Back of the envelope Dynamic range

[Elton Smith <elton@jlab.org>]

Hi Fernando,

Yesterday during your presentation you showed the dynamic range of the
preamp to saturate at 400 fC. We are a little confused about the range
required. I know that there was quite a bit of discussion at the time the
range was decided, but simple estimates give higher numbers and we need to
know what we are missing.

Fernando was correct in that the induced signal on the wire on the wire
coming from the electron signal is only about 20% of the avalanch charge
(in the first say 10 ns). Therefore, we obtain the following for a minimum
ionizing track:

Simon says that the gain of the FDC is about 0.8x10^5.

assume: 100 ions, 20% of Qtot, Gain=10^5, e=1.6x10^-19.
Signal charge (anode) = 0.2 x 100 x 10^5 x 1.6x10^-19 ~ 300 fC
Signal charge (cathode) ~ 300 fC x 1/2 x 1/4 ~ 40 fC.

My preliminary (rough) estimate concludes that this should work for the
FDC cathodes. The FDC anodes under the nominal design do not use the
FADCs, but would likely saturate if they do.

For the CDC, the anode signal will also be used for dE/dx, so a factor of
at least 30 is needed in dynamic range. These would clearly saturate at a
Gain of 10^5, but we might want to run them at lower gain.  At G=10^4
gain, and 30 dynamic range, we get a maximum signal of 900 fC, also
saturating the preamp.

Comments? What am I doing wrong?

Thanks, Elton.






Elton Smith
Jefferson Lab MS 12H5
12000 Jefferson Ave
Suite # 16
Newport News, VA 23606
elton@jlab.org
(757) 269-7625