The unit will respond to the negative going edge of

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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 Pulser Ct 2 pF 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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