AD8220 output voltage

I am running the AD8220BRMZ from +/-5V, with a gain set to 100 and the REF pin set to 2.5V. I notice that the output swing limits at 4.70V despite the datasheet indicating that I should get 4.85V as a minimum. Am I interpreting the datasheet correctly? This happens with a difference input of 22mV which is as expected i.e. (0.022V x 100) + 2.5V = 4.7V. The common mode max is 2.5V.


Another question I have is as follows. If I continue to increase the difference input beyond 22mV, the output remains clipped at 4.70V as expected but then above 40mV, the output begins to come down. At 45mV the output is 4.22V and at 49mV the output is 3.86V. Does anyone know what might be going on here?


Any comments appreciated. Thanks.

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  • Hi Scott,

    Thanks for your comments. I confess I've never paid attention to the diamond plots before (not being too sure how to interpret them). From your comment I can now see the answer to part 2 of my question. With a difference input of 40mV (when my output starts to drop), my common mode is actually 1.90V and figure 30 shows that's exactly what should happen. With a difference input of 49mV, my common mode is actually 2.50V and my output then measures 3.86V and figure 30 also confirms this.

    Going back to part 1 of my question, at the point that my output saturates (4.70V), the common mode input is actually only 1.10V. For these conditions, figure 30 suggests that I should be able to get up to 4.9V. I'm actually not too concerned about losing this 0.20V. To answer your questions, the 2.5V for REF comes from an ADR4525 reference, the -IN/+IN are driven from an AD8624 output, there's no series resistance on the supply pins and the output drives into a 2K load.

    Best regards,

    Paul

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  • Hi Scott,

    Thanks for your comments. I confess I've never paid attention to the diamond plots before (not being too sure how to interpret them). From your comment I can now see the answer to part 2 of my question. With a difference input of 40mV (when my output starts to drop), my common mode is actually 1.90V and figure 30 shows that's exactly what should happen. With a difference input of 49mV, my common mode is actually 2.50V and my output then measures 3.86V and figure 30 also confirms this.

    Going back to part 1 of my question, at the point that my output saturates (4.70V), the common mode input is actually only 1.10V. For these conditions, figure 30 suggests that I should be able to get up to 4.9V. I'm actually not too concerned about losing this 0.20V. To answer your questions, the 2.5V for REF comes from an ADR4525 reference, the -IN/+IN are driven from an AD8624 output, there's no series resistance on the supply pins and the output drives into a 2K load.

    Best regards,

    Paul

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