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AD8221 Input Common-Mode Range vs. Output Voltage


I've read the article below to discuss the Input Common-Mode Range vs. Output Voltage plot.

AN-1401 (Rev. 0) (

We have 3 boundaries, input range, preamp output range and output range.

The question is for the input range limitation of AD8221 as captured below,

why do we have only this limitation for positive input common-mode voltage, but no such limitation for the negative input common-mode voltage? I think it should be symmetrical

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  • Hi Justifice,

    Apologies I wasn't able to get back to you on this query.
    I asked for the designer's input regarding the shift on the VCM limits for the AD8221, and additional explanation on the input voltage range is present on newer datasheets such as the AD8421. It is stated that the input voltage range is based on a combination of a gained signal, common-mode signal and a diode drop.

    This diode drop corresponds with the transistor's Vbe at the inputs of these inamp as shown below:
    For the case of AD8221 we have NPN transistors:

    These NPN transistors shifts the Common-mode up a Vbe.

    PNP transistors on the other hand will shift the CM down as in the case of AD8226 below:

    I hope we were able to answer your inquiry Justifice.
    Keep safe.

    Best Regards,

  • Dann,

    thanks for your answer.

    I'm not inquiring about the shift on the VCM limits.

    I'm curious why the upper side boundary of internal Node is even higher than upper side boundary of input pin limits.

    Even if we have a diode there, then both boundary should all shift up a diode drop, right?