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INA199:Question about offset error

Part Number: INA199
Other Parts Discussed in Thread: INA149

The input bias curren is 28uA in the data sheet. Is this correct?

Considering that the internal resistor R1 is 1MΩ, the output offset will be quite large.

What I want to know is how input bias current and input offset current affect the output. I want you to teach me the calculation formula.

Basic Connections described in the data sheet is used as the circuit.

  • Hello user5855205,

    Welcome to the E2E forum and thanks for your question.

    The link kai klaas69 posted is appropriate. Here is another post as an article on IB error.

    https://e2e.ti.com/support/amplifiers/f/14/t/700466?tisearch=e2e-sitesearch&keymatch=faq:true

    Yes the internal input resistor is 1MOhm, but there is an input bias stage before the amplifier and this is what sets the input bias current. This input bias current block is a key difference between current sense amplifiers (e.g., INA199) and difference amplifiers (e.g., INA149). It is sometimes not shown in datasheet figures, but here it is:

    Errors from initial voltage offset (Vos), input bias current offset (Ios), and input bias current (IB) are all different and independent. Ios is only important when there is an input filter as shown in Figure 21 above.

    For a circuit with no input RCR filter, the error equations are:

    eVos = Vos/Vsense

    eIB = IB/Isense

    eIos = 0.

    Hope this helps.

    Sincerely,

    Peter Iliya

  • Thank you for your quick response.

    But there are a few things I still don't understand.

    1. Does the one with BIAS circuit shown in Figure 21 apply to all INA199 instead of optional parts?Should we consider that the bias current Ios was modeled to make it easier to understand?

    2. "e2e.ti.com/.../1727788 says "This input bias currents do not flow through the feedback resistors." Why is this?

    3. I don't understand eos = 0.

    4. I want to monitor a variable current of 10 to 600mA using a 47mΩ shunt resistor. Simple circuit without CRC filter.After all, the purpose is to estimate the error of the output voltage at each current value.I am trying to estimate from gain error and total error of offset voltage and offset current. Are there any other considerations? We believe that this estimation can be achieved if the effect of the offset current on the output becomes clear as the title of the question asks.

    I am sorry for the amateur's question, but thank you for your confirmation.

  • Hi,

    yes, all INA199 contain this BIAS circuit.

    The input bias currents don't flow across the internal feedback resistors, because the shunt and the load offer them a much much lower ohmic path.

    Kai

  • Hey user5855205,

    No problem with the questions. We are happy to help.

    1. Yes. The bias circuit applies to all INA199 gain versions. In fact, the majority of CSAs have a input bias stage. I don't understand your comment about the Ios.

    2. The bias current do not flow through internal resistors because the currents are sourced through the low-impedance inputs from the bus voltage.

    3. eIos = 0 because the difference in the IB+ and IB- currents is so small that the error from them is negligible.

    4. Consider using our error calculator tool on the INA199 product folder page (click the INA199 link).

    Hope this helps,

    Peter

  • Thank you for your polite answer.
    I understood the contents.

  • I'm sorry, but I have additional questions.
    When I referred to the training video, there was an attached explanation. A similar graph was included in the INA199 data sheet. However, since the measurement condition of IN199 is Vs = 5V, the graph does not match VCM = V + in the explanation of the video. (VCM rises at about 2.5V instead of 5V)
    What does that mean??
    Also, from many answers, it was found that the effect of the input bias current on the output voltage was small in this simple circuit, but please give me the detailed formula for calculating the output voltage error. The conditions are as follows. V + = 5V, VCM = 4V, Rshunt = 47mΩ, Ishunt = 50mA, 25 ℃, VREF = GND, high side current monitor.

  • Hello user5855205,

    I'm an unsure about what video and graph you are referring to. The VCM crossover voltage may differ from device to device. Sometimes the IB jumps at V+/2 or sometimes it can be at V+.

    Using Figure 33 of INA199 datasheet, we can approximate IB = 24uA at VCM = 4V for V+ = 5V and VREF = GND. The offset error due to IB (Vos_IB) can be easily calculated at (IB-)*Rshunt. This equates to (+20uA)*47mOhm = +1.128 uV.

    Hope this makes sense.

    Best,

    Peter

  • Thanks for your answer.
    Problem solved.