[gta02-core] ECN0032 (EMI and ESD protection for headset jack)

[Werner Almesberger]

Rene Harder wrote:
> Yes [ R4401 ] looks pretty much like a current limiter.

Joerg and I had a quick discussion on IRC:

- he agrees that R4401 is probably a current limiter to protect the
  codec's MIC1 input. Can't hurt to have it.

- he also agrees that C4405 should go. Also, R4404 can safely meet
  the same fate.

- he suggests to switch to a symmetrical configuration for the
  microphone, like in earlier GTA02 variants or in GTA01.

  The change to single-ended in GTA02v6 was an unsuccessful attempt
  to solve the buzz problem. A symmetrical design would have the
  advantage of better common-mode rejection.

  In the symmetrical design, R4301 and R4302 would be useless, so
  it's not any more complicated than the asymmetrical design. Plus,
  it would actually be a bit clearer what things do.

Regarding component choices, I looked for filter arrays which would
save even more space, e.g., Murata's NFA series. Alas, they all
seem to be difficult to source.

- Werner