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Unformatted text preview: e noise, the A225 input (Pin 1) has no
internal protection. External protection may be added to Pin 1
by connecting two back-to-back diodes to ground.
Typical Protective Circuit
DETECTOR 1 A225 NOTE: The input protection circuit provides a limited amount
of protection against transients generated in the detector and bias
network. Any circuit capable of providing absolute protection
would cause intolerable degradation of noise performance. For
this reason, care must be exercised in the use of any preamp with
high voltage detectors. Specifically, damage may result from
detector breakdown, breakdown of the high voltage coupling
capacitor or other component, excessively rapid rise or fall of
detector bias voltage, or the addition of uncharged capacitance
across the input with bias voltage applied.
In some applications, increased protection will justify an
increased noise level. In this case, a resistor may be added in
series with the input - normally a few hundred ohm will suffice.
The shaping amplifier of the A225 produces a unipolar pulse at
Pin 8 suitable for high resolution, high rate pulse height analysis.
This output has an AC impedance of approximately 30 Ω and
will drive 1 kΩ loads as well as several feet of unterminated
cable. In applications requiring highest linearity, load resistance
should be greater than 5 kΩ. This output has a quiescent D.C.
output level, or Baseline, of approximately 0.8 V. In most
applications the pulse should be capacitively coupled to external
The output at Pin 12 is a timing pulse with an unloaded risetime
of approximately 20 ns and a falltime of 2.8 µs capable of
driving a load of 500 Ω. This output has approximately the same
linearity characteristics as the shaped output at Pin 8. Grounding this pin could damage the unit.
In applications requiring a bipolar pulse, the unipolar output can
be differentiated with an RC differentiator. The time constant should be approximately 1.8 µs, suggested values are: C = 1 nF,
R = 1.8 kΩ. In most cases this pulse should be buffered in order
to drive subsequent circuitry.
The A225 can be tested with a pulser by using a s...
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This note was uploaded on 01/15/2012 for the course AAE 490 taught by Professor Andrisani during the Fall '09 term at Purdue.
- Fall '09