For ac circuit shown in figure, ${\rm{R}} = 100{\rm{k}}\Omega $ and ${\rm{C}} = 100{\rm{pF}}$ and the phase difference between ${{\rm{V}}_{{\rm{in }}}}$ and $\left( {{{\rm{V}}_{\rm{B}}} - {{\rm{V}}_{\rm{A}}}} \right)$ is ${90^\circ}$ . The input signal frequency is ${10^x}{\rm{rad}}/{\rm{sec}}$, where 'x' is ____.