Low voltage power supply for HCAL photodetectors in CMS

The compact muon solenoid (CMS) is one of the detectors for the new large hadron collider (LHC) of CERN. Because of the high level magnetic field inside CMS specialized photodetectors - hybrid photo diodes (HPDs) are used in its hadron calorimeter (HCAL). For their normal operation HPDs need two supply voltages - one high voltage of about 10 kV and a bias voltage from 50 up to 200 V. Because of some specific requirements as very low output noise, very weak load current (10 mu A), high precision measurement of the load current etc., a new linear mode voltage stabilizer is used as bias voltage power supply. The base distinction of the device is that a current mirror is included in its input, providing a precise measurement of the load current without any electrical contact with the output line. This feature is very important because about 500 power supplies of this type are included in the total HPD power supply system of the HCAL subdetector and can not be local earthed. (3 refs).