The important parameters defining ac behavior are the
transcapacitance, Ct, and the external feedback resistor (not
shown). The time constant formed by these components is
analogous to the dominant pole of a conventional op amp and
thus cannot be reduced below a critical value if the closed-loop
system is to be stable. In practice, Ct is held to as low a value as
possible (typically 4.5 pF) so that the feedback resistor can be
maximized while maintaining a fast response. The finite RIN
also affects the closed-loop response in some applications.