High impedance absolute telephone line voltage measurement circuit

The present invention generally relates to a telephone line interface circuit. More particularly, the present invention relates to measuring a characteristic of a telephone line. A telephone line measurement circuit is disclosed for detecting a variation of a characteristic between a first and a second leads of a telephone line. The circuit comprises a first circuit having a first and a second inputs and an output. The circuit also has a second circuit having a first and a second inputs and an output. The first input of the first circuit and the second input of the second circuit are coupled to the first lead of the telephone line and the second input of the first circuit and the first input of the second circuit are coupled to the second lead of the telephone line. The outputs of the first and the second circuits are then coupled together to provide an output for the measurement circuit.