# what is coupling capacitance

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#### rameshiloveu

##### Junior Member level 1
what is coupling capacitance in cmos? how is it different from parasitic capacitance?

could anyone please explain in detail?

#### kripacharya

##### Banned
Coupling capacitance allows you to pass a signal from point A to point B in a desired manner.
Parasitic capacitance is an annoyance and any unwanted capacitance would be called parasitic.

You know? Like a parasite ?

rameshiloveu

### rameshiloveu

points: 2

#### kenambo

##### Full Member level 6
Hi,

Generally coupling or decoupling capacitances are used for specific reasons for the proper operation...

Where as the parasitic capacitances are unwanted capacitances that add to our desired capacitance and make the results worse..

Thanks.....

#### timof

Suppose you have a conductor A, surrounded by other conductors - B1, B2, ... BN.

Apply a voltage V to conductor A, while all other conductors are grounded.

Measure/calculate charge change on each of conductors - Q_A, Q_B1, Q_B2, ... Q_BN.

Total capacitance of conductor A is C_A = Q_A / V.

Capacitance between conductors A and B1 is C_A_B1 = Q_B1 / V, between A and B2 is C_A_B2 = Q_B2 / V, and so on.

Capacitance between A and B1 is also called coupling capacitance between A and B1.

The total capacitance of A is equal to the sum of coupling capacitances: C_A = C_A_B1 + C_A_B2 + ... + C_A_BN.

Capacitance between A and B1 is equal to capacitance between B1 and A - so, capacitance is symmetric, in electrostatics.

You see, coupling or total capacitance, defined as above, has nothing to do with capacitance being parasitic or non-parasitic (i.e. intended).

I think the term "coupling" is meant to describe a partial capacitance between a conductor A and other conductor - as opposed to the total capacitance of A.

Now, capacitance can be intended, created on purpose - such as MOM capacitor, MIM capacitor, MOS capacitor, decoupling capacitor, energy storing capacitor, etc.
Or it can be unintended, unwanted, but present because of physical size and proximity of conductors - in this case, it is called parasitic.
The term "parasitic" can be applied to both total capacitance, and partial (coupling) capacitance.

I hope this helps.

Max
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