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Why is AD8553 instrumental amplifier not working? It only outputs a 1V even after changing the input.

Hey, I am currently using AD8553 with a gain of 1 to output a single signal that represents the difference between Vin+ and Vin- but it only outputs 1V even after changing the input.

The input to AD8553 comes from LTC1968; the LTC1968 outputs max DC voltage of 0.8V. Therefore, both the input( Vin+ and Vin- ) will be less than 0.8V.

I have checked the datasheet and it does mention that for a 5V source, the input range is between 0V to 3.3V which is what I want.

AD8553 datasheet:

I have used the following configuration, and I only get 1V output even after changing the input. I am not sure what I have missed.

Please help out.

I hope to hear your inputs on this.

I will try using a different instrumentation amplifier but not sure what I have missed here as I have taken into consideration the input voltage range and the single power supply.

  • Hi shurp,

    Thanks for reaching out to us regarding this.
    I checked the AD8553 datasheet, and on the spec table at 5V source, your common-mode voltage should be at 2.5V.

    I didn't see any input-common mode voltage vs output swing on the AD8533 datasheet though.
    Instrumentation amplifiers normally have a diamond plot spec as explained in this article, so, I think it's worth a try to isolate the AD8553 from the LTC1968s and try it out with a 2.5Vcm to see if we can get our expected output.
    Thanks shurp.

    Best Regards,

  • Just want to verify, Common Mode Voltage should be 2.5V; is this the input voltage??? On the image that you have provided, if you look below, it tells you that the input voltage range is 0 to 3.3V. I think I am missing a concept here. How is Common Mode Voltage different from Input voltage range?

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  • Hi shurp,

    The input voltage range could be the differential input voltage you can input on the instrumentation amplifier.
    It is also indicated at the top of the spec table Vin = Vinp - Vinn.

    It also confuses me as the datasheet don't have a diamond plot on its TPC and I still find it odd that you get 1V at the output. That's why it would be good to test it out when isolated from the LTC1968s, out of curiosity, have you checked the actual output of these LTC1968 through the scope along with the AD8553's output?

    I currently don't have an AD8553 but will try to work on getting a setup to test out the input characteristics myself. One more check shurp if you may, can you double check if your pin 6 is actually connected to Vs? because the datasheet explicitly say to not leave it floating.
    Thanks shurp. Will surely work this out, thank you for your patience. Slight smile

    Stay Safe.

    Best Regards,