Amplification from the additional Rref Is Very Annoying

I'm using AD8222 in my sensor design where it is used to interface with Wheatstone bridge. On the prototype, +IN1 and -IN1 are tied together to fix 2.50000V.  +Vs is set to 12.0v and -Vs is set to 0v. REF1 is tied to a unity buffer op-amp (REF495) with a voltage of 2.5V. The gain setting is 351 with RG1 set at 141 ohm.


Theoretically, i would expect the OUT1 of 2.50000V. However, there is always an additional amplification of 0.02V. I suspect this comes from the amplification from the additional Rref. How can i get rid of the additional amplification of 0.02V. i attach the circuitry of the design.

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  • Hi Tik Liang,

    What is your input range? When you apply the 2.5V at the inputs of the AD8222, did you short the +In and -In to make sure that there is really no differential voltage? Do you see the additional 0.02V as offset error or gain error?

    It seems that you are operating close to the diamond plot limit of the instrumentation amplifier as shown on this very useful tool. You can also learn more about input common-mode voltage vs output swing on this FAQ.

    You'll see that the output swing will be limited by the input common-mode voltage and depending on your target output range, you have to apply sufficient amount of input common-mode voltage which is ideally was set to Vcc/2 or 6V in your case.


    Any voltage at the reference pin will be amplified at gain of 1. Can you double check if you have accurate 2.5V going out from U4? I could see some resistor divider at the input pin of the buffer and its tolerances can contribute to 0.02V error that you are seeing.

    Also what is the purpose of TP1, TP2, TP3 and TP4? RG pins of the instrumentation amplifier are very sensitive to parasitics which if not properly taking care of, can results to ringing or worse oscillations at the output on the inamp.


    Best regards,

    Emman

Reply
  • Hi Tik Liang,

    What is your input range? When you apply the 2.5V at the inputs of the AD8222, did you short the +In and -In to make sure that there is really no differential voltage? Do you see the additional 0.02V as offset error or gain error?

    It seems that you are operating close to the diamond plot limit of the instrumentation amplifier as shown on this very useful tool. You can also learn more about input common-mode voltage vs output swing on this FAQ.

    You'll see that the output swing will be limited by the input common-mode voltage and depending on your target output range, you have to apply sufficient amount of input common-mode voltage which is ideally was set to Vcc/2 or 6V in your case.


    Any voltage at the reference pin will be amplified at gain of 1. Can you double check if you have accurate 2.5V going out from U4? I could see some resistor divider at the input pin of the buffer and its tolerances can contribute to 0.02V error that you are seeing.

    Also what is the purpose of TP1, TP2, TP3 and TP4? RG pins of the instrumentation amplifier are very sensitive to parasitics which if not properly taking care of, can results to ringing or worse oscillations at the output on the inamp.


    Best regards,

    Emman

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