[gta02-core] Audio ECN update

[Werner Almesberger]

Rene Harder wrote:
> > ECN0038: 39pF+18pF ~= 56 pF ? 
> You found it! ;-)

Just because I had 38+18 = 47 in my own notes as well ;-)

> Do you mean the EMI protection of the headset jack!  Sure using an
> integrated filter works with me, I just thought we do not want too many
> new components.

Yes. Ideally, I would use a "general-purpose" EMI filter, such as the
Murata NFL18ST series.
http://search.murata.co.jp/Ceramy/image/img/PDF/ENG/L0112S0119NFL18ST.pdf

They're 0603, and the data sheet promises truly magic characteristics,
including zero insertion loss at DC. The downside is that it seems only
Murata makes such things.

Also the Rohm MCF18 series, particularly the MCF182CN102 may be suitable
(and is a lot cheaper):
http://www.rohm.com/products/databook/emi/pdf/mcf18.pdf

Fun things I found along the way: a one-stop shopping EMI and ESD
solution for USB:
http://www.st.com/stonline/products/literature/ds/10950/emif02-usb01f2.pdf

The same for SDIO:
http://www.st.com/stonline/books/pdf/docs/13195.pdf

> However, I think discrete filter are more flexible regarding vendor or
> parameter changes.

What I'm not sure about is what parameters we really need. Most of
the signals we want to filter are slow: < 20 kHz for audio, < 1 MHz
for UART, < 4 MHz for SIM, <= 25 MHz for SDIO. The big interferences
are all in the GHz range. So a filter with a cut-off frequency around
100-300 MHz ought to be okay, right ?

- Werner