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INA219: Calculating Error due to Bias currents

Joel Krawczyk
Prodigy 30 points
Part Number: INA219

Hello,

I am trying to calculate total system error for my current sensing application using the INA219.

I have been using the Video Training series as a guide.

I see that Bias Current is included in the formula for calculating total system error but the video series does not explain how Bias Current error is calculated.

Can someone explain how to calculate Input Bias Current error for a current sensing application using INA219 or similar?

Thanks!

  • 0
    TI__Mastermind 26750 points

    Hey Joel,

    The INA219 is a unique part in that the IN- pin has an internal 830 kOhm resistance to ground, used to measure the bus voltage. This creates another input bias current source at the IN- pin which sums with the specified 20uA IB- that was already there. You can read about this in our FAQ article which discusses this error for INA219. The article also includes an input filter network, but still applies if you do not have this or plan to use an input filter.
    e2e.ti.com/.../3576.ina219-ina3221-input-impedance.

    Overall input bias current error can manifest in two ways: current siphoning and input-filter offsets. The first I explain a little the article below and this basically deals with the fact that the true current you may be measuring is actually your user-set current plus or minus the input bias current which is siphoning off current or adding onto the user-set current.
    e2e.ti.com/.../3593.how-vos-and-ib-affect-small-current-measurements

    The second is mentioned in the first attached article and many of our datasheets, but basically occurs when IB+ and IB- are different values and the inputs contain serial filter resistors with IN- and IN+. The difference between IB+ and IB- creates different voltage drops across these resistors. The difference between these voltage drops adds onto the input voltage offset of the device and thus decreases error if the resistors are too large (>10 Ohms). You can usually calculate this offset error as Ios*Rf, where Rf is the input filter resistor value (assuming you have equal resistors on the inputs and Ios = IB+ - IB-.

    Note in the first article with INA219, the offset error equation for "Vdiff" includes both manifestations of IB error discussed above.

    Sincerely,
    Peter Iliya
    Current Sensing Applications