LCL filter is widely used in grid connected converters for good harmonic mitigation performance. However, resonance poles of LCL filter challenge the system stability, especially in digital controlled system. Various active damping approaches have been proposed to solve the resonance issue, where the capacitor current based active damping control is an attractive option. In this paper, system stability with capacitor current based active damping control is analyzed with the consideration of digital control delay. A design principle for the active damping compensator is proposed in this paper, where maximum stability margin can be reached for active damping loop. It is shown that capacitor current feedback control with negative proportion gain, instead of positive gain, presents much better performance with considerable control delay. Simulative and experimental results validate the proposed control method.