Transcript Calorimeter Group

The ECAL base CW control
C1
C2
August 4th 2008
1
Jacques Lefrancois
ECAL CW base measurements
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The switching transistors commute a charge every 15 microsec
from C1 to the about 100 pfd stray capacitance of the 42 diodes.
This produces across C1 a V of 0.2% of the pumping voltage =>
about 200 mvolts
This V is applied through C2 to the input of the OP AMP and
turns off completely the OP AMP every 15 microsec. This means
that the OP AMP regulates "on average" in a pulse mode. It is a
small miracle that it works partly.
In the HeraB design C1 was 1 microF which allowed the OP AMP
to function in a DC mode. (verified on ECAL base)
In HCAL design C1 and C2 do not exist. The OP AMP is
stabilized at high frequency by a 100ns RC feedback. The OP
AMP operates in DC mode.
August 4th 2008
2
Jacques Lefrancois
Consequences and Conclusions
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For ECAL this switching mode explains the sensitivity to the +6VDC
It explains also the sensitivity of HV to anode current.
One must obtain the circuit diagrams of the PS/SPD CW HV
(from Whom?) and the PCB layout
One must study a "final solution" for ECAL with possible
modifications. Test must determine margins of safety from
switching mode; and safety vs "accident" like switching on off
the 100 volts and the +-6V. Test to be done on a large enough
sample of modified bases.
August 4th 2008
3
Jacques Lefrancois