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Unformatted text preview: mall capacitor
(usually 1 to 2 pF) to inject a test charge into the input. The unit
will respond to the negative-going edge of the test pulse, which
should have a transition time of less than 20 ns. This negative
going edge should be followed by a relatively flat part of the
waveform so that it appears as a step function.
For example, a square wave is a good test waveform. (Keep
the square wave frequency low enough that the response to the
positive-going edge can be ignored.) Alternately, a “sawtooth”
waveform or a tail pulse with long fall time (> 100 µs) may be
used. Charge transfer to the input is according to Q = Ct·V,
where Q = total charge, Ct = value of test capacitor, and V =
amplitude of voltage step. DO NOT connect the test pulser to
the input directly or through a large capacitor (> 100 pF) as this
can produce a large current in the input FET and cause irreversible damage.
Typical Test Circuit
+5 V Fast Rise
50 Ω 11
1 A225 8 Scope 2, 3, 4, 5, 6,
7, 9, 10, 13 Negative going pulse
Rise Time < 20 ns, fall time > 10 µs, or square wave.
Amplitude: 22 mV = 1 MeV (Si)
1 MeV (Si) = (1 x 106 eV x 1.6 x 10-19 C / 3.6 V = 0.044 x 10-12 C
V = Q / Ct = (0.044 x 10-12 C) / (2 x 10-12 F) = 22 mV
Hence, a 22 mV step into 2 pF test capacitor will simulate the
charge of a 1 MeV energy deposition into a silicon detector. Compensation
The A225 is internally optimized for detector capacitance up to
approximately 50 pF. In applications with large detector
capacitance and requiring short timing pulse risetime, a compensation capacitor from 0 to 250 pF may be connected from Pin 14
to ground. The exact value should be experimentally determined
with the detector connected to the input. Note that this compensation will not normally be necessary if the timing pulse is not
used, or its risetime is not critical. PC-25 TEST BOARD for the A225/A206 The PC-25 is a printed circuit board which accepts the
A225 charge sensitive preamplifier and shaping amplifier
and the A206 voltage amplifier and discriminator. It is
designed to be used for two purposes: to facilitate testing...
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- Fall '09