Pedestal and Common Noise
in the Test beam data
Rudi Ganz October
20th 1997
Here are some pictures generated from file #RUN00100.dat from
the 1997 test beam data.
Here is for the Telescope number #1 ADC value
versus channel number:
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before the subtraction of pedestals*
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after subtraction
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( *generated as channel by channel average over the "pedestal tagged"
events)
The question now is whether there is common noise which we then have
to subtract on an event-by event basis. Common Noise means a correlated
change of the pedestal. So if one plots one the ADC value of one channel
against the other channel (in the same Read-Out chip) one can look for
such correlation's:
The histograms (as suggested by Russell) show the correlation of
the noise between each channel of one readout chip with all the others.
(So I take the first channel on the chip and plot it against channel 2-128;
then I take second channel and plot it against 3-128 and so on ...) Each
telescope detector is read out by 5(4) 128 channel VA chips.
Telescope #1 chip 1
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Telescope #1 chip 2
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Telescope #1 chip 3
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Telescope #1 chip 4
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Telescope #1 chip 5
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One can see no significant correlation in this telescope detector ->
no significant common mode noise. In fact when one tried to subtract an
event-by-event average on the non-hit channels one can improve the already
very narrow pedestal peak width (RMS. = 2.1 ch) to an RMS of 1.9ch. As
expected from above, no significant improvement.