SCTA - A rad-hard BiCMOS analogue readout ASIC for the ATLAS semiconductor tracker

Two prototype chips for the analogue readout of silicon strip detectors in the ATLAS Semiconductor Tracker (SCT) have been designed and manufactured, in 32 channel and 128 channel versions, using the radiation hard BiCMOS DMILL process. The SCTA chip comprises three basic blocks: front-end amplifier, analogue pipeline and output multiplexer. The front-end circuit is a fast transresistance amplifier followed by an integrator, providing fast shaping with a peaking time of 25 ns, and an output buffer. The front end output values are sampled at 40 MHz rate and stored in a 112-cell deep analogue pipeline. The delay between the write pointer and trigger pointer is tunable between 2 mu s and 2.5 mu s. The chip has been tested successfully and subsequently irradiated up to 10 Mrad. Full functionality of all blocks of the chip has been achieved at a clock frequency of 40 MHz both before and after irradiation. Noise figures of ENC = 720 e(-) + 33 e(-)/pF before irradiation and 840 e(-) + 33 e(-)/pF after irradiation have been obtained.