Say you have a piece of electronic equipment which has an input impedance of 1Megohms, and an input capacitance of 10 picofarads as determined by a ceramic capacitor across the input resistor. This obviously creates an RC impedance filter which will act on any AC entering the device. Now enter into the equation a line capacitance (cable and/or source capacitance) of whatever is entering said device. Is the line capacitance ignored until it goes higher than 10pF? Or does it at all times sum with the input capacitance of the device?