User manual

Appendix
ComTec GmbH 7-16
7.6.4. Zero Channel Filling
Q: In the 4 parameter coincidence mode it seems to work OK for small pulse heights (up to 5V) but for
larger pulses the data gets put into the zero bin on all four ADCs. For a single/independent measurement
of just one ADC I can get data to show up properly throughout the full 10V full-scale ADC range. Do you
know what can cause this behavior?
A: When you get data into the zero bin for higher pulses, then probably you have to increase the data
ready timeout. It is in the ADC Settings dialog (icon showing the tools) just below the Coinc. time. The
7074 ADC has a large conversion time when used with high resolution (160 mikrosec for 16 k as it is a
100 MHz Wilkinson ADC) and the data ready timeout must be large enough.
Q: I already tried to study the influence of this parameter and my channel zero is always filled with a high
rate for every DRDY timeout values (50 µs, 200µs or much more !). I made one more test this mornig but
doesn't change nothing. I really worry about this. I describe you another test we made : We activated only
ADC number 1 in our chain and put it in the "Coinc. with any" configuration in the system configuration
dialog box. This way, each time an event occurs, the deadtime signal starts a coincidence window (and
only one). On the oscilloscope, we could see that a lot of windows start (aux. output) without no
corresponding deadtime signal. In our application, this zero channel filling is a real problem because we
are obliged to record very big List files with few relevant events. Here are our questions :
1/ Is that possible (by MPANT software or DLL) to reject the zero channel before the data reach the disk ?
2/ If no, is that possible to consider a dedicated version of theFPGA prom inside the module that would
reject the zero channel events due to the DRDY Time-out, instead of sending them to the PC. Dead time
correction and time tags accuracy is not so critical for us in comparison with the volume of data.
A: May be you have some noise on the ADC input? this could cause a deadtime signal also without a
valid conversion, especially if you use a high discriminator level LLD to discriminate the signal from noise,
as the deadtime may be sent also for such signals below the threshold. A lot of counts in channel zero
indicate that there are deadtime signals without data ready signals before the data ready timeout. Such
events are up to some degree normal. You can use a zoomed view to get rid of these counts for a
reasonable scaling of your spectra. In the single mode the deadtime is important only for the livetime
correction and such deadtime signals cannot cause zero events. Please check if the number of
reasonable counts per sec in your spectra without channel zero is about the same in single and
coincidence mode.
As this is so important for you I do what I can: We have already in the software a filter for reducing the
listfile size by writing only 1/10 (1/100, 1/1000) of the timer data: In the data operations dialog, if you
check 'Write Listfile', these options will appear. I introduce here now also an Option 'Drop zero events'. It
allows to drop coincidence events with all ADC data being zero. Such events will then also be eliminated
before writing it into a listfile. But if any ADC in this event has a non-zero value, the complete event
including all zeros is stored. (Version 1.48, Nov-08-2002) I see no chance to achieve this by a hardware
change.
(Version 1.52, Oct-07-2003) Now the 'Drop zero events' suppresses completely histogramming into
channel zero of any spectra.
Q: Please refresh me on what conditions in the ADC/ MPA-3 can result in counts (especially large
numbers of counts) appearing in MPA-3 channel 0. The customer is experiencing this. How can he
eliminate it?.
A: In short, these arise in coincidence mode when any ADC makes a Dead time signal but no Data
Ready before the Data Ready Timeout. The reason can be either a too short Data Ready Timeout, which
can be set in the ADC Settings dialog, or noise signals which are larger than the ADC threshold (about
120 mV for 7074 or 7072 ADCs) but below the Lower Level threshold. In case of noise it is recommended
to turn the amplification down to assure that the noise signals are below 120 mV. A Lower Level
Threshold higher than 120 mV is not recommended. It is possible to get rid of the zero events by crossing
the checkbox "drop zero events" in the listmode box of the data operations dialog, but these events are
then eliminated by software only, they are still in the data stream and produce dead time.