AD8220 Vref

Hello fellow engineers and enthusiasts,

I've been using this instrumentation amplifiers for a few months now, and I absolutely love it for its robustness and pricing. I'm about to do a design that will need to adjust the offset, and the AD8220 offers this feature using its Vref pin. I've read through the datasheet many times on parts that talk about Vref, but Vref is always a positive Voltage. So my question is that may I apply a negative voltage to the Vref pin? What might be the consequences if I do so? To be honest, I'm not even sure how the Vref pin works. I think it's Vo = A*Vin + Vref because Vo=A*(Vin+Vref) doesn't make much sense. Anyway, if someone else is more familiar with this IA, please help. I've attached the datasheet of the IA.

Thanks for reading!

AD8220.pdf
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  • 0
    •  Analog Employees 
    on Apr 7, 2011 11:19 PM

    Hi NothinRandom,

    Glad you like the part.  You can apply a negative voltage to the refence pin, as long as you have a negative supply voltage.  We say in the datasheet table that you can apply any voltage you'd like within the supply rails of the part.  And as you've probably seen in the "reference terminal" section in theory of operation, you can go a few 100 mV beyond the supply rails - there are ESD diodes on the reference pin, so you don't want to go far enough to turn these on.  Note that diode voltages get bigger at cold and smaller at hot.  The 500 mV stated in the datasheet is for room temperature.

    The transfer function of the part is Vo = A*Vin + Vref as you mentioned.

    And remember - don't drive the reference pin with a voltage divider.  The output impedance of the divider will mess up the CMR of the in amp.

    Matt

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  • 0
    •  Analog Employees 
    on Apr 7, 2011 11:19 PM

    Hi NothinRandom,

    Glad you like the part.  You can apply a negative voltage to the refence pin, as long as you have a negative supply voltage.  We say in the datasheet table that you can apply any voltage you'd like within the supply rails of the part.  And as you've probably seen in the "reference terminal" section in theory of operation, you can go a few 100 mV beyond the supply rails - there are ESD diodes on the reference pin, so you don't want to go far enough to turn these on.  Note that diode voltages get bigger at cold and smaller at hot.  The 500 mV stated in the datasheet is for room temperature.

    The transfer function of the part is Vo = A*Vin + Vref as you mentioned.

    And remember - don't drive the reference pin with a voltage divider.  The output impedance of the divider will mess up the CMR of the in amp.

    Matt

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