Circuit and method for reducing common mode interference in telephone instruments

A method of reducing cross-coupled interference between first and second telephone lines of a multi-line telephone instrument, comprising the steps of:

• receiving a first signal on said first telephone line into a first differential input amplifier having first tip and ring inputs coupled to conductors of said first telephone line and first tip and ring shunt resistors coupling said first tip and ring inputs to a signal reference of said telephone instrument;
• receiving a second signal on said second telephone line into a second differential input amplifier having second tip and ring inputs coupled to conductors of said second telephone line and second tip and ring shunt resistors coupling said second tip and ring inputs to said signal reference;
• and
• decreasing a common mode input resistance of said first and second differential input amplifiers with said first and second tip and ring shunt resistors, cross-coupled interference common to both of said first and second telephone lines thereby attenuated.

A multi-line telephone instrument, comprising:

• a base having a keypad and containing circuitry of said telephone instrument;
• a handset, coupled to said circuitry, for providing an interface for a user;
• a network interface for selectively coupling said telephone instrument to one of first and second telephone lines of a telecommunications network;
• and
• a circuit for reducing cross-coupled interference between said first and second telephone lines, comprising:
• a first differential input amplifier having first tip and ring inputs coupled to conductors of said first telephone line and first tip and ring shunt resistors coupling said first tip and ring inputs to a signal reference of said telephone instrument;
• and
• a second differential input amplifier having second tip and ring inputs coupled to conductors of said second telephone line and second tip and ring shunt resistors coupling said second tip and ring inputs to said signal reference, said first and second tip and ring shunt resistors decreasing a common mode input resistance of said first and second differential input amplifiers, thereby attenuating cross-coupled interference common to both of said first and second telephone lines and deleterious to an operation of said circuitry of said telephone instrument.