VFAT3: A Trigger and Tracking Front-end ASIC for the Binary Readout of Gaseous and Silicon Sensors

VFAT3 is the front-end ASIC designed specifically for the readout of GEM detectors within the CMS experiment during the high luminosity phase of the LHC at CERN. This paper presents the analog and digital design plus the measured functional and characterization results. Key design goals were optimization to GEM charge characteristics maximizing signal to noise, timing resolution and operation at high particle rate. There are 128 front-end channels comprising preamplifier, shaper and constant fraction discriminator (CFD). Features include programmable polarity (for use with gaseous MPGD or silicon detectors), programmable gain (for linearity 9.5 fC, 28 fC and 55 fC) and programmable shaping times (15 ns, 25 ns, 36 ns and 45 ns), plus the CFD reducing time walk to less than 0.4 ns within 3 fC to 30 fC of input charge, optimizing timing resolution. An internal calibration circuit allows calibration of each channel with "GEM like" or "Silicon like" input pulses, each having programmable amplitude, polarity and phase. The ENC measures 620 e + 33 e/pF in high gain. The hit rate capability is demonstrated to 2 MHz per channel. VFAT3 has 2 output paths; the first (Trigger Path) provides hit information for every LHC clock cycle. The second (Data Path) provides data packets upon receipt of a trigger, each data packet contains hit information and time stamps. The chip can operate with trigger latencies up to 25.6 µs and is capable of receiving consecutive triggers. VFAT3 can operate with up to 2 MHz trigger rate in default mode and has data packet zero suppression capabilities to go beyond this rate. An internal dedicated Comm-Port allows communication to and from the chip. Additional features include channel input protection, internal/external temperature measurement and design for a radiation environment. The design and measurements presented demonstrate the VFAT3 capability as a complete binary front-end readout ASIC optimized for GEM detectors in the high luminosity LHC.