A CMOS readout circuit for microstrip detectors

In this work, we present the design and the results of a CMOS analog channel for silicon microstrips detectors. The readout circuit was initially conceived for the outer layers of the SuperB silicon vertex tracker (SVT), but can serve more generally other microstrip-based detection systems. The strip detectors considered show a very high stray capacitance and high series resistance. Therefore, the noise optimization was the first priority design concern. A necessary compromise on the best peaking time to achieve an acceptable noise level together with efficiency and timing accuracy has been investigated. The ASIC is composed by a preamplifier, shaping amplifier and a Time over Threshold (T.o.T) block for the digitalization of the signals. The chosen shaping function is the third-order semi-Gaussian function implemented with complex poles. An inverter stage is employed in the analog channel in order to operate with signals delivered from both p and n strips. The circuit includes the possibility to select the peaking time of the shaper output from four values: 250 ns, 375 ns, 500 ns and 750 ns. In this way, the noise performances and the signal occupancy can be optimized according to the real background during the experiment. The ASIC prototype has been fabricated in the 130 nm IBM technology which is considered intrinsically radiation hard. The results of the experimental characterization of a produced prototype are satisfactorily matched with simulation.