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OPA2205: OPA2205: Error budget analysis

Phong Phan
Prodigy 60 points
Part Number: OPA2205
Other Parts Discussed in Thread: TINA-TI, OPA205

Hello everyone,

I am trying to do the Error budget analysis for the OPA2205 op amp. However, my calculations and simulation does not match. Taking a non-inverting amplifier circuit as an example.

Here is my simulation:

For this simulation, I used TINA-TI program. Because OPA2205 is not readily included in TINA-TI's Op Amp library, I have downloaded its model from TI's product website:

https://www.ti.com/product/OPA2205#design-development##design-tools-simulation

Once downloaded, I extracted the file SBOMB98.ZIP and follow the instruction from this video from TI:

https://youtu.be/UjDYO53l_KM

I have also attached the model file OPA205.TSM and the circuit file OPA2205.TSC

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As for my calculation, I tried to calculate the error resulting from different parameters of the OPA2205 op amp. For example, the input offset voltage of OPA2205 (for +/-15V operation) is Vos = 5uV.

Therefore, the error at output (due to input offset voltage) for the non-inverting circuit above will be:

This is only the calculation result from one parameter. I also intended to include other parameter's effect such as input bias & offset currents, CMRR & PSRR, etc. However, it is already 10 times larger than the simulation result. Therefore I am having a hard time trying to figure out why my calculation is not matching with my simulation result.

Thank you very much. I am trying to learn so any suggestions, corrections would be very much appreciated.

Phan Thanh Phong

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OPA205.TSMOPA2205.TSC

  • 0
    Prodigy 60 points

    Here is some example of the error budget analysis that I am trying to follow:
    https://www.dataforth.com/catalog/pdf/an102.pdf
    http://ww1.microchip.com/downloads/en/appnotes/01177a.pdf

    Any help and suggestions would be very much appreciated. Thank you.

    Phan Thanh Phong

  • 0
    TI__Guru* 93105 points

    Hello Phong Phan,

    The modern TI Precision Amplifier op amps simulation model applied to the OPA2205 uses "typical" parameter values and not maximums or minimums. Precision Amplifiers defines the typical value as the one standard deviation value from the normal distribution of the parameter. Therefore, for the simulation circuit you show the input referred voltage offset of the OPA2205 is set to the "typical" 5 uV, and not the 25 uV "maximum." The typical value applies to the input bias current, CMRR, PSRR, etc.If you use the OPA2205 simulation model to estimate the error budget derived would be that based on these typical values. The result would not be the worst case error budget.

    You could modify the simulation circuit such that external voltage sources representing the various op amp dc errors could be summed in series with the OPA2205 non-inverting input. For example, you could add one voltage source in series with the non-inverting input to represent the additional 20 uV of voltage offset such that the sum of the internal offset and external equal 25 uV. The sign of the voltage source would have to be observed to obtain the 25 uV total. This could be done for the other dc errors as well. The input bias current could be maximized by adding additional current sources from each input to ground. Again, the polarity of the bias currents would have to be observed such that the maximum bias current resulted.

    The two application notes that you cite provide the analysis methods needed to do the worst case budget analysis. I think that doing the analysis on paper, or in a spreadsheet is a more insightful and a more direct approach than using simulation.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 60 points

    Dear Mr. Thomas Kuehl,

    Thank you very much for your answer. I have tried to simulate with external error sources (modeled using voltage/current sources) and the results are very accurate.

    I will also look more into the worst-case error analysis.

    Thank you very much for your support!

    Best regards,

    Phan Thanh Phong