Constant impedance digital line interface circuit

Abstract

A digital line-interface circuit ( ) presents a constant impedance to a telephone line ( ) by emulating a short or a ground connection between the leads of the telephone line when the transmitter ( ) is disabled. The receiver ( ) includes a pair of differential op amps (U U ) connecting the receive line ( ) to the outputs of the transmitter's drivers (U A, U A) across a pair of switches (K K ). The switches may be replaced by a receive enable (RE) signal line connected to enable (EN) inputs of the op amps. Negative inputs of the op amps are connected to outputs of different transmitter drivers, positive inputs of the op amps are connected to a bias voltage source (Vbias), and the output of each op amp is connected to the receive line and is also fed back by a bias resistor (R , R ) to its negative input. When the transmitter drivers are disabled, the switches or RE signal line cause the op amps to be connected to the telephone line, where they simulate a short or a ground connection and thus present a low impedance to telephone line just like enabled transmitter drivers, yet still allow the receive line to receive signals from the telephone line. The impedance of the line circuit is matched to the impedance of the telephone line by using only series resistors (R and R ) and not using shunt resistors across the leads of the telephone line, thereby eliminating the waste of power caused by shunt resistors.