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DP4 User Manual D1.doc, 4/6/05
Use of the DP4 with Tail Pulse Preamps
The DP4 is configured at the factory for particular preamplifier properties, specifically for reset type
feedback, for negative input signals of tens of mV, etc. It can readily be used with other preamplifiers, but
this requires hardware reconfiguration. This section will guide the user in making these changes.
As shown in Figure 1, the DP4 contains an analog prefilter circuit. The output of this analog prefilter,
which is the input to the ADC and can be measured a test point marked AMP3 on the DP4 board, is shown in
Figure 13 as the Prefilter Output. The key to adapting the DP4 for various preamps is modifying the prefilter
to generate the Prefilter Output pulse shape shown in Figure 13. If the signal measured at AMP3 has the
proper characteristics, the DP4 will operate properly. This section provides suggestions regarding how the
analog prefilter should be adapted for a few common situations, but it cannot cover all possibilities. In any
and all cases, measuring the signal at AMP3 will allow the user to determine if the hardware has been
properly configured.
Prefilter Output
(ADC Input)
DP4 Output
DP4 Input
(Preamp Output)
Figure 13. Oscilloscope trace showing the proper signal at AMP3.
The important characteristics of the signal at AMP3 , the output of the analog prefilter, are:
•
Rise time should be >100 nsec but much faster than the peaking time to be set in the DP4 software
•
The decay time must be 3.2 µsec and must be a simple, single pole decay
•
The DC offset should be sufficient to keep the ADC input positive. Typically, 200 mV is good. The
ADC has a range of 0 to 2V, so if AMP3 drops below 0 V, signal processing anomalies will occur. The
DC offset may be increased if necessary. [AMP3 should only drop below 0V during detector reset.]
•
The amplitude of the pulse must be about 1V, positive going. The ADC has a range of 0 to 2V, so if
AMP3 exceeds 2V, signal processing anomalies will occur.
Amptek Inc.
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DP4 Digital Pulse Processor