To achieve the required per layer resolution of 150 micro-m for
ME1/2 (
The intrinsic noise for the preamp-shaper depends primarily on three
parameters: the input (strip) capacitance, the size (width and length) of the
preamp FET and the shaper peaking time. Anticipating the high rate
environment at the LHC - the estimated neutron and charged particle hit rate
can be as high as 1 kHz/cm**2 at the designed luminosity of 10**34/s/cm**2 -
the shaper peaking time is chosen to be 100 ns for fast recovery and for good
two-pulse time resolution. These considerations together with practical
choices for the FET size lead to the requirement that the rms equivalent input
noise be less than 7000 electronics for the strip with the highest capacitance
in the system (250 pF), or 25 e's/pF.
The required minimum signal to noise ratio of 100 can be met by
operating the chamber with a gas gain such that the induced cathode charge by
a normally incident minimum ionizing particle is about 7x10**5 electrons, or
112 fC. The overall DAQ system gain is 0.5 fC per ADC count. The electronics
noise from the largest chambers will result in an ADC pedestal rms width of
2.2 counts and the peak of the Landau distribution will be located at around
250 counts (8 bits). The dynamic range is 4000 (12 bit ADC), which is
necessary to accommodate the tail of the Landau distribution and variations
due to chamber and amplifier gains. The gain of the trigger output of the
preamp/shaper may be set higher in order to minimize any effects due to
comparator offsets. These specifications are listed in the following table.
The cathode preamp-shaper ASIC
( Buckeye chip) has gone through 5
submissions. Each of the submissions is a four channel prototype ASIC. The
foundry used is MOSIS-HP and the process is 1.2 micron CMOS with linear
capacitors. The test result of the latest submission shows that the Buckeye
chip has met all of the design requirements.
Cathode Preamp-shaper Specifications
Equivalent Input Noise (rms) Cstrip x 25 e/pF+700 e (7000 e at 250 pF)
Shaper Peaking Time 100 ns
Peaking Time of Shaped Chamber Pulse 170ns
Shaped Waveform: 5 pole semi-gaussian w/ tail cancellation
Nominal Input Charge (i,ii) 112 fC (=7000x100(S/N))
Preamp-Shaper Gain 0.9 mV/fC
Non-Linearity: < 1% 0 - 1.5 volt
Dynamic Range: 12 bits
Overall System Gain: 0.5 fC/ADC count
Two Track Time Resolution: 125 ns
Gain Variation: <4% channel-to-channel <10% chip-to-chip
Calibration Precision: 0.5% over full range
Footnotes:
i) Corresponding to the average ionization deposition (Landau peak) for a
normal incident minimum ionization particle. The operating voltage or the
chamber gas gain corresponding to this input charge should be determined
empirically.
ii) The gas gain required is 700,000/(N_ion x f_att x f_ind x f_sh),
where N_ion is the number of ion pairs generated by the traversing
particle; f_att is fraction of drift electrons lost due to attachment
to chamber gas molecules, f_ind is fraction of the anode charge induced on
either cathode; and f_sh is fraction of avalanche charge collected by
anode for a given shaping time. Simulation (CMS TN/95-194) of CSC response
gives N_ion = 180 (for 10 mm gas gap); F_att = 0.5; f_ind = 0.41; f_sh
(100ns shaper peaking time) = 0.19. Based on these numbers, the gas gain
is estimated to be about 10**5.