J. Colas, N. Dumont-Dayot, M. Moynot, P. Perrodo, G. Perrot,I. Wingerter-Seez (LAPP-Annecy)
C. De La Taille, N. Seguin-Moreau, L. Serin (LAL-Orsay)

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The ATLAS Liquid Argon electromagnetic calorimeter needs a very accurate calibration system in order to fulfill its physics requirements. 200000 channels need to be calibrated with a signal as close as possible to the detector signal which has a triangular shape with a rise time of ~1 ns and a decay time of ~450 ns. Dynamic range of this signal is from 200 nA (noise level) to 10 mA (almost 3 TeV in one cell).
The calibration signal is obtained by interrupting a precise DC current flowing into an inductor. The magnetic energy stored in the inductor produces a voltage pulse with an exponential decay across the cable characteristic impedance and termination resistor. This pulse is propagated inside the cryostat and is applied across precise injection resistors directly to the electrodes.
Ten calibration boards have been produced. Each board implements 128 pulsers. Amplitude is controlled by an 18 bits DAC. Calibration pulse timing is adjusted by programmable delays with 1 ns steps and Pulsers can be individually selected. A programmable sequencer allows the loading and execution of a complete calibration run with DAC values, delay values, pattern values and trigger number.
Signal uniformity has been measured to be better than 0.2% and sources of non uniformity have been identified.
Delays step and offset uniformity has been measured and Linearity over the 3 gains of the ATLAS Larg shaper has been controlled. Linearity is better than 0.1%, but in the high gain the signal for small amplitude is distorted by the injected charge. New developments to reduce this effect will be discussed.
Radiation tests with neutrons and gammas have shown many shortcomings. Commercial low offset amplifiers have shown an increase of their offset voltage and different propositions are under investigation like auto-zero amplifiers or custom made low offset amplifiers.
The 18 bits DAC offset voltage drifted by 1 mV and no commercial high precision DAC seems to be radiation tolerant. An homemade ramp DAC is currently under investigation. Its advantages and shortcomings will be addressed 
Control logic is implemented in Altera FPGAs which do not resist the cumulated dose. After investigating different types of FPGAs we have decided to develop an integrated circuit using the DMILL technology. Implications of this choice to the architecture of the control logic will be reviewed.
The replacement of the delay chip which is currently not radiation tolerant by the TTCRx chip is also under investigation as this chip will be used for clock and calibration signal distribution.
We would like to have a new prototype board by mid 2000 with a few "ATLAS like" boards available in 2001 for system test. Production being planned in 2002.

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The ATLAS Liquid Argon electromagnetic calorimeter needs a very accurate calibration system in order to fulfill its physics requirements.
A set of ten calibration boards has now been produced to be used during the tests of the calorimeter modules with electron beam. Board electronic will be described and results from systematic tests will be presented.
These boards are not radiation tolerant and some shortcomings have been observed. Developments in progress to improve on these limitations for the pulser and control logic will be reviewed.

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