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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

  • Why the upper side input limit pin limits line is crossing the internal node limit, but the lower side(negative) is touching the internal node limit(only 1 point of intersection)?

    It means that when considering the positive side, the internal node VCM limits can be higher than input VCM limit, why? Is it that the subtractor op amp's input VCM range is larger than the input buffer op amp's VCM range? But the datasheet doesn't give each internal op amp's input range.

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  • Why the upper side input limit pin limits line is crossing the internal node limit, but the lower side(negative) is touching the internal node limit(only 1 point of intersection)?

    It means that when considering the positive side, the internal node VCM limits can be higher than input VCM limit, why? Is it that the subtractor op amp's input VCM range is larger than the input buffer op amp's VCM range? But the datasheet doesn't give each internal op amp's input range.

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