Amptek A250CF “CoolFET”
Contents
1. FETs 1 and 2 are low Ciss 15 pF and FET 3 is high Ciss 30 pF Use FET 1 or 2 for low capacitance detectors and FET 3 for high capacitance detectors gt 100 pF To select FET 1 1 Set JP8 to Pin 5 2 Set JP7 to jumper 1 2 To select FET 2 1 Set JP8 to Pin 6 2 Set JP7 to jumper 3 4 To select FET 3 1 Set JP8 to Pin 7 2 Set JP7 to jumper 7 8 Amptek Inc am j Cool Fet Rev A4 DI 07703 h z sar eg aa eb Te te Tg Th iLE a Figure 4 4 2 Drain Current Resistors JP6 sets the drain current resistor There are three 3 selectable resistors e Position 1 2 R34 220 Ohms e Position 3 4 R35 110 Ohms e Position 5 6 R36 110 Ohms Under most circumstances the best performance is achieved with 220 Ohms for the CoolFet FETs If the user disables the CoolFET and uses and external FET then one or a combination of the other positions may yield the best results Also at higher detector capacitances a lower value may improve noise results Page 4 of 7 Charge Sensitive Preamplifier A250CEF Cool FET 4 3 AC and DC Coupling The A250CF is shipped from the factory configured for AC coupling with the Bias resistor and filter installed The Bias is connected through a teflon post to the BNC side of the input capacitor by a wire To DC couple the A250CF the user must remove the connection from this post to input capacitor and then solder a jumper wire across the input capacitor or remove it and make2. Jumpers are used to connect the gates to the input to connect the commensurate drains and to select the proper drain resistor to set the drain current There are additional circuit elements in the A250CF including 1 connections to detector bias 2 optional input protection circuitry enabled when shipped from the factory 3 a test input 4 an amplifier which buffers the energy output and provides for polarity and offset adjustments 5 an amplifier which buffers the timing output and provides for polarity adjustment and 6 power supply circuitry Page 2 of 7 Charge Sensitive Preamplifier A250CEF Cool FET Drain Current Drain IN Jumpers l l 100 MQ A Offset Adjust Zz Gain of 2 Energy VvV Optional Invert Gate Jumpers a on d A250 DC or AC Coupling Pe Cam Input Protection a i E gt Jumper i Input i l z a i ae i s ermoelectric we CoolFET l av ee Vo aena Timing Test 8V Sa 1 GQ Power 3 3V 5V E amp E Supply ml TEC Pwr lt gt pF Figure 1 Bock Diagram of A250CF CoolFET 2 2 CoolFET Hybrid The CoolFET hybrid contains three 3 FETs that are placed on top of a thermoelectric cooler and enclosed in a TO 8 package There are two main advantages to c
3. a charge sensitive preamplifier must be tested together with the post amplifier shaper The A250CF noise characteristics given in the specifications are associated with a 2 us shaping time constant in the post amplifier The post amplifier must have very low input noise as in the case of NIM electronics amplifiers or the Amptek A275 so that its contribution to the measurement is minimal The function of the post amplifier is not only to preserve and amplify the linear information received by the charge amplifier but also to provide a band pass filter to eliminate frequencies that contribute to noise Two methods are normally used to measure noise in the preamplifier The first is by using a Multichannel Analyzer MCA and the second is with a wide bandwidth RMS AC voltmeter 6 1 Noise Measurement Using an MCA Connect a calibrated capacitor 1 2 pF not provided to the Input of the A250CF Stimulate with a pulse of known amplitude as in the Section 5 1 Connect the A250CF Energy Outpu E to the post amplifier shaper with the correct shaping time constant 1 3 us for solid state detectors 2 Connect the post amplifier output to the MCA input 3 Calibrate the MCA in pC channel or keV channel by observing two peaks formed by two different known amplitude test pulses 4 The Full Width at Half Maximum FWHM of a particular energy peak can now be read directly from the analyzer 6 2 Noise Measurement Using RMS Voltmeter Conn
