• State TI Thinks Resolved
  • Locked Locked
  • Replies 9 replies
  • Answers 2 answers
  • Subscribers 49 subscribers
  • Views 897 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

OPA445: CMRR: Vcm continuously changes

Loris Drago
Prodigy 130 points

Part Number: OPA445

Hello,

I'm using my op amp in an application in which the common-mode voltage Vcm ( Vcm = (Vp + Vn )/2 ) is continuously changing. Since to evaluate the offset voltage Vos due the variation of Vcm I have to use the common-mode rejection ratio CMRR times ΔVcm, what is the value of ΔVcm that I should use? Should I use the variation of Vcm with respect to 0? or perhaps with respect to the initial value of Vcm?

Thanks,

Loris

  • 0
    Guru 140200 points
    Hi Loris,

    why opening a new thread?

    e2e.ti.com/.../2879399

    Kai
  • 0 in reply to kai klaas69
    Prodigy 130 points
    Hi Kai,
    because the question is different. Here I'm asking how I should evaluate ΔVcm if Vcm continously change in my application. In (e2e.ti.com/.../2879399 I asked at which points of the circuit I should evaluate Vcm.
    Regards,
    Loris
  • 0 in reply to Loris Drago
    TI__Guru* 93105 points
    Hi Loris,

    The OPA445 CMRR is production tested using a very large VCM voltage change. The OPA445 Electrical Characteristics table indicates that VCM is −35V to +35V for the CMRR specification (Vs = +/-40 V), and the CMRR must be at least 80 dB. That is the level of CMRR performance the datasheet assures and no other assurances about that parameter are provided.

    The VCM change is most often accomplished in an actual op amp CMRR test by moving the supplies together, rather than actually applying a changing VCM voltage to the non-inverting input. For the +35 VCM condition Vs+ would move from +40 V up to +75 V, and Vs- would move from -40 V up to -5V. For the -35 VCM condition Vs+ would move from +40 V down to +5 V, and Vs- would move from -40 V down to -75V.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    Prodigy 130 points

    Hi Thomas,

    Thank you for your answer. I get your point with the example on the Vs variation (even if Vs can not excide +/-45V). However in my application I have fixed V, while Vcm continuously varies, thus I would like to know how to evaluate the Vcm change: should I use the variation of Vcm with respect to 0V? or perhaps with respect to the initial value of Vcm (time = 0s)?

    Regards, Loris

  • 0 in reply to Loris Drago
    TI__Guru* 93105 points

    Hello Loris,

    The CMRR specification listed in Electrical Characteristics table for TI's Precision Op amps is measured with respect to mid-scale. For the OPA445 using a +/-45 V supply the midscale voltage is 0 V. The VCM is moved to +35 and -35 V relative to that midscale level. Therefore, if you wish to determine the CMRR performance of your OPA445 circuit you can drive the non-inverting input +/- 35 V about the 0 V midscale level.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 130 points

    Hello Thomas,

    thank you for your explenation. I have a final question. the CMRR value reported on the datasheet has a positive value. However I learned for the "TI Precison Labs" that CMRR can lead either to a positiveor a negative variation of Vos, given a Vcm change. How can I know the sign of the change of voltae Vos to be applied to my op amp input pins?

    Regards,

    Loris

  • 0 in reply to Loris Drago
    TI__Guru* 93105 points

    Hi Loris,

    The OPA445 has typical CMRR of 95 dB (56,234 V/V) listed in the Electrical Characteristics table. In reality, this CMRR number represents rejection of a common-mode response. The output referred offset change will be the inverse of the 56,234 V/V, or 1/56,234 V/V (-95 dB).

    The voltage offset change in a CMRR measurement, measured at the op amp output may be positive, or negative. What needs to be applied is the absolute value of the total offset change, as VCM is exercised over the specified test input voltage range. Doing so will result in a positive value for the offset change that can be divided by the input VCM voltage change.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 130 points

    Hello Thomas,

    Could you explain your point by using Vcm (mean voltage at the input pins), Vos (voltage added to an input pin of the op amp to account for the CMRR effect), Vout (output voltage of the op amp).

    Regards, Loris

  • 0 in reply to Loris Drago
    TI__Guru* 93105 points

    Hello Loris,

    It appears the explanations I have provided you regarding op amp common-mode rejection ratio (CMRR) and common-mode input voltage (Vcm) are leaving you with even more questions. I think at this point it is best I refer you to some of the TI resources that discuss the subjects. They may be able to help you gain a better understanding of the subjects and answer better answer your questions. Please see the following resources:

    Op amps for Everyone - Section 18.4 Input Common-Mode Range and Section 11.3.9 Common-Mode Rejection Ratio

    web.mit.edu/6.101/www/reference/op_amps_everyone.pdf

    TI Analog Wire blog: "Tust-but-verify-spice-model-accuracy-part-1-common-mode-rejection-ratio"

    https://e2e.ti.com/blogs_/b/analogwire/archive/2017/07/27/trust-but-verify-spice-model-accuracy-part-1-common-mode-rejection-ratio-cmrr?

    Also see TI Precision Labs – Op amps

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

    https://training.ti.com/ti-precision-labs-op-amps#section-7

    Hopefully, these resources will provide the answers you need.

    Regards, Thomas

    Precision Amplifiers Applications Engineering