Patent ID: 6950514

Claim:
An apparatus comprising: an input port to which an input signal is applied; an output port from which an output signal is derived; a signal processing circuit coupled between said input and output ports, and being operative to process said input signal applied to said input port and to generate said output signal, in accordance with a prescribed relationship between said input signal and a DC reference applied to a reference port of said signal processing circuit, said DC reference being subject to the presence of unwanted noise components; and an AC filter having an input coupled to receive said DC reference and an output coupled to a signal transport path through which said input signal is coupled to said input port, said AC filter having a transfer function that is operative to pass said noise components that may be present on said DC reference to said input port, but to block the DC component of said DC reference from application to said input port; said signal processing circuit performing common mode rejection of said noise components that may be present on said DC reference, so that said output signal at said output port has substantially reduced noise components which, when coupled to downstream differential mode processing circuitry that is coupled to said DC reference, are effectively canceled, wherein said signal transport path contains an amplifier having a differential pair of inputs coupled to receive a differential signal, and wherein one of said differential pair of inputs is coupled to the output of said AC filter, said amplifier having an output thereof coupled to said input port, and wherein said amplifier comprises a sense amplifier of subscriber line interface circuit, coupled to receive tip/ring signals interfaced with a wireline pair, and wherein said signal processing circuit comprises an operational amplifier having a first input coupled to receive said DC reference from a external device to which said subscriber line interface circuit is coupled, and a second, opposite polarity, input coupled to receive the output of said sense amplifier, said operational amplifier having an output coupled as an input to said external device.