Common mode differential mode signals

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Common mode signals can be either AC or DC and appear equally on both inputs of a differential amplifier, while differential mode signals vary between the two inputs. An example of common mode is 60Hz noise affecting both wires in a twisted pair cable. Differential mode signals occur when one input is more positive than the other, which can also be around a common mode voltage. Both types of signals can coexist as AC or DC, and operational amplifiers have specifications for both common mode and differential mode ranges. Understanding these concepts is crucial for effectively using differential amplifiers in circuits.
FOIWATER
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I am having trouble getting this.

I have been introduced to common mode and differential mode signals.

I assumed the inputs to a differential amplifier is an ac signal...

my instructor gave instruction on how to input such a signal to a circuit using SPICE, he used DC.

is the common mode dc while the diff mode is ac? can the diff mode be dc as well
 
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Common mode is a signal component that appears on both inputs. It can be AC or DC. An AC example would be 60Hz noise from a transformer coupled into a twisted pair cable. The twisted pair could be carrying any sort of signal, but they are both changing in unison from the 60 Hz common mode signal.

Differential mode signals are when the two signals are changing (or not) differently. One is more positive that the other. It could be around an AC or DC common mode voltage, or even ground.

Common mode and differential mode signals can be AC or DC or any combination.

opamps and comparators have both differential and common mode range specifications. The common mode range would be the range over which the amplifier can reject the common mode signal.
 
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Excellent, thanks
 

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