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(AD8418A)Vout ≠ Vref

Hi !

I have a question about AD8418A.

This is my test circuit.

I measured the Output voltage but it is not Vref level.

=> I don't have AD8418A so I measured AD8418.

Why this happens?

Our customer said that  this won't happen when they are using AD8206

but I measured AD8206 at my Lab and same thing happened.

But the output voltage offset is smaller than AD8418.

I think that maybe the +IN pin leakage current is big 

Can you explain that why this happens?

Best regards

Kawa

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  • Hello Kawa, 

    Yes, the AD8418 can have high bias currents at high common-mode voltages (See Figure 8 on the datasheet). Using a shunt resistor that's too large would cause a high offset when that bias current goes through the 4.7k resistor.  

    I have been trying to go through the results and your setup, and the measurements with the 4.7k resistor do not really make sense to me. The output is much too high, even when the possible errors are accounted, i.e. bias current, offset voltage, and the error from the reference voltage. With your setup, you would still be operating close to 0Vcm, and Figure 8 from the datasheet shows you that the bias current will be very small at this point. It is usually around 1uA, based on experience.

    The second set of results makes more sense -- you are still seeing the reference voltage coming through correctly, albeit with some error. Can you try shorting the inputs together and connecting to ground? Is the output accurate when you measure with a reference voltage? I just want to make sure that the device is working correctly in a known configuration. 

    I will check the AD8206 bias current and see how it compares to the AD8418, and see whether we can conclude anything from there. I must reiterate though, that in the applications the AD8418 is designed for, large shunts are not really used due to the error they can contribute. Why does the design call for such a large shunt?

    Thanks, 

    Kris

Reply
  • Hello Kawa, 

    Yes, the AD8418 can have high bias currents at high common-mode voltages (See Figure 8 on the datasheet). Using a shunt resistor that's too large would cause a high offset when that bias current goes through the 4.7k resistor.  

    I have been trying to go through the results and your setup, and the measurements with the 4.7k resistor do not really make sense to me. The output is much too high, even when the possible errors are accounted, i.e. bias current, offset voltage, and the error from the reference voltage. With your setup, you would still be operating close to 0Vcm, and Figure 8 from the datasheet shows you that the bias current will be very small at this point. It is usually around 1uA, based on experience.

    The second set of results makes more sense -- you are still seeing the reference voltage coming through correctly, albeit with some error. Can you try shorting the inputs together and connecting to ground? Is the output accurate when you measure with a reference voltage? I just want to make sure that the device is working correctly in a known configuration. 

    I will check the AD8206 bias current and see how it compares to the AD8418, and see whether we can conclude anything from there. I must reiterate though, that in the applications the AD8418 is designed for, large shunts are not really used due to the error they can contribute. Why does the design call for such a large shunt?

    Thanks, 

    Kris

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