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    Why we should choose even number of finger

    Recently I have question about why we should choose even number of finger.
    As far as I know, the even finger is easier to layout in Current Mirror, Differential Pair when we need matching. (Because we should abut, place common centroid etc,...)

    But I want to know how about in schematic view point. What is advantages of even number of finger?

    I have searched and found out that there is more parasitic capacitance when drain is connected with dummy. But could you tell me more detail?

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    Re: Why we should choose even number of finger

    What in heavens name are you talking about ? What is an 'even finger'... it sounds quite rude.



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    Re: Why we should choose even number of finger

    Even numbers of fingers are better because it makes the device layout symmetric. You have two sources and one shared diffusion. In the case of a source coupled differential pair, since the sources are common, you can share source diffusions of device A with device B. Something that isn’t easily doable with odd number of fingers (can make use of dummy devices).



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    Re: Why we should choose even number of finger

    Quote Originally Posted by kripacharya View Post
    What in heavens name are you talking about ? What is an 'even finger'... it sounds quite rude.
    My mistake, even number of finger. Does it make more convinent for you to discuss?

    Quote Originally Posted by ljp2706 View Post
    Even numbers of fingers are better because it makes the device layout symmetric. You have two sources and one shared diffusion. In the case of a source coupled differential pair, since the sources are common, you can share source diffusions of device A with device B. Something that isn’t easily doable with odd number of fingers (can make use of dummy devices).
    Thank you for your replying. As I said, I know what is benefit from layout view point. Could you give some opinion form schematic view point?



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    Re: Why we should choose even number of finger

    I think it’s primarily done for layout matching. As long as your schematic is readable, doesn’t violate pdk requirements, meets specifications, and matches your layout, it doesn’t matter too much what you do with your devices from a schematic point of view. Ultimately the layout is what gets fabricated. The only other thing I’ll say on odd fingers, is that there may be some instances where you won’t be able to create clean ratios. For example, if your current mirror has 3 fingers and you want to mirror half the current to another branch, you can’t create a 1.5 finger device.



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    Re: Why we should choose even number of finger

    Quote Originally Posted by ljp2706 View Post
    I think it’s primarily done for layout matching. As long as your schematic is readable, doesn’t violate pdk requirements, meets specifications, and matches your layout, it doesn’t matter too much what you do with your devices from a schematic point of view. Ultimately the layout is what gets fabricated. The only other thing I’ll say on odd fingers, is that there may be some instances where you won’t be able to create clean ratios. For example, if your current mirror has 3 fingers and you want to mirror half the current to another branch, you can’t create a 1.5 finger device.
    Thank you. It's just an exception but still intresting for considering.
    If you have any other point of view. Please let me know.



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    Re: Why we should choose even number of finger

    How you put in the number of fingers in the schematic may effect your simulations. A transistor with a width of 1000u, mult of 1 is different then a 1000u transistor with a mult of 10 which if different then a transistor that has a width of 100 but iterated <1:10>.



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    Re: Why we should choose even number of finger

    Quote Originally Posted by rangermad View Post
    How you put in the number of fingers in the schematic may effect your simulations. A transistor with a width of 1000u, mult of 1 is different then a 1000u transistor with a mult of 10 which if different then a transistor that has a width of 100 but iterated <1:10>.
    Thank you for your reply.
    But my question is comparison between 1000 and 1001 number of finger in schematic viewpoint. I mean the main ppint is odd and even number of finger.



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