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 WA98 detector segment number #1    ADC value versus channel number
for the telescope detector 1 click here:
 
 
 
 before the subtraction of pedestals* 
    after subtraction
( *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 WA98 detector is read out by 8  128 channel VA chips.
 

 
 
chip 1
 chip 2
chip 3
 
 
 
chip 4
chip 5
chip 6
 
  
 
 
chip 7
chip 8
 
One can see also no significant correlation in the WA98 detector(may a small hint of it) -> no significant common mode noise.