An 80Mbit/s radiation tolerant Optical Receiver for the CMS digital optical link

The CMS tracker slow control system will use approximately 1000 digital optical links for the transmission of timing, trigger and control signals. In this system, the 80Mbit/s optical receiver at the detector end of each optical link has to be radiation hard since it will operate in the severe radiation environment of the CMS tracker (10 Mrad in 10 years). We have developed a prototype circuit in a 0.25 μm commercial CMOS process using radiation tolerant layout practices to achieve the required radiation tolerance. This effective technique consists in the systematic use of enclosed (edgeless) NMOS transistors and guardrings, and relies in the natural total dose hardness of the thin gate oxide of deep submicron processes. The circuit features an Automatic Gain Control (AGC) loop allowing detection of wide dynamic range input signals (-20 to – 3 dBm) with minimum noise, compatible with the maximum expected radiation-induced drop in quantum efficiency of the PIN photodiode. A second feedback loop compensates a photodiode leakage current up to 100 μA, and the circuit outputs an LVDS signal. Four receiver channels were integrated in a 2x2mm chip, out of which two were simultaneously bonded to two PIN photodiodes, and their Bit Error Rate (BER) performance was measured before and after an irradiation with 10 keV X-rays up to 20 Mrad(SiO2).