Offset null with AD8221

Hello,

I am working on some transducer application and need to measure very low voltage 0~800mV DC to convert it into 0~5VDC. I use AD8221 here with gain of 6.25 to achieve this. AD8221 is supplied with +/- 6VDC.

Problem is, there remains about 2mV offset in DC signal coming from field (0~800mV) and I need to eliminate this offset so that o/p of AD8221 remains 0V and not 12.5mV (2mVX6.25) when there is no signal in actual.

Please anyone advise some cost effective solution to nullify this offset.

Thanks

  • 0
    •  Analog Employees 
    on Oct 22, 2013 6:23 PM

    Hi, Jatin.

    Sorry for the delay. The offset of AD8221 can be corrected in three ways.

    1.  

    1. Through hardware. To compensate it through this method, you have to apply an equal but opposite voltage to the REF pin. This can be done by using DAC, potentiometer or configuring an op-amp as buffer to drive the REF pin. So in your system, applying a voltage of -12.5mV to the REF pin will cancel out or return to zero the 12.5mV output offset (2mV input offset X 6.25 gain). However, this method may introduce errors, noise and may affect your Vos tempco.
    2. Through software. This is the most cost-effective way to null the offset. With this, you have to subtract the 12.5mV output offset through software. However, this might require a slight gain reduction to account for the offset.
    3. Through hardware and software. Only some use this method but I think this is the best method among the three. A mechanical potentiometer or digital potentiometer may be used to adjust the offset as close to zero as possible. Once this method is complete, your software must determine the residual offset then subtract this from each of your future measurements. With this combination, the offset null compensation is more optimized.

    I hope this helps.

    Regards,

    Anna

  • Thanks for your detailed explanation.

    Yes at present we are considering 2nd option to nullify offset with software.

    Jatin