Patent ID: 6625276
Filing Date: 2003-09-23
Classification: H04M

Abstract:
A circuit for distinguishing between a first voltage signal having a high amplitude and a second voltage signal having a low amplitude, said first and second voltage signals comprising respective AC components superimposed on a DC level, the circuit comprising:a capacitor for blocking the DC level of the first and second voltage signals, an attenuator for attenuating the respective AC components of the first and second signals by an attenuation factor which is voltage dependent and substantially frequency independent, so that the respective AC components of the first and second voltage signals after attenuation have similar amplitudes, and a processor for processing the attenuated respective AC components of the first and second voltage signals so as to distinguish between the first and second voltage signals according to their respective frequencies; wherein the attenuator comprises: an operational amplifier having an output terminal, an inverting input terminal and a non-inverting input terminal, a linear gain setting resistor for coupling a first voltage rail of a voltage source feeding the respective AC components of the first and second voltage signals to the inverting input terminal, and a non-linear gain setting resistor coupled between the inverting input terminal and the output terminal having a low value to the AC component of the high amplitude first voltage signal and a high value to the AC component of the low amplitude second voltage signal.