The TPSF12C1 has a built-in input line filter. The high-pass
filter stage attenuates the large line-frequency (50 Hz or 60 Hz) components
of the power-line voltages, both line-to-line and line-to-earth, thus
maximizing the useful voltage range of the low-voltage output at
INJ.
The circuit also sums the signals in a CM
combiner, rejecting the DM components of the voltages and extracting a
signal that represents the CM noise signature without line-frequency
components. Combined with the action of the high-pass filter, the net result
is that the COMP1 pin voltage represents the sensed high-frequency CM noise
that the device attempts to cancel. Because the entire filter is integrated
in the device, matching is better than what can be achieved using discrete
components.