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INA208: The worse case calculation of INA208AIDGSR pin2

Red Yen
Expert 4360 points
Part Number: INA208


Hi Support team 

my Customer is asking the worse case calculation of INA208AIDGSR pin2 under

Vbus is 12V, shunt resistor is 10mohm and Max 4A current rating, 

BTW, may i have mathcad calculation spreadsheet for this part 

thanks a lot 

  • 0
    TI__Mastermind 32700 points

    Hello Red,

    There is not enough information to calculate the worst case error.  We have a whole training series on understanding the errors seen.  You can see the training videos here.  These videos help explain a lot about current sensing error.

    In some application you need to worry about some of the error while not in others.  The best way is to apply the error that matters for the application. 

    We also have an RSS error calculator on the INA208 product page.  The calculator assume an ideal Shunt resistor.  I have applied it below.  You will notice the error is higher as the current is lower.  This is due to the VOS of the INA208 being higher.  VOS is normally the dominating factor at the low current measurements and Gain error will normally dominate at higher currents.  I have placed a screen shot below of the calculator.

    I will assume your error is only for room and I would say VOS is a max of 2.5mV  and gain error is 1%. 

    VOS contribution 2.5mV/(4A*10mΩ)*100 = 6.25% at room assuming 12V VCM and Vsupply = 12V(This means CMRR and PSRR do not contribute to error).

    Gain Error is 1% so combining both we get an max error of 7.25% under those conditions.  Please verify the calculations based on the training videos and the applications conditions.

  • 0 in reply to Javier Contreras4
    TI__Expert 4360 points
    Hi Javier

    it's really confuse me since the reply from William as following link
    e2e.ti.com/.../803855
    Input offset voltage IS included in the output accuracy (less than ±3.5% max) spec. Please see my response on your other post below. The ±3.5% includes the effects of input offset voltage and gain error.

    why the worse case from ±3.5% max to 7.25%? which one is correct?
  • 0 in reply to Red Yen
    TI__Mastermind 32700 points
    Red,

    For your condition I would use the calculated value of 7.25% for the worse case. Let me explain below why this is the case. Also read last section regarding Vsense<20mV

    The specification of 3.5% is under the condition of Vsense = 120mV. This is where Vos is less of a factor. If you factor the same calculation for the Vsense voltage of 120mV you get the following.

    2.5mV/(12A*0.01Ω)*100 = 2.08% from VOS error
    1% Gain Error

    1%+2.08% Vos error at 120mV = 3.08% total error.

    Now this would be at room temperature. This is much lower than the previous calculation but you can see this larger than the 2.2% specified. Under the specified conditions I would go with the 2.2% at room and 3.5% over temperature.

    Be sure to understand that this is done with 120mV Vsense voltage and that VCM = 16V and VS = 12V. Any conditions way from this like the videos state for PSRR and CMRR need to be taken into account.

    Also this is at higher Vsense voltage. You need to take a look at section 8.4 of the datasheet to understand how the part works when Vsense<20mV. The output does not behave as expected and all there is detailed explanation in the datasheet.