Today, the main challenge of Positron Emission Tomography (PET) systems dedicated to small animal imaging is to obtain high detection efficiency with a highly accurate spatial resolution which depends as well on design of the detector as on the electronics readout system. In this paper, we present a new design of a front-end readout chip with full custom sub-micrometer CMOS implementation in order to obtain a high accuracy on the charge-energy information and to give a time trigger to the Time-to-Digital Converter (TDC) with a resolution lower than nanosecond. This chip has been implemented with AMS 0.35 mum process. The readout architectures will be presented and the main simulation and measured results will be shown in this paper.