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What is TI op amp datasheet noise standard practice?

Farron Dacus
Prodigy 30 points
Other Parts Discussed in Thread: OPA1611, OPA170, TINA-TI, OPA209

Is it the Texas Instrument standard for noise and current noise density graphs to reflect a differential model where the noise is relative to both inputs?  If so, then the numbers would need to be multiplied by root 2 to get single ended (i.e. relative to non-inverting input) noise.  That seems to be closer to SPICE noise sims.

  • 0
    TI__Guru 67601 points

    Farron,

    The Input Voltage Noise spectral density is measured in a single-ended configuration and their graphs shown in the datasheets, and simulation models, are also shown as single-ended specs (in series with non-inverting input and not differential) and thus they do NOT need to be mutlipled by sq-rt(2). 

    The Input Current Noise spectral density graphs in CMOS and JFET op amps are typically modeled as differential and their magnitude is set to the RMS value equal to the sum of two independent current noise sources.  However, because of their small magnitude, the datasheets typically show only the calculated value of Inoise=sq-rt(2*q*IB) which applies to a single-ended measurement.

  • 0 in reply to Marek Lis
    Prodigy 30 points

    Hello Marek:

    Thanks for the prompt reply. 

    What had led to this question is the TINA simulated noise in the OPA1611, which was running higher than datasheet.  However, I now see that this is a function of common mode input voltage.  Near the datasheet minimum of 2.0V the noise begins degrading, at least according to the SPICE model.  Would you know if that is trustworthy?

    Regards,

    Farron

  • 0 in reply to Farron Dacus
    TI__Guru 67601 points

    Farron,

    The OPA1611 simulated input voltage and current noise is in line with PDS graphs (see below) BUT you are correct about it degrading with low Vcm - however, I do not believe this is real especially since I do not see the same kind of problem on low dual supplies, Vs=+/-2.25V, which effective bring the Vcm within 2.25V of negative rail. Therefore, I believe this is a modeling problem where voltage noise get worse while current noise gets better. 

  • 0 in reply to Marek Lis
    Prodigy 30 points

    Hello Again, Marek:

    OK, thanks. 

    You might like to know of what seems to be additional modeling problems on the OPA170:

    1.  The OPA170 is not in the latest download of TINA-TI V9. 

    2.  When the available model is loaded into LT-SPICE, it runs fine for AC and transient, but the noise analysis appears way off.  I don't think this is an incompatibility between LT SPICE and the TINA Texas Instruments subcircuits, since the OPA209 and OPA1611 models run fine for noise in LT SPICE. 

    Regards,

    Farron