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  1. #1
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    Expected linearity of diff pair

    Hi,

    Can anyone give me numbers from their experience of which linearity a plain diff pair in a modern process should be able to achieve?

    Using a 28nm SOI process I built a simple diff pair with ideal current source loads and tail source (i.e., the output voltage is given by gm/gds). I am using pss and paq to measure IIP3 which is about -20dBm!!! (does not change significantly with different W and currents). This sounds suuuuuper low to me, I was hoping more for +20dBm ...


    Thanks!

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  2. #2
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    Re: Expected linearity of diff pair

    I think the standard definition for IIP3 is
    The third-order intercept point relates nonlinear products caused by the third-order nonlinear term to the linearly amplified signal
    (see e.g. here), so should be < 0 dB , isn't it?



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  3. #3
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    Re: Expected linearity of diff pair

    I am talking about dBm ... so can be > 0.

    For example, for a mixer (only) gives me +30dBm which is very good.

    So I am asking mainly about your experiences and if -21dBm can really be true.

    If it's easier I would also be happy with HD3 numbers + input amplitude because there is a relationship:

    IIP3 = 30 - 10 log6R - 0.5 HD3 + 10 log vi^2



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  4. #4
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    Re: Expected linearity of diff pair

    Quote Originally Posted by exp View Post
    I am talking about dBm ... so can be > 0.
    Ok, overseen, sorry! So you're talking about the IIP3 point, measured in power units.

    Quote Originally Posted by exp View Post
    For example, for a mixer (only) gives me +30dBm which is very good.
    1W ≙ 7V @ 50Ω is rather extreme for a mixer, I'd say.

    Quote Originally Posted by exp View Post
    So I am asking mainly about your experiences and if -21dBm can really be true.
    This would mean an intercept point at an output voltage of about 0.5V at an impedance of 25kΩ. Possible ...



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