Patent ID: 6681011
Filing Date: 2004-01-20
Classification: H04M

Abstract:
A circuit for determining connection status of a phone line including respective tip and ring lines, the circuit comprising:a first operational amplifier coupled to receive a signal indicative of a voltage difference between the respective tip and ring lines; a lag network coupled to impart a predetermined delay to the output signal from the first operational amplifier, the lag network comprising an RC network; an oscillator for supplying a dither signal having a predetermined frequency; a second operational amplifier coupled to receive the voltage difference signal, the output signal from the second operational amplifier having a sufficiently fast time response relative to the output signal from the lag network; and a differential amplifier having an inverting terminal and a non-inverting terminal, the inverting terminal coupled to receive the dither signal and the output signal from the lag network, and the non-inverting terminal coupled to receive the output signal from the second operational amplifier, the differential amplifier further having an output terminal for supplying a signal indicative of the connection status of the phone line.