A time-over-threshold machine: The readout integrated circuit for the BABAR silicon vertex tracker

A low-noise, mixed-signal, 128-channel CMOS integrated circuit containing the complete readout electronics for the BABAR Silicon Vertex Tracker has been developed. The outstanding feature of the present implementation is the ability to perform simultaneously low-level signal acquisition, derandomizing data storage, sparsification and data transmission on a single monolithic chip. The signals from the detector strips are amplified, shaped by a CR-RC2 filter with digitally selectable peaking time of 100 ns, 200 ns, 300 ns, or 400 ns, and then presented to a time-over-threshold processor to implement a compression type analog-to-digital conversion. The digital information is stored, sparsified and read out through a serial link upon receipt of a command. The digital section operates from a 60 MHz incoming clock. Noise measurements at 200 ns peaking time and 3.5 mW total power dissipation per channel yield an equivalent noise charge of 600 el, rms at 12 pF added source capacitance, The chip measures 5.7 mm x 8.3 mm and contains 330k transistors. The first full-scale prototype was fabricated in a radiation soft 0.8 mu m, 3-metal CMOS process. The same circuit is now being fabricated in an analogous radiation hard technology.