In optical performance monitoring system,the analog to digital converter is needed to detect the peak of nanosecond pulse and get the signal envelope.A scheme based on a designed anti-aliasing filter and analog to digital converter is proposed to broaden the nanosecond pulse and make it easier for the analog to digital converter to catch the peak of the nanosecond pulse.The experimental results demonstrate that,with the proposed scheme,the optical performance system needs less time to get the recovered eye-diagram of high speed optical data signal,and is robust to phase mismatch in the analog to digital converter circuit. In optical performance monitoring system, the analog to digital converter is needed to detect the peak of nanosecond pulse and get the signal envelope. A scheme based on a designed anti-aliasing filter and analog to digital converter is proposed to broaden the nanosecond pulse and make it easier for the analog to digital converter to catch the peak of the nanosecond pulse. The experimental results demonstrate that, with the proposed scheme, the optical performance system needs less time to get the recovered eye-diagram of high speed optical data signal, and is robust to phase mismatch in the analog to digital converter circuit.