The high voltage protection boards for the RICH detectors of LHC

We present the circuit protection system for the monitoring of the High Voltage bias for the Hybrid Photon Detectors (HPDs), of the Ring Imaging CHerenkov (RICH) detectors of LHCb. The protection system buffers the voltage lines, attenuated by about 12700 V/V, in normal operating conditions and limits the output voltage excursion to the ADC monitoring system to a safe range (between -2.5 V and +5 V) in case of discharge. The circuit is designed and tested to be radiation tolerant. The protection system has been fully characterized with radiation and temperature under the whole expected working conditions of LHCb. Results have shown that the developed protection boards are fully adequate for the whole LHCb lifetime.