Lpc2000

----- Original Message ----- 
From: "charlesgrenz" <charles.grenz@...>
To: <lpc2000@yahoogroups.com>
Sent: Saturday, January 28, 2006 1:28 PM
Subject: [lpc2000] EMI - was Re: lpc2138 5V tolerant is not really tolerant!


> --- In lpc2000@yahoogroups.com, Rob Jansen <rob@m...> wrote:
>>
>> I'm not an EMI expert,
>> > I do have a 51
>> > ohm series resistor on all I/O lines including the RXD and TXD of
>> > USART0 to make sure that no EMI problems occur.
>> >
>> > regards,
>> > Charles
>> >
>>
>> I did do some ESD research/design once for a web tablet. Although
> the IC
>> used (forget which one) was said to be ESD safe, firing at the touch
>> screen with a Human Body Model ESD tester resulted in error. I then
>> added R/C combinations to resolve this problem.
>>
>> But I'm definitely not an EMI expert and would even think that adding
>> resistors may make the problem worse ...
>> Are there any special considerations to take into account with (apart
>> from PCB design and use of separate ground/power planes) ?
>>
>> Regards,
>>
>>     Rob
>>
>> P.s: let me state that I was _not_ the original designer of that web
>> tablet :o)
>>
> Hi Rob,
>
>  I am not either even though I have been designing our products to
> eliminate EMI for CE approvals for the past 4 years on our products.
>
>  I have also used what Leon suggested as well (the AVX transguards),
> but I am using some resistor packs from CTS that are 0.5mm pitch which
> allows me to put them about 3mm away from the processor pins.
>
>  We also designed a motion control stage for a large Canandian
> company and they wanted 47 ohm resistors on all I/O's from a large 12
> x 14 inch, multilayer board. They still had problems with the EMI, but
> then we added some ferrits to the motor stages and that helped them
> drop to 5db below class B.
>
>  We do have clients (one in particular) who are and suggested adding
> an resistor to each processor line to eliminate any EMI and to help
> with ESD. They suggested a 30 to 51 ohm resistor on all processor
> lines along with 100pf to 1nF capacitor on ports that extend off the
> board. We have followed this advice 3 times and each of our boards
> that went through CE, class B with no problems. There are still other
> considerations like board layout, capacitor and other components to
> help reduce the noise as well.
>
>  We design allot of motion control and there is plenty of noise that
> is generated that we have to take into account. One saving grace is
> the CE approved power supply. We ran experiments with one at an
> approved lab and found that it eliminates all the conducted emmissions
> from the system if one is there. That saved us from having to put
> chokes and such on our boards since we could be pumping as much as 12
> amps through the board at any one time.

Where I used to work we had to put ferrite beads and capacitors on the 
serial port lines to get our units through the emissions tests. Connectors 
are available with ferrite built-in, which could be useful.

Leon