An apparatus for sensing a button-pressed state in a mobile device comprising a switch unit formed by combining rotary switches in a switch unit and a controller connected to the rotary switches within the switch unit, for sensing a switching operation according to a connected state between contact points of the rotary switches. The high-speed bit stream interface module includes a line side interface, a board side interface, and a signal conditioning circuit. To prevent instability in the main loop caused by the duty cycle modulator and FIR filter, the delta sigma modulator also includes a stabilizer loop.