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 Emman,

    Just to clarify, I would like to made this design to have differential output. I assume AD8222 dual channel are symmetrical internally. The gain resistors for both channels are 141 ohm at the prototype testing. Is my design methodology correct in order to obtain the differential output?

    The input range is 2.5V (+-10mV), it is actually just a strain gauge transducer output. Yes, when I apply 2.5V at the inputs of the AD8222, I short the +In and -In and there is not voltage difference at the input.

    And thanks for the diamond plot tool, I had also reduce the VCC from 12V to 5V. So, input common mode voltage of 2.5V is actually sufficient now.

    I have also make sure the output from U4 to be very accurate at 2.500V.

    I did further investigation today and noticed that there is always some slight difference between Out1 (U2.15) and Out2 (U2.14). The difference could raise from 0.01V to 0.02V although I had shorted the both + and - inputs to 2.5V. As a result, the noise performance of this circuitry is bad. However, when I short both of them to Ground, the difference will go away.

    This clearly shows the gain resistor is the problem. The data pointed towards the gain error. i have actually want to made TP1, TP2, TP3 and TP4 as test points to solder an external potentiometer for gain tuning. But it looks like it backfires. The extra stub or traces could contribute to this imbalance and parasitic. I never expect the in-amp is so sensitive to the gain resistor. In order to made channel1 and channel 2 symmetrical, looks like I also need to make sure the Rg path symmetrical and as short as possible?

Reply
  • Hi Emman,

    Just to clarify, I would like to made this design to have differential output. I assume AD8222 dual channel are symmetrical internally. The gain resistors for both channels are 141 ohm at the prototype testing. Is my design methodology correct in order to obtain the differential output?

    The input range is 2.5V (+-10mV), it is actually just a strain gauge transducer output. Yes, when I apply 2.5V at the inputs of the AD8222, I short the +In and -In and there is not voltage difference at the input.

    And thanks for the diamond plot tool, I had also reduce the VCC from 12V to 5V. So, input common mode voltage of 2.5V is actually sufficient now.

    I have also make sure the output from U4 to be very accurate at 2.500V.

    I did further investigation today and noticed that there is always some slight difference between Out1 (U2.15) and Out2 (U2.14). The difference could raise from 0.01V to 0.02V although I had shorted the both + and - inputs to 2.5V. As a result, the noise performance of this circuitry is bad. However, when I short both of them to Ground, the difference will go away.

    This clearly shows the gain resistor is the problem. The data pointed towards the gain error. i have actually want to made TP1, TP2, TP3 and TP4 as test points to solder an external potentiometer for gain tuning. But it looks like it backfires. The extra stub or traces could contribute to this imbalance and parasitic. I never expect the in-amp is so sensitive to the gain resistor. In order to made channel1 and channel 2 symmetrical, looks like I also need to make sure the Rg path symmetrical and as short as possible?

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