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OPA1612: THD model

Remy HOFF
Prodigy 230 points
Part Number:

Hi all,

i am building an input stage design with OPA1612 with TINA for TPA3255 (PFFB) and quite surprised by somme results in terms of THD.

Fourrier analysis show a very high THD calculation of 0.11163% !

Several calculation shows that THD depends of R*C products.

Better values in terms of output noise for bipolar AOP should be 1K/1K. With 330pF, the THD calculation shows 0.008531%.

Following to what i am seeing, i tried with 22pF. In this case, gain pole fequency is quite high and the signal integrity should not be modified (gain/phase), then i do not understand why the THD calculation shows a better result (0.00075097%).

Then, my question is : is OPA1612 TINA model be accurate for THD calculations ?

Second question. we often see schematic with a R3 bias resistor (with value = R1//R4) on positive input to reduce/cancel output offset voltage. Since long years, i thought that the goal is to low the THD but TINA does not show that. Moreover, it ups the total noise (about 3dB in my case).

Do i forget something ?

Thank you in advance for your help and best regards.

  • 0
    Prodigy 230 points

    [quote user="Remy HOFF"]

    Part Number: OPA1612

    Hi all,

    i am building an input stage design with OPA1612 with TINA for TPA3255 (PFFB) and quite surprised by somme results in terms of THD.

    Fourrier analysis show a very high THD calculation of 0.11163% !

    Several calculation shows that THD depends of R*C products.

    Better values in terms of output noise for bipolar AOP should be 1K/1K. With 330pF, the THD calculation shows 0.008531%.

    Following to what i am seeing, i tried with 22pF. In this case, gain pole fequency is quite high and the signal integrity should not be modified (gain/phase), then i do not understand why the THD calculation shows a better result (0.00075097%).

    Then, my question is : is OPA1612 TINA model be accurate for THD calculations ?

    Second question. we often see schematic with a R3 bias resistor (with value = R1//R4) on positive input to reduce/cancel output offset voltage. Since long years, i thought that the goal is to low the THD but TINA does not show that. Moreover, it ups the total noise (about 3dB in my case).

    Do i forget something ?

    Thank you in advance for your help and best regards.

  • 0 in reply to Remy HOFF
    TI__Guru* 93105 points

    Hello Remy,

    Regarding your questions:

    Then, my question is : is OPA1612 TINA model be accurate for THD calculations ?

    • No. the simulation model on which the OPA1612 model is based is a behavioral model (controlled sources and mathematical relationships)  and does not include the ability to replicate THD. We looked into ways of including THD behaviors in op amp simulation models a few years ago, but their were so many variables and operating conditions that it just proved impractical. I don't know of a single op amp model that includes THD.

    Second question. we often see schematic with a R3 bias resistor (with value = R1//R4) on positive input to reduce/cancel output offset voltage. Since long years, i thought that the goal is to low the THD but TINA does not show that. Moreover, it ups the total noise (about 3dB in my case).

    I hope this helps.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 230 points

    Clear explanations, much appreciated !