Speaker
Description
Summary
On detector electronics of the CMS electromagnetic calorimeter (ECAL), which consist
of 75,848 radiation hard scintillating crystals PbWO4, contain almost 16,000
Very-Front-End (VFE) boards that process signals from Avalanche Photodiodes (APDs) in
the central barrel region and Vacuum Phototriodes (VPTs) in the forward endcaps
regions, respectively. The VFE board was designed in two types: covering a dynamic
range up to 50 pC corresponding to incident particle energy of ~1.7 TeV for barrel
and 16 pC for energy up to ~3 TeV for the endcaps. Both types comprises five
identical and independent read-out channels. Each channel, processing the signal from
one crystal, consists of a Multi-Gain Pre-Amplifier (MGPA), a multi-channel ADC, and
two level adapters LVDS-RX. The MGPA contains a pre-amplifier and three parallel gain
stages with nominal gains 1, 6, and 12 that shape and amplify the photodetector
signal. The three analogue output signals of the MGPA are then digitized in parallel
by the multi-channel 40 MHz 12-bit ADC (AD41240). An ADC internal logic determines
whether a gain is saturated and then outputs the data from the highest non-saturated
channel. In addition, the barrel VFE board also incorporates a Detector Control Unit
(DCU) chip for measuring the APD leakage current and the crystal temperature.
All the VFE boards have to pass an extensive quality and assurance program to
guarantee their functionality and reliability. The program includes an optical
inspection by the manufacturer, a power-on test - the first electrical test that
measures voltages, currents and performs a functional test, a burn-in for 72 hours
at a temperature of 60°C, which is followed by a complete calibration and
characterization of each channel. The calibration procedure covers an absolute
calibration of each channel for the three gain stages in ADC counts per pC,
channel-to-channel relative calibration, and gain ratios and linearity studies.
Moreover, other relevant tests, such as a simulation of leakage current of the APDs,
a test of temperature read-out channel on its complete dynamic range are also
performed. Results are registered in a database and are used for a first
intercalibration of the ECAL. To date, the test program for all the 12,800 barrel and
the ~3,000 endcap VFE boards has been completed. The dispersion in the gains
is found to be small (~ 1%) complying with the CMS detector specifications.
Only around 2% of them failed the test criteria and have been rejected, the rest
have been assembled into barrel supermodules and endcap supercrystals.
The results obtained during the Q&A program have been verified in summers 2006
and 2007, when several fully equipped supermodules and supercrystals were
tested and calibrated with high energy electrons.
