Can a so called "common mode voltage" be a dc voltage?? Say there is a simple series circuit... a DC source.. a resistor.. and finally a motor. The point where the motor meets the DC source is the ground. Neither side of the resistor is at ground potential. So if we want to measure the voltage across the resistor.. we can connect it to differential inputs of some type of amplifier. Is there a common mode DC voltage existent at the inputs? Say one side of the resistor is at 50V (DC source's voltage) and the other side is at 48V. The differential voltage is 2V... is the common mode voltage 48V? Since this 48V is undesired in our measurement, and is a component of both voltages.. is it considered a common mode voltage? I am asking you guys this so I can better understand application notes that sometimes use the term COMMON MODE as if they are strictly ac noise signals.. and other that talk of them as DC voltages.. some people would say that the common mode DC voltage in my example is not 48V... but it's 49V (because one side of the resistor is actually at +1 , and the other side is at -1... when subtracting the common mode voltage). Help me out here.