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OPA445: CMRR: Vcm evaluation

Loris Drago
Prodigy 130 points

Part Number: OPA445
Other Parts Discussed in Thread: TINA-TI

Hello,

I'm using a model of my op amp that accounts for the basic offset voltage Vos_0, the offset voltage due to the CMRR Vos_CMRR and the input impedances (Zd: differential impedance; Zcm: common-mode impedance). I have a doubt about the evaluation of the common mode voltage Vcm. Shuld I evaluate Vcm as (Vp+Vn)/2 or as (Vp'+Vn')/2?

thanks,

Loris

  • 0
    Guru 140200 points
    Hi Loris,

    why aren't you using the Spice model of OPA445 itself for your simulation? Use TINA-TI and the TINA-TI model of OPA445 TI has already prepared for you. You can even use the TINA-TI reference design which you will find here:

    www.ti.com/.../toolssoftware

    Kai
  • 0 in reply to kai klaas69
    Prodigy 130 points
    Hi Kai,
    I need this kind of information because I have to develop a general model for my application, so it has to be function of the standard parameters used in the characterization of the op amp.
    So shuld I evaluate Vcm as (Vp+Vn)/2 or as (Vp'+Vn')/2?
    thanks,
    Loris
  • 0
    TI__Guru* 93105 points

    Hi Loris,

    An ideal voltage source has zero impedance, and that should be the case for the Vos_0 and Vos_CMRR sources. They simply add or subtract a voltage to Vp', relative to Vn. In  actual op amp testing the CMRR is measured with these offsets in place, and the device must deliver a specified level of CMRR performance. Therefore, it appears the Vn and Vp are the points you would use to evaluate the CMRR.

    Regards, Thomas

    Precision Amplifiers Applications Engineering