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PCM2912A: Sizing capacitors

Amy Domino
Prodigy 190 points
Part Number: PCM2912A


Hello,

I am using the PCM2912A in a design and I have a few questions. 

1. The application example in the datasheet has a cap on the VIN mic input. My mic is the MR-28406-000 by Knowles Electronics. I am wondering how to size the cap? I know it depends on the required frequency response of the mic input. But the mic datasheet does not mention required freq response. There is a sensitivity graph across freq though.  Do you have any guidance here?

2. I have a similar question on the caps on VOUTR and VOUTL. I have the freq response of my speaker, but I am not sure how to size the caps based on it. Again, any guidance would be helpful. 

Thanks!

  • 0
    Intellectual 550 points
    The mic or the speakers would not specify "required frequency response" --- *you* decide / specify your required frequency response (well, the *actual* freq. response of the microphone can affect your decision --- if the mic does not respond below 100Hz, there's no point in you "requiring" a freq. response down to 10 or 20 or 30Hz. Still with this hypothetical example: maybe if you want a good margin so that you have little phase-distorsion at 100Hz, then yes, you could design your cutoff frequency to be 5 or 10 times below the required frequency).

    For the mic, the frequency response depends on the input pin of the chip (its resistance to GND). Table 7.5 indicates a resistance of 20k; C9 in the application example (0.22uF) makes a high-pass filter here, with cutoff frequency of 36Hz. So, if *your* required frequency response should extend to below 36Hz (or if you want a clean phase response and want to leave a larger margin between the cutoff frequency and the lowest frequency of interest), then you can use something higher than 0.22uF. I recommend that you stick to film capacitors for this one.

    For the speakers output, the idea is the same --- only in this case the freq. response does depend on the speaker impedance. If it is a 32-ohm headphone, with the 100uF the example circuit shows (C13 and C16), you'll get a cutoff of 50Hz. Personally, I find 50Hz a bit too high, so I would maybe go up to 220uF or even 470uF for those capacitors. You'll probably have little or no choice other than electrolytic capacitors for C13 and C16 (some of these are advertised as "for audio applications" --- to be honest, I'm not really sure whether they indeed lead to higher audio quality).

    Good luck with your design!

    Carlos
    --
  • 0 in reply to Carlos Moreno1
    Prodigy 190 points

    Hey Carlos,

    Great! Thanks for the response that is really helpful. I have a few additional questions.

    1. Do you suggest a film cap because of non linearities in ceramic caps?

    2. My speaker has an impedance of 8Ohm. But it only has a frequency range of 400Hz to 10kHz ( the part is DB Unlimited SW280408-1). So a cutoff of 50Hz is probably a bit lower than I need. Is there any downside to a 50Hz cutoff other than a larger capacitance?

    Regards,

    Amy

  • 0 in reply to Amy Domino
    Intellectual 550 points
    Glad I could help!

    For (1) --- yes, I've seen in general the recommendation to avoid ceramic capacitors in the audio path. Some sources specify that it is the X7R's only (meaning that NP0 are ok); I prefer to avoid the ceramics across the board when we're talking about the audio signal's path.

    (2) It certainly is up to you in terms of what you want to obtain; but it does look like something as low as 22uF would still be more than ok (that would give you 90Hz cutoff, a bit above two octaves below, so you'd have very low phase distorsion and very low attenuation at 400Hz. 10uF may not be crazy (you're still more than an octave below the frequency of interest). However, but be careful on how the speaker specifies the freq. response --- there may still be a lot of valuable frequency response below 400Hz. Again, it will be your decision as the designer.

    All the best,
    Carlos
    --
  • 0 in reply to Carlos Moreno1
    Prodigy 190 points
    Wouldn't I need 220uF for 90Hz cutoff? Fc = 1/2pi*R*C where R is 8 Ohm and C is 220uF?
    I assume you read 8 Ohm as 80?
  • 0 in reply to Amy Domino
    Intellectual 550 points
    Hahaha --- yes, I did read 80 !!! Even though I found it a curious value for a speaker impedance, I still fell for it! (anyway, my suggestions are still the same, with the capacitance values multiplied times 10, of course)

    Regards,
    Carlos
    --
  • 0 in reply to Carlos Moreno1
    TI__Guru 57800 points

    Thanks for your response Carlos!, It is very complete.

    Amy, Please let us know if  you have further questions.

    Best Regards,

      -Diego Meléndez López
       Audio Applications Engineer