  The water Cherenkov detector array (WCDA) is one of the key detectors in the
large high altitude air shower observatory (LHAASO), which is proposed for very
high gamma ray source survey. In WCDA, there are more than 3000 photomultiplier
tubes (PMTs) scattered under water in an area of 80000 m2. As for the WCDA
readout electronics, both high precision time and charge measurement is
required over a large dynamic range from 1 photon electron (P.E.) to 4000 P.E.
To reduce the electronics complexity and improve the system reliability, a
readout scheme based on application specific integrated circuits (ASICs) is
proposed. Two prototype ASICs were designed and tested. The first ASIC
integrates amplification and shaping circuits for charge measurement and
discrimination circuits used for time measurement. The shaped signal is further
fed to the second ADC ASIC, while the output signal from the discriminator is
digitized by the FPGA-based time-to-digital converter (TDC). Test results
indicate that time resolution is better than 250 ps RMS, and the charge
resolution is better than 10% at 1 P.E., and 1% at 4000 P.E. which meets the
requirements of the LHAASO WCDA.
