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Common Centroid & Interdigitization

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vbhupendra

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common centroid

what is the difference b/w Interdigitization & Common Centroid layout ?
when to use Interdigitized and when Common Centroid.
 

common centroid layout

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.
 
common centroid layout technique

Check this book: "The Art of Analog Layout" by Alan Hastings
 

common centroid matching

"A disadvantage of common-centroid layout is apparent.That is the need to cross-connect the devices increases the separation between matched devices and may worsen matching in cases where linear process gradients are not the main limitation."
 

common centroid differential pair

is this disadvantage really? I think most of input device of differentia ampifer use this kind of method,common centroid, right?
 

common-centroid layout

In the case of matching vs area, the winner is definitely matching. so, when going for common centroid method , area is of least inportance. if the transistors are not matched correctly then that would really screw up the whole circuit. so common centroid is really a boon . its not possible to always go for common centroid but when ever possible the best thing is to go for it.
 

common centroid

Hi,

I think the question should be "common centroid vs symmetry". Both use Interdigitization. By defination, symmetry is always common centroid, but common centroid can be non-symmetry.

Symmetry layout is good, but you should know what you are "balancing", and not just bindly applied it. In the book I wrote "CMOS Transistor Layout Kungfu" I describe some principles on this topics. However, this chapter is not include in the free release in my website www.eda-utilities.com ...

Regards,
Eng Han
www.eda-utilities.com
 

interdigitization

U may assume that interdigitization is a special case of a common centroid but with only one row

In transistor matching if the total length of the mirror exceeds 80um or 100um, try to use common centroid for better matching
 

interdigitate or common centroid

common centroid layout provides better matching as in this method gradients are canceled in both X and Y direction.
Interdigitized method provides gradient cancellation in one direction only.

Ok
 
common centroid current mirror

I like this topic about interdigi' vs common centroid. One of my concerns, which none mentioned in the previous posts, is that: when the input differential signal frequency is pretty high (like in RF or high speed IO/CML), the interdigi' structure will cause cross-coupling between gate figners. Obviously, common centriod is sortof free of this problem. The structure can be redrawn as below:

interdigi': ABAB

commoncentriod: A B
commoncentriod: B A

I am not much certain of my concern so I'm all ears to any comments on that.
 

interdigitated and common centroid layout

I think common centroid can be used with following conditions:
1. area is not a concern
2. transistor is not big
 
interdigitising

Hi, upstairs

Don't you think the two conditions you listed are kinda in conflict w/ each other?
BTW, how do you think about that in terms of high frequency?
 

common-centroid current mirror

the former is prefered
 

current mirror common centroid layout

hi ,
when we go for interditization and common centroid. how it will work
 

linear gradient in centroid

I usually do common centroid on match transistors used in differential amplifier and current mirrors.

While interdigitation on resistors.
 

interdigitising in layout

Hi,
inter digitization is inter mixing of required matching units, if we use this we can not match in all directions.In common centroid, the center for two require pairs should be same. so it match the two pairs in all directions.

for e.g if we want match two txrs with 2 fingers.
by using inter digitization: ABAB here the center for both pairs is different the temp& linear oxide gradients will differ.
by using comon centroid: ABAB or
AB
BA

in this method center for both pair is same, so all temp and linear oxide gradients will same for both.


coming usage(Generally):
1. for biasing, current mirrors and resistors we will use inter digitization
2. For input diff pair and cap-array we will use common centroid
 

resistor common centroid layout

If drain or source is shared in common centroid, the layout could be very compact as well. For current mirror with common centroid, gate connection can be very easy.
 

common-centroid

Guys,

For high speed (GHz) differential amplifier, should I use common centroid or interdigitized? Both seems to add additional parasitics to the high speed line. What's your experience in this?

Regards.
 

layout common centroid current mirror

In my opinion the common centroid is the best choice in terms of matching, but of course in High Frequency is all more difficult.
You have to choice the best compromise between matching-simmetry-parassitics.


Good luck
 

common centroids

I use common centroid for critical layout, use interdigitization for 2nd critical one.
Give layout guys some vacation when I've got loose spec.
 

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