We use a dispersion representation based on unitarity and analyticity to study the low energy $\gamma^* N\rightarrow \pi N$ process in $S_{11}$ channel. Final state interactions among $\pi system are critical this analysis. The left-hand part of partial wave amplitude is imported from $\mathcal{O}(p^2)$ chiral perturbation theory result. On right-hand part, final interaction calculated through ...