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TAC5242EVM-K: AC coupling input capacitor

aforino
Prodigy 120 points
Part Number: TAC5242EVM-K


Tool/software:

In the TAC5242EVM-K a relatively low quality X5R ceramic capacitor with 10uF is used for input ac coupling (C4, C5, C7, C8). Since this is directly in the audio path is this capacitor not very critical in terms of performance for distortion and should be a much higher quality and more suitable capacitor such as a X7R or electrolytic capacitor as outlined in this document: https://www.ti.com/lit/an/slyt796a/slyt796a.pdf?

  • +1
    TI__Mastermind 47945 points

    Hi,

    You're referencing the right document. For cost purposes the EVM used lower quality input caps. But if audio distortion is a concern for your design, then we do recommend using an X7R cap or even better a film capacitor. The advice in the app note applies to this codec as well.

    Best regards,
    Jeff McPherson

  • 0 in reply to Jeff McPherson
    Prodigy 120 points

    Could I add a 100k resistor in series after the capacitor to then reduce the capacitor size to a value where a C0G is available (like 0.15uH) or is this not recommended?

  • 0 in reply to aforino
    TI__Mastermind 47945 points

    Hi Aforino,

    I assume your plan is to add the resistor to keep the cutoff frequency the same (formed by the cap + input impedance). But this would still add the non-linearity of a ceramic cap to the audio path. 

    The material is the dominating factor rather than the size of the cap.

    Best regards,
    Jeff McPherson

  • 0 in reply to Jeff McPherson
    Prodigy 120 points

    Hi Jeff, 

    Thank you for your quick response.

    So my plan is to downsize the capacitor to something like 0.15uH so I can use a C0G capacitor, because C0G capacitors are not available with 10uF values. C0G ceramic capacitors should give very low distortion compared to X5R and X7R capacitors.

    Best,

    aforino

  • +1 in reply to aforino
    Prodigy 120 points

    I answered my own question. Significantly increasing the input resistance reduces noise immunity which is exactly whats not preferred in this case.