Evaluation of a front-end ASIC for the readout of PMTs over a large dynamic range

The Large High Altitude Air Shower Observatory (LHAASO) project has been proposed for the survey and study of cosmic rays. In the LHAASO project, the Water Cherenkov Detector Array (WCDA) is one of the major detectors for searching for gamma ray sources. A Charge-to-Time Convertor (QTC) ASIC (Application Specification Integrated Circuit), fabricated with Global Foundry 0.35 μm CMOS technology, has been developed for readout of photomultiplier tubes (PMTs) in the WCDA. This ASIC provides both time and charge measurement of PMT signals. The input charge is converted to a pulse width based on the Time-Over-Threshold (TOT) technique and linear discharge method; as for time measurement, leading edge discrimination is employed. This paper focuses on the evaluation of this front-end readout ASIC performance. Test results indicate that the time resolution is better than 400 ps and the charge resolution is better than 1% with large input signals and remains better than 15% @1 photoelectron (P.E.), both beyond the application requirement. Moreover, this ASIC has a weak ambient temperature dependence, low input rate dependence and high channel-to-channel isolation.