OH, The IC on XCM box should be LMH 1251, good.
But where is the buffer op-amp?
like this
If there is no buffer, the output may be overloaded.
e.g. 0.7V output with 75 ohm loading, by ohm's law, the current should be 9.3mA
BUT the designed output current is only 3mA!
And I don't understand why...