A subscriber line interface circuit apparatus includes tip/ring sense
circuitry generating a tip sense signal and a ring sense signal from
three sensed currents, wherein the tip sense signal and the ring sense
signal correspond to subscriber loop tip and ring currents, respectively.
In one embodiment, the tip/ring sense circuitry includes a current mirror
generating first and second mirrored sense currents from a first sense
current proportional to a battery feed node voltage of a subscriber loop.
Current differencing circuitry provides the tip sense signal from a
difference between the first mirrored sense current and a second sense
current associated with a tip line of the subscriber loop. The current
differencing circuitry provides the ring sense signal from a difference
between the second mirrored sense current and a third sense current
associated with a ring line of the subscriber loop.