Model A225 is a high performance thin film hybrid charge sensitive preamplifier and shaping amplifier developed especially for high resolution systems employing solid state detectors, proportional counters, photo multiplier tubes and other charge producing detectors in the pulse height analysis or A/D mode of operation.

Test Conditions: (Vs = +5 V, T = +25 °C, RLOAD = 10 K)

A225 Mechanical Dimensions

   Due to the high sensitivity and ultra low noise of the A225, care should be taken in circuit layout. The PC-25 Test Board may be used as an example of appropriate layout technique. In general, ground plane construction is recommended. Input and output lines should be kept well separated and in most cases shielding will be necessary. Particular attention should be paid to the detector ground connection to avoid oscillation due to feedback. The supply voltage isinternally decoupled.While thisis normally adequate, in some applications external bypassing may be helpful.

   To facilitate noise minimization in certain applications, two separate ground connections are provided. Pin 4 is the ground connection for the input stage and is also connected to the case. Pin 5 is ground for the remainder of the circuit. In most applications Pins 4 and 5 may be connected to the same ground (preferably a ground plane under the unit) along with Pins 2,3,6,7,9,10 and 13.

   The A225 may also be used in applications where the input from the detector is positive by connecting Pin 5 (output ground) to anegative supply voltage instead of ground. This voltage can range from 0 to -10 V. A negative output pulse at Pin 8 will result from a positive input. The dynamic range in this case will be approximately 0.5 Volts greater than Pin 5. For example, with Pin 5 operated at -5 Volts, negative pulses of up to 5.5 Volts amplitude can be obtained. This mode of operation will result in increased operating current. The A225 is not specified in this mode and critical parameters should be verified by the user.

   While specifications are given for operation at +5 V, the characteristics of the A225 are relatively unaffected by changes in supply voltage from +4 to +25 VDC. Parameters critical to a particular application should be checked at the actual operating voltage.

Typical Protective Circuit

   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 ohms will suffice.

irreversible damage.

   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 ? and will drive 1 k? loads as well as several feet of unterminated cable. In

applicationsrequiring highest linearity, load resistance should be greater than 5 k?. 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 circuitry.

   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 ?. This output has approximately the same linearity characteristics as the shaped output at Pin 8. Grounding this Pin could damage the unit.

   The A225 isinternally 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.

   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 k ?. In most cases this pulse should be buffered in order to drive subsequent circuitry.

Up to 100 MeV (Si) or 500 picocoulomb can be analyzed with the A225 without overloading the unit, resulting into a dynamic range of 40,000.The x10 overload characteristics of the A225 are: < 20 μs @ Vs = +25 VDC and < 40 μs @ Vs = +5 VDC.

The sensitivity of the A225 can be reduced by adding the following external network:

   This note is intended as a guide in the use of the A225 to prevent damage due to overstress of input or output circuitry.

   Input protection is described above. Please read that section carefully. The A225 input

connects directly to the gate of a sensitive field effect transistor (FET). In general, any large, rapid change in voltage in the detector circuit can cause excessive input currentsto flow and must be avoided. Such transients can be caused by the breakdown of the detector or the coupling or filter capacitor. Also,raising or lowering the detector bias voltage too quickly can have the same effect. The following procedures should be followed to ensure safe operation of the unit.

     4. To minimize the energy which can be dissipated at the input in the event of a breakdown, do not use a coupling capacitor of much larger value than necessary. 1 nF is adequate in many applications.

     2. If it is desired to use the direct output without capacitive coupling, place a 100 ohm

resistor in series with the output close to Pin 8. This will prevent damage in the event of a short.

A225 Applications

   The PC25 is a printed circuit board which acceptsthe 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 of these devices, together or separately,and to simplify their use in many applications.The board provides input test circuitry, gain adjustment, voltage reference for theA206, and convenient access to all active device pins. Ground plane construction minimizes noise pickup.