Common mode differential mode signals

[FOIWATER]

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

 

[meBigGuy]

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.

 

[FOIWATER]

Excellent, thanks