We present the low noise design of an 8-channel CMOS fast readout circuit for CZT (CdZnTe) x-ray detectors. The one channel of the readout circuit is composed the continuous discharged preamplifier, the gain amplifier and the comparator. The Hspice noise simulations are performed in the time domain. The calculated RMS noise voltages of the gain stage output for the time domain is 15.5 mV. A test chip has been fabricated by using AMI (American Microsystems, Inc.) 1.5 μm low-noise analog CMOS process through the MOSIS service. The measured RMS noise voltage of the gain stage output is 23.5 mV. The charge-to-voltage gain of this readout ASIC is 130 mV/fC and the signal to noise ratio at the gain stage output is 34.
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