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AD8221 Input Common-Mode Range vs. Output Voltage

Hi,

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

AN-1401 (Rev. 0) (analog.com)

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,

    Good day!
    As the article stated, the overall plot consists of those three limitations (input, output and internal node) mentioned.
    And looking at the AD8221's spec table we have this input voltage range:

    The min and max spec are not similar, so, I wouldn't expect a symmetrical input range. but looking deeper at the AD8221's limitations through the Instrumentation Amplifier Diamond Plot Tool:

    The difference between the positive and negative input common-mode range limitation is also caused by the internal node limits being different on both positive and negative which is inherent with the AD8221. This document gives more detail on the different inamp architecture.

    I hope this helps and let me know if you have more questions.

    Best Regards,
    Dann

  • Hi,

    could you please help reply my post?

  • 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

  • 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?

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