A Protection Scheme for the Calibration Resistive Networks

The High Voltage discharge incidents during the test of the LAr barrel modules at Saclay damaged several of the calibration resistive networks on the Mother Boards. This effect was reproducible in a controlled test-bench environment. Bench tests have demonstrated that an energy discharge of 120 mJ is sufficient to change the resistance beyond its tight tolerance of 0.1%. This corresponds to a discharge at a HV of 500 Volts, assuming a detector capacitance of 1nF. A diode protection circuit at the MB input has been devised to minimize accidental damage to the calibration resistive networks. This protection network is now being used for both the barrel and endcap EM calorimeters. The design and performance of the protection network under various conditions have been studied and summarized in this note.