Off-hook detector with reduced sensitivity to common mode

Claim 1

An off-hook detector for use in a subscriber line interface circuit to provide a loop signal to indicate the open-loop or closed-loop status of a subscriber line, comprising; a transmission bridge comprising a first terminal and a second terminal, a first branch connected between said first terminal and one side of a central battery, a second branch connected between said second terminal and the other side of said central battery, said branches normally being symmetrical; means for introducing an asymmetry in said second branch in response to a control signal; a difference detector having inputs connected to said two terminals and an output that supplies a first logic signal the state of which indicates the open-loop or closed-loop status of the subscriber line; a comparator having a first input connected via a low pass filter to said second terminal, a second input connected to a voltage threshold, and an output that produces a second logic signal the state of which indicates the open-loop or closed-loop status of the subscriber line; a logic circuit connected to said difference detector and said comparator and responsive to said control signal to provide said first logic signal as the loop signal when said branches are symmetrical and to provide said second logic signal as the loop signal when an asymmetry is introduced in response to said control signal.