4. a direct connection to the input node 4 4 Input Protection The A250CF is shipped with the protection network enabled ah shown in figure 5 The best noise performance is obtained without any protection but the risk of damage is much higher Follow the precautions outlined in Section 1 when disabling the protection The diode protection degrades the noise performance more the higher the dectector capacitance _ Figure 5 Shows the protection network enabled D1 is soldered and the R5 jumper is not soldered Refer to shcematics at end of manual gt 586066 j M a zza Ho 017 Figure 6 Shows the protection network disabled D1 is not sodlered and the R5 jumper is sodlered Amptek A250CF CoolFET User Manual Preliminary 4 5 Energy Output E The Output of the A250 is buffered by an amplifier with a Gain of 2 Both the polarity and the offset of the Energy Output E can be user adjusted Output Impedance is 50 Ohms The DC offset of the energy output can be adjusted by using potentiometer R9 It is zeroed when shipped from the factory If a grater dynamic range is needed in one direction 1 e positive or negative the user can adjust this For example if a greater positive dynamic range is needed this can be adjusted negative The Energy Output E can be inverted by using switch S1 4 6 Timing Output T The Timing Output T is buffered by an amplifier with a gain of 2 The polarity of the timin
5. keV FWHM 0 1 1 10 Detector Capacitance pF 100 1000 Figure 2 Noise as a function of FET and detector capacitance D C Coupled Amptek A250CF CoolFET User Manual Preliminary Page of 7 Charge Sensitive Preamplifier A250CEF Cool FET 1 Warnings Under normal operation a high voltage bias is applied across the detector that is used with the Amptek A250CF preamplifier The user must exercise care when the detector is biased Moreover the bias circuitry has a very long time constant and so this circuitry including the A250CF s input node can remain at a high voltage for a very long time If the user does not exercise suitable caution this voltage can possibly cause personal injury More likely however is that this voltage could damage or destroy either the detector or the A250CF and void warranty if proper care and operation are not exercised as discussed in this manual The input circuit will be destroyed if the detector input capacitor is shorted while the detector bias components are charged These capacitors retain charge for a long period of time A short circuit which can result from connecting a detector cable or capacitor will cause the applied bias voltage stored on the input capacitor to be coupled directly to the input FET causing catastrophic damage to the FET inside the CoolFET hybrid To avoid the possibility of such damage please observe the following precautions 1 Compl
6. Charge Sensitive Preamplifier A250CEF Cool FET Amptek sets the New State of the Art Again with Cooled FET RUN SILENT RUN FAST RUN COOL Performance Noise 670 eV FWHM Si 76 electrons RMS Noise Slope 11 5 eV pF High Ciss FET Fast Rise Time 2 5 ns Features Thermoelectrically Cooled FET 3 FETs to match detector Lowest Noise and Noise Slope AC or DC coupling to the detector Easy to use Both Energy and Timing outputs Optional input protection The Amptek A250CF Cool FET Charge Sensitive Preamplifier is the lowest noise general purpose preamplifier available It is designed to give the ultimate performance when used with either low or high detector capacitance detectors and is a direct replacement for other higher noise charge sensitive preamplifiers The A250CF CoolFET technology uses a thermoelectric Peltier cooler to keep the input FET s at 50 C Cooling is totally transparent to the user Hence the A250CF CoolFET operates like a room temperature preamplifier Based on the successful A250 designed for high performance satellite instrumentation the A250CF CoolFET re defines the new state of the art Power to the A250CF CoolFET is provided by an external stable power supply included Detector bias can be applied via an SHV connector Input Energy E Output Timing T Output and Test are provided via BNC connectors 100 10 Noise Si equivalent
7. a 27 U 1000v 10U AK NI AD8014 1K wy BIAS RET QQQ iii B Buffer Amplifier a J26 Drain Op ip APB se Gain of 2 tl i eee ee Resistor 100UF Optional Invert Selection 8 499 R5 gt eae DC Offset Adjust 49 9 YOY Input BNC U3 Input Capacitor l i 1K l 10 nF 1000 i S1 i 0J C A C and D C coupling i B ins a a 5 O i D1 Selection C18 1K l 49 9 TP A C Coupling i rp j oir Ap J24 Connect H V to the Input TE PO R BNC TTT I a i i p aay L11 C1 R2 NI D C Coupling 1uF 499 n Short the Input Capacitor i i C13 and do not connect H V to i h Io o wae C11 TONF the Input BNC 8V u lt Py 470pF H arses ere aaa SRO ie SL a T o NPO Optional Input L L E is Protection 7 a n a i 10U AD8014 wees eel TEME FET Gate zi C28 To enable protection R5 should we CoolFET Hybrid not be shorted 1 2 open and D1 Selection VY 499 TP should be connected 2 3 Thera IN J25 connected Seat f selection must Differentiator Amplifier 0 25 ti tant To disable protection R5 should pit er h pii Gai es iania be shorted 1 2 connected and So Eroa DYNE lt GND aa are aves D1 not connected 2 3 open DS i R16 1K AO ear J21 C2 Jl k 0 SpF Op R1 49 9 Amptek A250CF CoolFET User Manual Preliminary Page 7 of 7
