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

Werner Almesberger werner at openmoko.org
Wed Oct 7 22:39:32 CEST 2009

Rene Harder wrote:
> My main idea of using discrete filters though, was to keep new
> components (with different packages) as small as possible to reduce SMT
> assembly cost.

Yes, I was hoping that we could have one "universal" filter that's
suitable for all low-frequency signals (audio, UART, etc.), given
that most of the noise lives >> 100 MHz and those low-frequency
signals are << 100 MHz.

> Also you are more flexible with a discrete filter than with a single
> component one; If we mess things up, we can just replace the caps or the
> inductor and fix the problem but with a single component filter we would
> need a different one and might need to change the pcb pattern as well.

That's true, yes. Of course, I fervently hope that we can avoid the
joy of precision-tuning those filters ;-)

> However, I think we are better off using a single component filter, this
> decreases complexity and risk of possible misbehavior.

Great ! 

When we discussed integrated filters, we came up with the following

Murata NFL18ST (0603, "T"):

Rohm MCF182CN102 (0603, "for power supply"):

TDK MEM2012P101R (0805, "Pi"):

Which one would be closest to the kind of "universal" filter we're
looking for ? As usual, smaller would be better :-)

> I've no idea what the purpose of this resistor is/was. This seems to be
> an ordinary pull-down resistor but for what I can't tell for sure.
> Nothing makes completely sense to me.

Are we talking about the same part ? R4401 is 220 Ohm and series, while
the pull-downs in the area (R4416, R4117) are 10 k. Looks more like a
current-limiter for problem conditions to me.

Joerg, do you remember what it does and whether we still need it ?

- Werner

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