Patent ID: 7995743
Filing Date: 2011-08-09
Classification: H04M

Abstract:
1. A circuit for detecting whether a telecommunication line is off-hook, said telecommunication line comprising tip and ring signal lines, said circuit comprising: a voltage divider for coupling between said tip and ring lines and having first and second nodes across which appears a scaled version of a voltage across said tip and ring lines; a differential analog-to-digital converter having first and second analog input terminals and a digital output terminal; a signal line for selectively enabling said circuit when said signal is in a first state and disabling said circuit when said signal is in a second state; a first transistor having a control terminal coupled to said signal line and first and second current flow terminals coupled between said first node and said first input terminal of said analog-to-digital converter; a second transistor having a control terminal coupled to said signal line and first and second current flow terminals coupled between said second node and said second input terminal of said analog-to-digital converter, wherein: when said signal line is in said first state, said analog-to-digital converter receives a scaled version of the voltage across said tip and ring lines, and, when said signal line is in said second state, said analog-to-digital converter receives no signal from said voltage divider; said voltage divider comprises a first resistor having a first terminal coupled to a first one of said tip and ring lines and a second terminal coupled to said first node and a second resistor having a first terminal coupled to said first node and a second terminal coupled to said second node; and the circuit further comprises a diode coupled between said second node and an other one of said tip and ring lines, such that said diode biases one of said current flow terminals of said second transistor to a voltage that is greater than a voltage of said other one of said tip and ring lines.