8. ect a calibrated capacitor 1 2 pF not provided to the Input of the A250CF Stimulate with a pulse of known amplitude as in the Section 5 The FWHM noise using the RMS voltmeter is given by FWHM keV Si 2 35 Vims CVin CV out Where e Vims 18 the noise in volts from the voltmeter e Viis the input test pulse in keV equivalent e Vou iS the output pulse in volts from the post amplifier Example With a 2 pF test capacitor 1 Set Vin 22 mV 1 MeV Si 2 Set Post amplifier s gain to obtain 2 35 Volts output pulse 3 The RMS voltmeter is now calibrated to 1 mV RMS 1 keV FWHM Si 4 Remove the test pulser cover the input BNC and read the RMS voltmeter Amptek A250CF CoolFET User Manual Preliminary Conversion 1 keV FWHM Si 113 electrons RMS 1 81 x 10 Coulombs RMS When measuring noise of the entire system either by the MCA or the RMS voltmeter method the detector must be simultaneously connected with the test circuit to the input of the A250CF The noise measurement in this case will include the contribution from the detector due to both its capacitance and its leakage current Page 6 of 7 Charge Sensitive Preamplifier A250CEF Cool FET 7 Schematic RQ L1 iy sy R12 P R10 Q Q 15mH C22 5 R18 R21 402K 7 15k High Voltage th ress 4 Low Pass Filter 250 250V 48V Fa 5V 15MH 1N C7 5V c3 T e C26 C14 BIAS s O 410NF 47NF 47NF J10 R8 GL il R47 110 L10 10UF s39
9. ed during unprotected operation is not covered under warranty unless the precautions outlined here are followed Amptek A250CF CoolFET User Manual Preliminary 2 Overview 2 1 Description The A250CEF is a charge sensitive preamplifier The input to the preamplifier is the signal from a radiation detector a current pulse of short duration mseconds or less The total energy deposited is proportional to the total charge generated The primary output of A250CF the energy output is a voltage step proportional to the input charge the time integral of the current pulse with a gain of 176 mV MeV Si This step has a fast rise the rise time of the A250CF is 2 5 nsec for 0 capacitance and a slow decay to baseline SOO msec This energy output is generally sent to a shaping amplifier and is used for spectroscopic measurements The timing output has a much faster decay 1 msec and is used as the input to a timing circuit The A250CE consists of several main function blocks The core of the preamplifier is the charge amplifier itself which consists of the CoolFET hybrid the A250 amplifier and the feedback components This circuit produces the voltage output for a current input and determines the output noise and risetime The series noise of a preamplifier is at its minimum when the input capacitance is comparable to the FET capacitance The A250CF includes a jumper that permit the user to select 1 of 3 FETs to match capacitance
10. etely discharge the detector bias circuit by turning off the bias supply before connecting a cable capacitor or any other device to the Detector Input connection on the front panel of the preamplifier 2 To discharge the circuit if a variable HV supply is used turn the voltage control to zero and leave it there for are least 30 seconds The bias circuitry will discharge itself through the output of the bias supply Otherwise connect a low impedance ideally a short circuit across the Detector Bias SHV connector on the rear panel of the preamplifier for at least 30 seconds Do not short the Detector Input connector on the preamplifier The preamplifier includes a protection circuit that is enabled when the unit ships from the factory which protects the input FET from destruction due to large transients under abnormal operating conditions It is important to note that this circuit protects the FET and not the detector If the detector breaks down due to over voltage the protection circuit prevents damage to the preamplifier FET The protection circuit does add to the electronic noise of the preamplifier so if the lowest noise is required this circuit can be removed as described later in this manual However this leaves the FET susceptible to damage by a variety of transients The half life of an unprotected FET is much shorter than that of a protected FET so there is a trade off between the very lowest noise and risk Damage caus
