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Analog layouts are more concerned about matching rather than area constraints. U have to keep in mind the current flowing in each path, critical devices, noisy devices,shielding required. interdigitation and common centroid techniques deal with the matching concept. I would suggest you to read "Art of analog layout" by Alan Hastings, it should give you some idea about this vast field. Also, practice makes man perfect.So work on layouts to get a hang of it.
The layout techniques to be followed are
firstly ,find out which are the blocks that require matching(either common centroid or interdigitized) , also try fingering if possible
secondly ,if the analog block has to be protected from the surrounding blocks provide guardrings .
shield the signals which are very weak and critical
placement of the analog blocks is important
The main problems of layout for analog/mixed signal IC's are device matching and unwanted parasitics reduction.Generally speaking the principle of mixed signal layout is noise reduction(offset can be treated as a sort of noise)As previously told the techniques for matching are common centroid and interdigitized which can be used separately or in combination. Also place dummys while matching.If there are no parasitics ,then there is no noise coupling.The parasitics reduction techniques include shielding,guarding ,long distance b/w noisy digital signal and quiet analog one.
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