[Halld-tracking-hw] ADC125 as a TDC...

[Beni Zihlmann]

Hi All,
attached are 3 plots. The first is a 2d plot for FDC wire 11 on cosmic
data. The vertical axis is the strip number and the horizontal axis
is the time difference between the strip and wire 11. The times are
determined using Gerard's algorithm with interpolation on the FDC
data for both the wire and the strip. Note that the time difference
is calculated as "t_wire - t_strip" so the long tail to the negative times
is due to the strip while we have a rather sharp edge on the right side.
this can be seen more clearly when looking at the projection of the 2d
plot to the horizontal axis. This is shown in the next to plots individually
for the channel 0-23 and 24-47 which represents the two preamp cards
for the top and bottom cathode planes. I try to extract a width by fitting
mostly only the upper edge. As you can see the width is between 1 and
1.5 ns. So the individual resolution may be about sqrt(2) better. You can
also see that the timing resolution depends on the average signal amplitude
which is smaller for the second preamp (channel 24-47). I do not know what
causes the shift in time between the two preamp card. This shift is 
certainly
much larger than any cable length miss match etc. But Lubomir observed
this behavior already earlier.
What causes the long tail to negative time difference is not completely 
clear
to me. One reason might be that there are considerable gain variations 
between
strip channels and I plot all hits not just the one with the largest 
amplitude
in a hit.
Let me know if you have additional ideas what else one could tests.

cheers,
Beni
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