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OPA211: The error voltage for OPA211

user4674678
Intellectual 490 points

Part Number: OPA211
Other Parts Discussed in Thread: OPA188

Hi team,

I want to get the error voltage for the op amp. For exmple, OPA211, Vos is 20uV, Ib is 50nA,CMRR is 120dB. 

Q1: If Ib causes an error voltage Vib and CMRR causes an error voltage Vos_CMRR, the closed gain is G, then the total error voltage = G(Vos+Vib+Vos_CMRR), correct? 

I use a  non-inverting configuration in this case. Would you explain how to calculate the total error voltage in the op amp circuit in detail?

Q2: I note this blog: 

What you need to know about CMRR - the operational amplifier (Part 1)
I think about common-mode rejection (CMRR) often, even outside of work! I am an avid college football fan and when watching games on Saturday at my house I am often interrupted by the common noise of my…
By in Technical articles > Analog

 I would like to know why if  in a non-inverting configuration, VCM=VIN while in a inverting configuration, VCM=0V, not the VIN voltage which is in the inverting input pin ? What is the common voltage of the op amp?

  • 0
    TI__Guru* 93105 points

    Hello user4674678,

    I suggest you have a look at the TI Precision Labs series  which are designed to assist customers with questions such as these abut the errors contributed by voltage offset, input bias currents and CMRR. The specific sessions that I suggest are:

    2. TI Precision Labs - Op Amps: Input Offset Voltage and Input Bias Current

    7. TI Precision Labs - Op Amps: Common Mode Rejection and Power Supply Rejection

    Please follow the link:

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Intellectual 490 points
    Hi Thomas,
    I have finished watching this video. For my understanding, I think the total error voltage = G(Vos+Vib+Vos_CMRR) is correct.

    I still want to verify if my undstanding is correct. Would you help me?
  • 0 in reply to Thomas Kuehl
    TI__Guru* 93105 points

    HI,

    Thank you for spending the time revieweing the TI Precision Labs materials. Hopefully, they were of use to you.

    The equation you stated for the total error is correct keeping in mind that the errors contibuted by Ib, Vos, and Vos_CMRR have polarities associted with each one of them. It is entirely possible for the offset contributed by the Ib currents to cancel some, or part of the voltage offset due to the mismatch in the input differential pair VBE or VGS.

    Therefore, when applying the equation the polarites of each effect needs to be taken into account. There is a section in the TI Precision Labs, lab portion that illustrates the issue. Quite fortunately, the lab uses the bipolar input OPA211 for one of the examples, as well as the CMOS input OPA188. Calculations are provided for both the "typical" and "maximum" cases. I have extracted the slides from the lab manual and have attached them for your review.

    VOS & IB – Lab.pptx

    You include the Vos contribution due to CMRR in the equation. Keep in mind that if the the common-mode voltage is static (not moving) that voltage offset and Ib of the differential pair are going to reside at some static value as well. Only if the VCM is moved to a different voltage level will a small shift in the overall Vos be observed at the output. This is because the Ib and Vos may change with the VCM level.

    As the slide calculations show the total error at the output is calculated based on the Ib and Vos contributions to offset, multiplied by the closed-loop gain of the system. The CMRR effect is not included.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    TI__Genius 11135 points
    user4674678

    We haven't heard back from you so we assume this resolved your questions. If not, post another reply below.

    Thanks
    Dennis