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OPA818: OPA818 Sim Anomaly!

Peter Partridge
Intellectual 290 points
Part Number: OPA818
Other Parts Discussed in Thread: TINA-TI

Hello,

I have emailed the help-desk regarding my question and although there has been a good attempt I am still looking for answers. So I hope someone could help me get an answer. Below are my original questions:-

I have simulated 3 OPA818 circuits in TINA. All circuits are identical with a gain of 10 with specified feedback resistors and driven with same source of 20mV (The -330uV source is to correct the output offset). The only difference between them is the amount of capacitive loading. First amp has zero capacitance. The 2nd has 2p and the third has 20p. If you run a transient simulation for 0 - 20n you will see that there is little overshoot from 0 - 2p loading and about 22% overshoot for the 20p loading. The data sheet specifies that under the same conditions of 2p loading and gain etc there should be a 30% overshoot! This seems to agree more with the 20p loading result!
So which do I believe the data sheet or the simulator? It is important that I have confidence in the Sim because my multistage wide-band amp must amplify signals without excessive ringing.

I was not satisfied with the answer given so I responded further:-

If the OPA818 is a 'preview' part and the specification of the device is not final does this mean that data sheet information is only a guide or approximate at this stage?

If that is the case then I’m not too concerned but does that also mean that your model for the OPA818 is also only approximate? If that is the case then I am concerned! Why produce a model that doesn’t give real world results?

I need to have confidence in the model otherwise there is not point using it and my design will not agree with the results Tina predicted!

So I rephrase the question – is the model of the OPA818 as accurate as your other opamp models?

I was informed that the OPA818 is only a 'preview' part and therefore the data sheet is only a 'guide'. That's OK but the question is - if the OPA818 is a 'preview' part then is the Tina model for it only a 'preview' as well and only approximate and not for accurate modelling? I was informed that the ringing with 2p loading on the Sim is well within the spec on the data sheet so all is OK - but to me it is not OK!!

The funny thing is that there is an evaluation board for it for sale, which I'd like to use and cascade 2 for higher amplification but if the modelling is not accurate then how can I determine a desired result before I purchase the boards?

I tried to attach the TSC file for anybody's reference but not sure whether it attached properly.

Thanks.

Best regards,

Peter J Partridge

OPA818test.TSC

  • 0
    TI__Mastermind 23206 points

    Hi Peter,

    Thank you for your question. The datasheet currently online is advance information and is subject to change. The capacitor drive specification you mentioned was determined with via silicon level simulation. The only measured data we currently have available pertaining to this specification is for a gain of 7V/V with no load. For an output step of 2V/V, the overshoot is 5%. For an output step of 100mV, the overshoot is 16%. I expect the measured results for a 2pF load would be similar to these results for 0pF.

    The TINA-TI model shows 8% and 23% overshoot for these conditions. The overshoot from the model is based on the interaction of gain, capacitor load, and isolation resistance, and reflects the Aol and series output impedance given by design. The behavior model always ends up somehow being a little less stable than the real amplifier, which is helpful to design an unconditionally stable circuit. Hopefully this is within the accuracy range you require to try the evaluation module.

    Best regards,

    Sean

  • 0 in reply to Sean Cashin
    Intellectual 290 points

    Hi Sean,

    Thanks so much for your prompt reply. If you don't mind, just a bit of clarification:-

    I gather your 'Silicon level simulation' is a much deeper level of simulation than Tina? Therefore the capacitive load drive specs in the data sheet are not measured results but only simulation results? It seems to give much worse data than what Tina predicts! Hence that is why I was concerned about the accuracy of Tina!

    What threw me was the results I was getting from Tina for a 2p load and G=10 being much less than that in the data sheet.

    I neglected to note that the overshoot test was done with G=7, however the data sheet indicates you did it with G=10 as well but no magnitude stated for Vout which I assumed to be the same.

    I did a quick sim with G=7 and of course the overshoot is much higher than G=10 when you consider the difference in the Bode plots.

    It is good to know that Tina gives results that are more worse case than real world results but when compared to the data sheet info it was the reverse! Now I can proceed with my wide-band high gain design of 2 cascaded modules using Tina with more confidence!

    In reality the Evaluation boards come with an isolation resistor of ~ 50 ohms, which will deal with any capacitively induced instability from the next stage's input/coax etc anyway.

    Best regards and thanks for restoring my confidence in Tina!,

    Peter