11. g output can be toggled by using switch S2 Output Impedance is 50 Ohms 5 Testing via Test Input The A250CF can be tested with a pulser to inject a test charge into the Test input The unit will respond to both the negative and positive edge of the test pulse which should have a transition time of less than 20 ns A square wave or a tail pulse with long fall time gt 100 us may be used Charge transfer to the input of the A250CF is being applied only during the transition time according to Q C V where Q total charge transferred C value of test capacitor and V amplitude of voltage step DO NOT connect the test pulser to the input directly or through a test capacitor greater than 100 pF as this can produce a large current pulse at the input FET and cause irreversible damage Input waveform Square wave or Tail pulse T lt 20ns T gt 100 us Amplitude V Q C 1 V picoCoulomb for C 1 pF Example To simulate 1 MeV in Si detector 1 MeV Si 0 044 pC 1 V pC 0 044 pC 44 mV Hence a 44 mV step into 1 pF test capacitor simulates the charge generated in a silicon detector by a particle when it loses 1 MeV of its energy NOTE The internal test capacitor 0 5 pF is not a calibrated capacitor It is provided to facilitate the testing of the preamplifier and not to make absolute noise measurements Page 5 of 7 Charge Sensitive Preamplifier A250CEF Cool FET 6 Noise Measurement The noise of
12. ooling the FET it reduces the leakage current and increases the transconductance both of which reduce the electronic noise of the system The increased transconductance provides a much improved noise slope eV pF over un cooled systems which is especially important for large capacitance detectors 3 Specifications 3 1 Performance Noise at 0 pF keV 2 us shaping CoolFET DC 100 MQ Type coupled Bias Table 1 Typical noise data for A250CF The noise depends on 1 the detector capacitance 2 input configuration AC vs DC coupling use of input protection and 3 which CoolFET is used Gain With a 0 5 pF feedback capacitor and a gain of 2 in the output buffer 176 mV MeV Si 220 mV MeV Ge 144 mV MeV CdTe 152 mV MeV HglI2 Amptek A250CF CoolFET User Manual Preliminary 4 V pC 0 64 uV electron FET Capacitance Ciss FET 1 and 2 Low Cis 15 pF FET 3 High Cis 30 pF Rise Time 2 5 nsec at 0 pF Integral Nonlinearity lt 0 03 3 2 Inputs Detector Input Positive or negative input signals from semiconductor detectors Bias Detector bias voltage maximum 1 kV from a power supply Test Input voltage pulses for system test and calibration 3 3 Outputs Energy E Provides an output voltage step proportional to the input charge Nominal gain is 176 mV MeV Si Polarity and DC offset are adjustable Output Impedance is 50 Ohms Dynamic Range 4 V offset adjustable to 6 V Timing Pr
13. ovides a tail pulse 1 msec decay for system timing measurements Polarity is adjustable see below Output Impedance is 50 Ohms Page 3 of 7 Charge Sensitive Preamplifier A250CEF Cool FET 1 0 O K Energy gt gt O N Normalized Amplitude V gt P ih gt SS O e NW 100 150 200 250 Time msec Nn Nn Energy Output 5 os Timing Output aN 0 0 eoa 1 11 0 100 200 300 400 Time nsec Figure 3 Timing and energy outputs from a typical test pulser 3 4 Connections Input Test Energy Timing BNC Bias SHV 3 5 Power 3 3 V DC at 1 6 A AC power adapter included 3 6 Mechanical Size 3 5 x 2 5 Weight 28 g 4 Preamplifier Configuration Adjustment and FET Selection IMPORTANT JP6 drain current resistor selection and JP7 FET Drain selection have jumpers installed that facilitate selection The Input Protection selection and the FET Gate selection however have posts to which the user must solder in order to make the appropriate selection This is to ensure the best noise performance The user must exercise EXTREME caution while soldering these posts Amptek A250CF CoolFET User Manual Preliminary Please contact the factory if there are any questions about the proper selection soldering procedure 4 1 FET Selection The A250CEF contains three cooled FETs
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