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OPA1632: "headroom" and dynamic specifications improvement

user4626547
Prodigy 70 points
Part Number: OPA1632

Hello,

Could you please provide some advice on this design?

I would like to improve the specifications of my OPA1632 in the audio range. In this condition, if the input voltage is "too high" (more than a third of the full range I wanted to use) I get some high harmonics on my output. I noticed that those harmonics get lower when I remove C2 and C4 (resulting in a better THD and SINAD) but I would like to improve further.

Is there some "obvious flaw" of the design that I missed? Do you have any recommendations on R5/R6/C3 values? Is it "wrong" if I remove C2 and C4?

Thanks in advance.

Sincerely yours,
Theo

  • 0
    TI__Genius 15975 points

    Hi Theo,

    I don't think there is an obvious flaw in your design. However, I would not recommend removing the C2 and C4 because your circuit will oscillate without them while driving the large 10.1nF capacitor. I would also recommend including a 1nF cap across the input pins, as shown below for stability. 

    For improving the THD and SINAD, I think the way you can improve it is by reducing the load at the OPA1632 output. You can try to select a higher value for R7 and subsequently a low value for C3 instead while keeping the same corner frequency. Also, if you can try to increase the Rf values (R8 and R9 in your circuit) and subsequently reduce the Cf values (C2 & C4 in your circuit) keeping the same corner frequency, you might get some improvement in the THD. However, I would be wary of increasing the resistor values too much or else you might end up with a lot of thermal noise at the output.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Prodigy 70 points

    Hello Rohit,

    My initial THD problem came from the clamp diodes that I did not include in this particular simulation. I gained about 15dB of THD after switching them with higher performance ones.

    I will keep C3 and C4 removed. They do not affect the performance that I am looking for in the audio band, and keeping them removed improves my THD spec. The ADC driven by this circuit is "hard to drive" so keeping C5 at this value is necessary unfortunately.

    Thank you for your feedback.

    Best regards,
    Théo