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# The difference between Interdigitization and Common Centroid

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#### vbhupendra

##### Full Member level 4
Layout Matching

Interdigitization & Common Centroid matching are different or same.
what is the difference b/w two techniques ?
when to use Interdigitization and/or Common Centroid ?

Re: Layout Matching

Interdigitation just means that the devices (transistors, resistors, capacitors, diodes) are distributed in an interleaved maner. For instance imagine that you must match two devices (A and B) composed by 4 unit devices each:
ABABABAB
---------> If there is a gradient in this direction component B is more affected than component A

A common centroid structure would be
ABAB|BABA

Notice that in this case there is an axis of symmetry (I added character | for the axis) so any "linear" gradient is compensated. If now A and B are divided into 16 unit devices, a more complex common centroid structure would be
ABAB|BABA
BABA|ABAB
--------------
BABA|ABAB
ABAB|BABA

Notice in this case that the structure has vertical and horizontal symmetry axes, or more general central symmetry.

Have a look at the Hskings book "The Art of Analog Layout"

siddeshb

### siddeshb

Points: 2
Layout Matching

hi we can match the transistors in two methods.

1.interdigitised
2.common centroid.

in interdigitised pattern all the transistors are in interleaved pattern like suppose there are two transistors with 2 fingers each.then
AB|BA
here the first half is exactly mirrored.here the variations for A and B are same.
in common centroid pattern we care for X and Y variations unlike in interdigitised we care only X variations.then
A|B
B|A
common centroid is more precision than interdigitised .ALL differential pairs are matched using this.
current mirrors are matched using interdigitised pattern since they need minimal matching of vgs.

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