AD8226 offset voltage

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Hi

I've selected AD8226 in my design for self diagnosis of 0-20mA current loop.

I simulated AD8226 for sensing a constant current 1mA across 40E resistor. Below is the result for same.

Below are the parameters set. +Vs=30V , -Vs=-6V G=1 Sense resistor=40E current flowing=1mA Observing output voltage of 40.78933mV(Fig 1 and 2) but Vout=I*R = 1mA*40 = 40mV

Again I simulated AD8226 for sensing a constant current 20mA across 40E resistor. Below is the result for same.

Below are the parameters set. +Vs=30V , -Vs=-6V G=1 Sense resistor=40E current flowing=20mA Observing output voltage of 800.7578mV(Fig. 3 and 4) but Vout=I*R = 20mA*40 = 800mV

It is observed that nearly 0.7mV is additionally added to output, which is 700uV, But in the datasheet it is mentioned that 50uV is the input offset voltage. As gain is 1 output offset also should be 50uV.

Please let me know if I'm missing out anything.

Thanks and Regards

Nidhi P Shetty

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    •  Analog Employees 
    on Feb 28, 2020 1:41 AM over 1 year ago

    Hi Nidhi P Shetty,

    The offset you're seeing at the output of your simulation is the offset voltage of the device plus the offset introduced by your common-mode voltage.
    On the datasheet you have a maximum offset (without common-mode) of 700uV or 450uV depending if you have ARZ or BRZ respectively.

    On LTspice, AD8226 is modeled with an offset voltage of around ~313uV.

    Also, I've checked the CMRR of AD8226's model and it is around ~94dB.
    The offset introduced by the common mode voltage is given by:

    On your first case, you have a common-mode voltage of 23.98V, therefore you'll have an additional 478.46uV on your output.
    Add this to AD8226's offset voltage of 313.36uV. You now have a total offset of around 791.82uV. Also shown in this simulation.

    On your second case you have a 23.6V common-mode voltage.
    Similar with the first case you'll have an additional offset around 470.882uV, the total offset now will be 784.242uV.

    I hope this answers your question.
    For more information about the common-mode rejection ratio, please see this article https://www.analog.com/media/en/training-seminars/tutorials/MT-042.pdf.

    Thanks Nidhi P Shetty.

    Best Regards,
    Dann

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