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# Common mode voltage significance.

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#### Rahul Soni

##### Member level 1
What is the significance of common mode voltage(CMV)?
In the datasheets, common mode voltage is given with the differential voltage.
What is the use of common mode voltage. What will be the effects if we don't consider CMV. ?

#### crutschow

You don't "use" common-mode voltage, it is just the DC voltage that's common to both halves of a differential signal.
Thus you could have a 1mVac differential signal that's riding on a 3V common-mode DC level.
Differential amplifiers are rated as to the common-mode voltage they can tolerate on their inputs while still operating properly, so you must be sure the input signal stays within those limits.
If you do, then the differential signal will be amplified but the common-mode signal will be ignored (rejected).
Since no amplifier is perfect the amplifier spec will list the rejection value for the common-mode voltage (how much of the common-mode voltage appears at the output). Typically this value is very small (>80dB at DC)
So called rail-to-rail input op amps can operate with a common-mode voltage anywhere between the positive supply rail and the negative supply rail.

Rahul Soni

### Rahul Soni

Points: 2

#### KlausST

##### Super Moderator
Staff member
Hi,

the term "common mode voltage" is used for many applications.

Klaus

#### Rahul Soni

##### Member level 1
Hi,

the term "common mode voltage" is used for many applications.

Klaus

While using ICS85104I fan out buffer, I found this table

I am unable to understand why this Vcm is provided and where will it be used?

#### KlausST

##### Super Moderator
Staff member
Hi,

VPP says the difference between the two inputs must not be more than 1.3V.

But referenced to GND both inputs may be near GND and also near VCC. (Common mode).

With differential inputs the absolute voltage level referenced to GND is not that important.
If CLK0 is at 0.8V and nCLK0 is at 0.5V, then the output is HIGH.
If CLK0 is at 1.5V but nCLK0 is at 1.8V, then the output is LOW, although now the CLK0 level is much higher than before.

See both inputs as comparator inputs. CLK0 is the noninverting input, nCLK0 is the inverting input.

Klaus

Rahul Soni

Points: 2