensitive preamp, SHV, 2kV/10nF, 150mV/pC||CSPA112S|
|CSPA12-2S||Charge sensitive preamp, SHV, 2kV/10nF, 15mV/pC||CSPA122S|
|CSPA13-2S||Charge sensitive preamp, SHV, 2kV/10nF, 1.5mV/pC||CSPA132S|
|CSPA10-4S||Charge sensitive preamp, SHV, 4kV/4.7nF, 1.4V/pC||CSPA104S|
|CSPA11-4S||Charge sensitive preamp, SHV, 4kV/4.7nF, 150mV/pC||CSPA114S|
|CSPA12-4S||Charge sensitive preamp, SHV, 4kV/4.7nF, 15mV/pC||CSPA124S|
|CSPA13-4S||Charge sensitive preamp, SHV, 4kV/4.7nF, 1.5mV/pC||CSPA134S|
|CSPA10-1B||Charge sensitive preamp, BNC, 1kV/10nF, 1.4V/pC||CSPA101B|
|CSPA11-1B||Charge sensitive preamp, BNC, 1kV/10nF, 150mV/pC||CSPA111B|
|CSPA12-1B||Charge sensitive preamp, BNC, 1kV/10nF, 15mV/pC||CSPA121B|
|CSPA13-1B||Charge sensitive preamp, BNC, 1kV/10nF, 1.5mV/pC||CSPA131B|
|CSPA10-1BS||Charge sensitive preamp, BNC/SHV, 1kV/10nF, 1.4V/pC||CSPA101BS|
|CSPA11-1BS||Charge sensitive preamp, BNC/SHV, 1kV/10nF, 150mV/pC||CSPA111BS|
|CSPA12-1BS||Charge sensitive preamp, BNC/SHV, 1kV/10nF, 15mV/pC||CSPA121BS|
|CSPA13-1BS||Charge sensitive preamp, BNC/SHV, 1kV/10nF, 1.5mV/pC||CSPA131BS|