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Interdigitatio means using a single multi-fingered transistor to implement two or more transitor that share a source or a drain.
Common centroid layout is generally used with Diff pairs. It is a matching method in which the two transistors of the Diff pair are symetrically laid out about a certain axis. This guarantees that both transistors see the same process variations so they'd be matched under all conditions.
Common centroid Diff pairs may be interdigitated too.
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
An interdigitised 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 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.
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