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OPA189: CMRR inconsistency

Jonathan Navor
Genius 9475 points
Part Number: OPA189

Hi Team,

I am posting this on behalf of the customer.

The datasheet for OPA189 has an inconsistency regarding CMRR. The CMRR is guaranteed >146dB for a +-18v supply and -18.1V<Vin<15.5V . I therefore calculate that the offset voltage should vary no more than 1.7uV over this range. However, Figure 7-6 shows 5 typical units which all exceed this value, with the worst being about 16uV variation (CMRR of only 126dB). Which portion of the datasheet is correct?

 Thank you in advance for your support.

Best regards,

Jonathan

  • +1
    TI__Guru 63790 points

    Jonathan,

    You are correct that Fig 7-6 is inconsistent with the OPA189 minimum CMRR of 146dB at 25C since its calculation based on the red curve Vos change of around 12uV would result in around 129dB.  However, the CMRR curves are within OPA189 over temperature min specification of 120dB.  Thus, I believe the sample of 5 units below includes temperature variation above room temperature caused by power dissipation inside the package.  

  • 0 in reply to Marek Lis
    TI__Genius 9475 points

    Hi Marek,

    I appreciate your response. However, I don't think it fixes the inconsistency. Fig 7-6 is specified at Ta=25C, according to the top of the page. This is the same condition as specified for the 140dB CMRR in the specification table. For both, Ta is specified (not Tj) which suggests that self-heating is included in the specification.

    Could you check it again?

    Best regards,

    Jonathan

  • 0 in reply to Jonathan Navor
    TI__Guru 63790 points

    Jonathan,

    A part performs according to its own junction temperature, Tj, and thus self-heating would be included if the temperature was specified as Tj and not Ta. Just for your understanding: during the production final test, which may last less than one second, there is little time for the part to heat up; thus, the Tj is pretty much the same as Ta - this is how min CMRR limit in datasheet table is specified (at Tj~Ta=25C).  However, the Vos vs Vcm curves shown in Fig 7-6 require DC sweeps that last much longer, which allows the part to warm up - this results in Tj being higher than Ta causing discrepancy between minimum CMRR limit of 146dB (where Tj=Ta) and Fig 7-6 (Tj>Ta).  Having said that, in the last seven years since the introduction of OPA189 our characterization team has improved procedures for taking those measurements and is now looking into retaking them for OPA189.   

  • 0 in reply to Marek Lis
    TI__Genius 9475 points

    Hi Marek,

    I'll inform the customer about this information. Thank you.

    Best regards,

    Jonathan

  • 0 in reply to Jonathan Navor
    TI__Guru 63790 points

    No problem.  Please let us know in case your customer has further questions.