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    gate connection using poly vs metal

    Dear friends,

    When I do layout I usually connect the gate of the single matched array by extending the poly. it is easy to implement and I think that extending poly might be better than pitting via per every gate to move to metal connection as the via also introduce resistance,

    however in the book of the Art of layout design he suggested to connect the gates using metals

    I would like to have your opinion according to your experiance

    thank you very much


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    Re: gate connection using poly vs metal

    Quote Originally Posted by Junus2012 View Post
    Dear friends,

    When I do layout I usually connect the gate of the single matched array by extending the poly. it is easy to implement and I think that extending poly might be better than pitting via per every gate to move to metal connection as the via also introduce resistance,

    however in the book of the Art of layout design he suggested to connect the gates using metals

    I would like to have your opinion according to your experiance

    thank you very much


    Click image for larger version. 

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    You can't route with poly unless the technology is ~20 years old. It's a non-starter.
    Really, I am not Sam.


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    Re: gate connection using poly vs metal

    Dear NotSam,

    I am using the 0.35 technology.



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    Re: gate connection using poly vs metal

    There will be a "master" and N "slaves" in the current mirror
    rack and you want to pay attention to where the reference
    current flows. For example if you push the current in at the
    left end of a S-S-M-S-S rack, the M device's drain current
    makes a successive I*R drop between S1.G and S2.G, S2.G
    and M.{G,D}, and then there is no further drop to S3.G and
    S4.G; so you would have three slightly different (or not so
    slightly, depending) current outputs with a systematic mismatch
    (S1>S2>S3,S4).

    You should understand the poly resistor options and for an
    analog layout, attempt to make the poly-route look more like
    some number of poly resistors so effects are represented that
    you might have overlooked.


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    Re: gate connection using poly vs metal

    Dear freebird,

    As I understood from you you are not in favor of connecting the gain using the poly, surely it gives more resistance no doubt about it.. but I I am matching 10 gates for example then I need to put 10 via to make the connection with metal one or 20 via if I use metal 2 which I usually use in the horizontal routing,

    20 via would also add contact resistance but I am not sure if it is still less than comparing if I use the poly,,,, I need to know from your experience please.. if you confirm me that metal+via is better than only poly then I will not use the poly contact any more for connecting gates

    Note: I am only using poly for connecting gates, all other circuit parts are connected with metal



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    Re: gate connection using poly vs metal

    There are good reasons for and against, as the engineer
    the value judgment is up to you.

    For example maybe it's better to route poly in a standard
    cell logic library, than "bake in" metal2 obstructions that
    cost you routing efficiency and time. More true back in
    the few-metal-layers days (but these are not entirely
    over - especially if you live on the trailing edge).

    Via resistance has a spec. It's going to be lower than a
    square of low doped or undoped poly; maybe more,
    maybe less than silicided. You can parallel vias.

    Whatever you do, you can insert presistor elements to
    assess the effect and use that feedback to decide.
    Since poly has the highest capacitance to bulk, of the
    conductors, you might look into that as well.

    One other impact of routed gate poly is the elevated
    effective Rg. That can impair high frequency PSRR and
    gain, allow Miller kick to more easily / significantly perturb
    current mirror output (presuming any decoupling / stiffness
    on the reference gate node is remote). Again realism in
    modeling at the schematic would show this, if you looked.


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