A 12-Channel (Ch) CMOS preamplifier-shaper-discriminator ASIC designed for avalanche photodiode (APD) and gas counter readout has been fabricated on a 2.4 mm × 2.4 mm die area using ROHM 0.35 μ CMOS technology. This mixed signal ASIC consists of both analog and digital devices and a window type discriminator is easily implemented through the use of a digital encoder to encode outputs from two comparators. The charge sensitive preamplifier which is based on gain-boosted (regulated) cascode topology has been tested to have a low optimum Equivalent Noise Charge (ENC) of about 900 e- + 75 e - /pF FWHM at a shaping time of 0.5 μs and the gain (voltage output to charge input) is 0.9/pF. The gain of the shaper is about 2.5/pF and the shaping time can be varied slightly from an external pin. This chip is capable of sensing bipolar signals and is linear at least up to 320 fC for negative charge and 150 fC for positive charge. The average ENC of each channel has been calculated to be about 1600 e- + 80 e-/pF FWHM. Finally, the power consumption of the chip was approximately 0.13 W.