Specific Infrared (IR) detection systems are a solution for non-invasive monitoring of polymer aging in electrical power plants. For such application, the target photo-detector has a low shunt impedance of 40Ω and produces a large photo-current of 60uA. This paper proposes a readout circuit designed in XFAB 0.18um technology. Based on a CMDI architecture, a bi-cascode current mirroring direct injection circuit is proposed. It offers a 98% charge injection efficiency. The structure performs the DC current cancellation and integrate the resulting current in a 20pF capacitance. The 20us duration pulses are integrated with a linearity error of 0.01%