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commom centroid geomentry

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Newbie level 6
Dec 13, 2005
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centroid matching

what is actually the common centroid geomentry and where is it used

common centroid layout technique

common centroid geometry is often used in instances like current mirror or any form of mirroring.

split the fingers of the transistors evenly in order for both sides of the mirror to achieve matching

see attached article for more info


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common centroid technique is used for matching purposes. Where, two transistors that need matching .matching means,, the process gradients should be equally felt by the two transistors that need to be matched. For this to be achieved ,, especially in the case of Differential pair matching, common centroid layout techniques are used. Common centroid is analogous to centre of mass. ie,,
if M1 and M2 are two transistors that needed to be matched, then they are digitated to fingers. And the fingers of M1 and fingers of M2 are placed in such a fashion that , the centre of mass of M1 and centre of mass of M2 are both same. This is analogous to common centroid. While providing the interconnects with metals and poly, care has to be taken to ensure that even the mass of the metals also have the same centre of masss. This kind of architecture gives a good matching.

To know what are the various styles of putting it,, refer Alan Hastings Art of Analog Lyout

this technique is used for matching transitors such that the process variations cancels in either direction. For more information read Alan Hastings book.

as the name implies this technique constructs two devices symetrically about a common centre in the layout

the effect of process gradients is cancelled ib both x & y directions

while manufacturing is done due o the accuracy of the tool there exists a problem of mismatch btween the transistors.

so in the case of differential pair the should be matched

it is used for matching purpose for any alements in an IC.
because mismatch is due to different things that the most
important of them is ingrediants.

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