Amptek CoolFET A250CF Handmatig - Pagina 6
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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.
The DC offset of the energy output can be adjusted by
using potentiometer R9. It is "zeroed" when shipped from
the factory. If a greater dynamic range is needed in one
direction (i.e. positive or negative), the user can adjust this.
The Energy Output (E) can be inverted by using switch S1.
Output Impedance is 50 Ohms.
4.6
Timing Output (T)
The Timing Output (T) is buffered by an amplifier with a
gain of 2. The polarity of the timing 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 (>100 µs) may be used.
Charge transfer to the input of the A250CF is being applied
only during the transition time according to Q = C
Q = total charge transferred, C
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
> 100 µs)
Amplitude: V = Q/C
= 2 V/picoCoulomb for C
t
Example: To simulate 1 MeV in Si detector: 1 MeV (Si) =
0.044 pC, (2 V/pC)( 0.044 pC) = 88 mV
Hence, an 88 mV step into 0.5 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.
6 Noise Measurement
The noise of 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 s 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
Amptek A250CF CoolFET User Manual –Rev. A5
Charge Sensitive Preamplifier
V, where
t
= value of test capacitor,
t
< 20ns, T
r
= 0.5 pF
t
A250CF CoolFET
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 Output (E) to the post
amplifier/shaper with the correct shaping time constant
(1-3 µs 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
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. The FWHM noise using the
RMS voltmeter is given by:
FWHM (keV, Si) = 2.35 (V
Where:
f
• V
is the noise in volts from the voltmeter.
rms
• V
is the input test pulse in keV equivalent.
in
• V
is the output pulse in volts from the post
out
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.
Conversion: 1 keV FWHM (Si) = 113 electrons RMS =
-17
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.
) (V
)/(V
)
rms
in
out
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