  Time and charge measurements over a large dynamic range from 1 Photo Electron
(P.E.) to 4000 P.E. are required for the Water Cherenkov Detector Array (WCDA),
which is one of the key components in the Large High Altitude Air Shower
Observatory (LHAASO). To simplify the circuit structure of the readout
electronics, a front end ASIC was designed. Based on the charge-to-time
conversion method, the output pulse width of the ASIC corresponds to the input
signal charge information while time information of the input signal is picked
off through a discriminator, and thus the time and charge information can be
digitized simultaneously using this ASIC and a following Time-to-Digital
Converter (TDC). To address the challenge of mismatch among the channels
observed in the previous prototype version, this work presents approaches for
analyzing the problem and optimizing the circuits. A new version of the ASIC
was designed and fabricated in the GLOBALFOUNDRIES 0.35 um CMOS technology,
which integrates 6 channels (corresponding to the readout of the 3 PMTs) in
each chip. The test results indicate that the mismatch between the channels is
significantly reduced to less than 20% using the proposed approach. The time
measurement resolution better than 300 ps is achieved, and the charge
measurement resolution is better than 10% at 1 P.E., and 1% at 4000 P.E., which
meets the application requirements.
