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    Offset nonlinearity of Rail-to-Rail OpAmp

    Hello!
    While designing a SAR ADC i found out that Rail-to-Rail OpAmp that i used, have non-linear offset in relation to common mode. That has caused in high INL(though DNL is good).
    I think that this happened because input make up from 2 differential pairs nfet and pfet. When common mode near gnd or vdd is working only one differential pair and offset only from this pair affecting ADC. When common mode in the middle of range, offsets from both pair adding to each other and overall offset through all common-mode range became non-linear. Could this be the cause of the bad INL?
    If this the case, did somebody know what can i do about this? Except using dynamic offset compensation like auto-zero or chopper.

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  2. #2
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    Re: Offset nonlinearity of Rail-to-Rail OpAmp

    Yes, your explanation sounds quite reasonable. Are you using that opamp to do the comparator in the SAR?



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    Re: Offset nonlinearity of Rail-to-Rail OpAmp

    Maybe not so much that it's one or two pairs, but
    moving from one to the other you transition from one
    Vio to another (over some range of common mode).
    The "both active" might be the best (and normal) as
    the two Vios will be averaged somewhat.

    Since this is a sampled data system, why are you
    opposed to auto-zero? You surely have phases that
    you can "hide" that activity behind. Ping-pong, even,
    if one has to be active for a full cycle.



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    Re: Offset nonlinearity of Rail-to-Rail OpAmp

    Yes, i use OpAmp like a comparator.


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    Re: Offset nonlinearity of Rail-to-Rail OpAmp

    I'm not really against auto-zero, i thought that this some common problem and there some easy way to fix it.



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    Re: Offset nonlinearity of Rail-to-Rail OpAmp

    Quote Originally Posted by ICnow View Post
    Yes, i use OpAmp like a comparator.
    Why don't just use a dedicated comparator instead of an opamp?



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  7. #7
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    Re: Offset nonlinearity of Rail-to-Rail OpAmp

    Many SAR ADCs work simply on a comparison to GND
    (if you have split supplies this is likely). If so then you
    may find a lot easier task, maybe just a clocked
    comparator whose common mode range includes GND.
    Much simpler than a RRIO op amp to design, and the
    "latched" phase can hide the zero action behind it.



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