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COMMON MODE VOLTAGE? Can it be a DC voltage?

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Jul21-04, 09:56 PM   #1

COMMON MODE VOLTAGE? Can it be a DC voltage?

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.
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Jul22-04, 07:02 PM   #2
It is implied that a common mode signal is measured with respect to ground. I believe common mode usually refers to an AC voltage but is not necessarily a hard and fast rule. You could say that the input to your amplifier has a signal of 2 volts and a common mode OFFSET of 48 or 50 volts or whatever....
Jul25-04, 09:24 AM   #3
Common Mode refers to what is common about a differential ( voltage say) wrt. ground
Your example is not typical of where such a term would be used -- still you could say that the mid point of the resistor voltage is at 49 volts wrt ground , clearly this is a little artificial since there maybe no accesible mid point, and it does not really effect the circuit anyway.
A more typical situation would be a transformer feed in an AC circuit where the input terminals are NOT connected in any way to the secondary feeding the circuit. If the
circuit is grounded then you can ask what is the common mode voltage wrt ground of the transformer primary which in a simple case has no effect on the circuit.
However this voltage can have effects when for instance the primary to secondary isolation is not perfect ( interwinding capacitance is often responsible) and this can cause 'ground currents'.
The telephone system is an example where the wires to your phone come as a twisted pair and what you hear is their differential voltage , but a ground lightening strike can induce very large 'common mode' voltages which must be attenuated to keep the phone safe.
Jul25-04, 03:58 PM   #4
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COMMON MODE VOLTAGE? Can it be a DC voltage?

A differential signal of one volt might be present on two leads, one at 1V with respect to ground, and one at 2V with respect to ground. (2 - 1 = 1 of course). The common mode voltage here is volt.

The same differential signal could be present on two leads, one at 4V and one at 5V. While the differential signal is the same (5 - 4 = 1), the common mode voltage is now four volts.

- Warren
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